Improved Boron 10 Quantification via PGNAA and
ICP-AES
by
Kent J. Riley
B.S., University of Michigan, Ann Arbor (1993)
Submitted to the Department of Nuclear Engineering
in partial fulfillment of the requirements for the degrees of
MASTER OF SCIENCE IN NUCLEAR ENGINEERING
AND
NUCLEAR ENGINEER
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June 1997
Copyright © 1997 Massachusetts Institute of Technology.
All rights reserved.
Signature of Author:
Department of Nuclear Engineering
May 9, 1997
(
Certified by:
Prof. Otto K. Har n , 'thesis supervisor
Professor of(Nuclear Engineering
Dr. Guido R. Solares, Thesis Reader
BIDMC - Harvard Medical School
Accepted by:
//
I
Prof. Jeffrey P. 4 reidberg
Chairman, Departrent ComA ittee on Graduate Students
C.
JUL 10 1997
LUBARES
Science
Improved Boron 10 Quantification via PGNAA and
ICP-AES
by
Kent J. Riley
Submitted to the Department of Nuclear Engineering on May 9, 1997
in partial fulfillment of the requirements for the degrees of
Master of Science in Nuclear Engineering
and
Nuclear Engineer
Abstract
Boron 10 quantification is a vital part of the clinical trials and other
research that is in progress under the Massachusetts Institute of Technology
- Beth Israel Deaconess Medical Center (MIT/BIDMC) joint BNCT project.
For example, accurate knowledge of the Boron 10 content of blood as a
function of time and immediately prior to irradiation is needed to calculate
the total dose delivered to healthy tissue. The MIT/BIDMC group continually
seeks to improve and refine quantification methods to achieve greater
sensitivity and faster analysis time. Rapid analysis is desirable so that
irradiation can be initiated quickly, while high sensitivity will be needed to
analyze very small samples. Brain tumor biopsies may be quite small,
especially in cases where the tumor is not resectable, or blood samples may
be very small when repeated samples are taken from small animals.
As a major part of this thesis, an upgraded version of the MIT prompt
gamma neutron activation analysis (PGNAA) facility has been completed.
The upgraded facility boasts a threefold increase in the neutron flux at the
sample position. The increased flux results in a higher sensitivity and
improved detection limits. Efforts to reduce the background count rate of the
PGNAA facility have resulted in a background count rate that is dominated
by interactions that occur in the sample. Although the gross integral
background count rate is higher than before the improvements, system dead
time is still tolerable and the increased thermal neutron flux results in a net
performance gain.
The improved PGNAA facility is well suited for rapid analysis, as it is
able to quantify 1 ppm of boron in a 0.5 ml sample in less than 5 minutes
with a statistical uncertainty of 10%. The PGNAA facility is also able to
quantify blood or tissue samples as small as 0.05 ml, with concentrations as
low as 5 ppm in less than an hour.
An inductively coupled plasma atomic emission spectroscopy (ICPAES) machine employing a cross-flow nebulizer, and later a High Efficiency
Nebulizer (HEN), has also been implemented for routine sample analysis.
The cross flow nebulizer is able to analyze samples as small as 0.05 ml with
concentrations as low as 1 ppm, while the HEN can quantify samples as
small as 0.01 ml and with concentrations of approximately 1 ppm. The AES
facility is more sensitive than the PGNAA facility and is therefore able to
analyze smaller samples and lower concentrations. The time required to
complete analyses with the AES facility is, however, dependent mostly on the
time required to prepare the sample. Sample preparation times can be as
long as 40 minutes, depending on the sample type.
This thesis will outline the demands of quantification schemes for
BNCT and evaluate PGNAA and ICP-AES in light of these demands. In
particular, the accuracy, sensitivity, speed, convenience and relative
advantages of the two methods will be compared. The improved PGNAA
facility described in this thesis will also be compared with other PGNAA
facilities of its kind.
Acknowledgments
It has been a rewarding experience to work with Professor Otto K.
Harling, my thesis advisor. Throughout my research I have learned how to
conduct thorough scientific measurements, and learned the importance of
collecting and interpreting data. It has been my honor to work with such a
diligent and scrupulous scientist, his lessons and values will serve me for
years.
Much of my work would have not been possible without the
tremendous help of John DiCiaccio, NRL Maintenance and Ron St. Jean, now
retired Machinist. Their patience with and ability to work around my many
mistakes was often a lifesaver. They also took the time to teach me basic
machine shop skills that will make me a better engineer.
A very special thank you goes to Professor Tony Nunes from the
University of Rhode Island Physics Department. Professor Nunes was a
tremendous help in designing, testing and implementing the focusing
monochromator described in this thesis. Thanks also to Professor Cliff Shull
for his input and assistance.
Reactor Operations has always been ready to help me in my seemingly
endless endeavors at the 4DH3 beamport. Nearly everyone from Operations
has helped me in some way at some point in my research, and I am grateful
to everyone. I would like to specially thank Don Kelly for his constant
support and friendship.
The Reactor Radiation Protection Office handled my renovations at
4DH3 expertly. I thank them for their support and care in what was
potentially dangerous work. Special thanks to Fred McWilliams for his ideas
and answers to my endless stream of questions.
The BNCT Group at MIT is full of talented people, I would like to
collectively thank them all for their support and assistance. I would
especially like to thank Dr. Guido Solares for serving as reader of this thesis,
and for his support and encouragement throughout my work on the
MIT/BIDMC BNCT project.
I would like to thank my family for their support throughout my
research. Finally, I would like to thank my fiancee, Heather Multhaupt.
Her endless motivational, emotional, financial and, at times, technical
support carried me through difficult times. Without her, this certainly would
not have been possible.
This research was supported by the U.S. Department of Energy under
Contract No. DE-FG02-87ER 60600.
Table of Contents
CHAPTER ONE
19
Introduction
1.0 BACKGROUND
19
1.1 THE ROLE OF 10B QUANTIFICATION
20
1.2 0oB QUANTIFICATION FACILITIES
24
1.2.1 PGNAA
24
1.2.2 ICP-AES
1.3 RESEARCH GOALS
34
1.4 REFERENCES
36
CHAPTER TWO
41
Prompt Gamma Neutron Activation Analysis
2. O0 FIGURES OF MERIT
41
2.0.1 SENSITIVITY
2.0.2 LIMIT OF DETECTION
43
2.0.3 BACKGROUND COUNT RATES
44
2.1 DESIGN CONSIDERATIONS
2.1.1 DESIGN GOALS
2.1.2 BACKGROUND ASSAY
44
2.1.3 PROPOSED MODIFICATIONS
51
2.1.4 DESIGN CONSTRAINTS
57
2.2 PORT PLUG DESIGN AND CONSTRUCTION
58
2.2.1 CONCEPTUAL DESIGN
58
2.2.2 ENGINEERING DESIGN
62
2.2.3 PORT PLUG INSTALLATION
70
2.3 FOCUSING MONOCHROMATOR DESIGN
79
2.3.1 CONCEPTUAL DESIGN
79
2.3.2 DIFFRACTION ANGLE OPTIMIZATION
89
2.3.3 CONSTRUCTION
96
2.3.4 CRYSTAL ALIGNMENT
99
2.4 SAPPHIRE FILTER CRYSTAL MEASUREMENTS
106
2.4.1 CRYSTAL UNIFORMITY TEST
109
2.4.2 TUNING MEASUREMENTS
111
2.4.3 TRANSMISSION MEASUREMENTS
113
2.4.4 CONCLUSIONS
121
2.5 FINAL CONFIGURATION AND PERFORMANCE OF
UPGRADED FACILITY
122
2.5.1 SAPPHIRE FILTER CRYSTAL MEASUREMENTS
122
2.5.2 FOCUSING MONOCHROMATOR MEASUREMENTS
127
2.5.3 BACKGROUND MEASUREMENTS
132
2.5.4 SENSITIVITY AND DETECTION LIMIT MEASUREMENTS
2.6 REFERENCES
CHAPTER THREE
138
144
147
Inductively Coupled Plasma Atomic Emission
Spectroscopy
3. 0 FIGURES OF MERIT
147
3.0.1 SENSITIVITY
148
3.0.2 LIMIT OF DETECTION
149
3.0.3 BACKGROUND COUNT RATES
149
3.1 ROUTINE ANALYSIS WITH CROSS FLOW NEBULIZER
150
3.1.1 SAMPLE PREPARATION
151
3.1.2 FIGURES OF MERIT
155
3.1.3 CROSS CALIBRATION WITH THE PGNAA FACILITY
158
3.2 SMALL SAMPLE ANALYSIS WITH HIGH EFFICIENCY
NEBULIZER
163
3.2.1 DESCRIPTION OF THE HIGH EFFICIENCY NEBULIZER
163
3.2.2 FIGURES OF MERIT
165
3.2.3 CALIBRATION OF THE HIGH EFFICIENCY NEBULIZER
167
3.3 REFERENCES
170
CHAPTER FOUR
173
Comparison and Conclusions
4. 0 FIGURES OF MERIT
173
4.1 OTHER CONSIDERATIONS
177
4.1.1 SAMPLE PREPARATION
178
4.1.2 DESTRUCTIVE VS. NON-DESTRUCTIVE
179
4.1.3 MATRIX EFFECTS
181
4.2 ANALYSIS SCENARIOS
183
4.2.1 ANALYTICAL RANGE
183
4.2.2 ANALYSIS SPEED
186
4.2.3 OTHER CONSIDERATIONS
187
4.3 CONCLUSIONS
188
4.4 POSSIBLE IMPROVEMENTS
191
4.4.1 PGNAA
191
4.4.2 ICP-AES
193
4.5 REFERENCES
APPENDIX A
Engineering Drawings of the Port Plug
Constructed for 4DH3
196
197
APPENDIX B
Biodistribution Data From Human Clinical
Trials
213
List of Figures
CHAPTER ONE
Figure 1.1:
19
Example of a biodistribution curve. The first shaded
region represents the period during which the boron drug
is infused.
Subsequent shaded regions represent
irradiation times.
22
Figure 1.2: Decay Scheme for 11B*
25
Figure 1.3: Cross sectional View of the MITR-II reactor showing the
4DH3 beamport and adjacent experimental facilities.
(Drawingcourtesy of Todd Date, RRPO)
27
Figure 1.4: PGNAA Facility Prior to Renovation
28
Figure 1.5: Schematic of the ICP-AES facility
33
CHAPTER TWO
41
Figure 2.1: Summary of Background Components seen by Detector at
4DH3
Figure 2.2: Side view of port plug assembly
59
Figure 2.3: Schematic of the reservoir system used to open and close
the water shutter at 4DH3. A vacuum pump can be
connected at positions 1,2, and 3 depending on whether or
not the water shutter is to be filled or drained.
67
Figure 2.4: Photographs of the final port plug assembly. The end cap
is removed (left photo), revealing the lead collimator that
sits inside the water shutter. The rear side of the end cap
(to the right of the plug) shows the water inlet and outlet
holes and the nitrile o-ring seal. With the end cap
attached (right photo) the aluminum inlet and outlet
tubing welded to the cap is visible.
70
Figure 2.5:
A photograph of the insertion jigs containing the
rectangular sapphire crystal bars. The jigs were used to
insert the sapphire crystals into the water shutter at the
4DH3 beamport.
76
Figure 2.6: Composite drawing depicting the final configuration of the
port plug at 4DH3, including the two 15 cm sections of
sapphire crystal. The water inlet and outlet lines have
been omitted since the water shutter is no longer
functional.
78
12
Figure 2.7: Schematic illustrating the concept of Bragg diffraction
Figure 2.8:
Schematic illustrating the focusing effect of curved
reflecting surfaces.
Figure 2.9: Rocking curve used to calculate the mosaic spread of the
single crystal graphite piece #1.
Figure 2.10: Slow neutron attenuation coefficient for sapphire single
crystal as a function of neutron energy, showing the fourth
order polynomial curve fit parameters. Data was taken
from Reference 13.
Figure
The Maxwell-Boltzmann thermal neutron flux
2.11:
distribution as a function of energy (E), speed (v) and
Bragg angle (0), plotted versus neutron energy. The
differential elements for each curve are not linearly
related and therefore change the shape of the curve.
Figure 2.12: Composite function for PGNAA sensitivity plotted versus
Bragg angle and corresponding neutron energy. The peak
occurs at approximately 190.
Figure 2.13:
The
The fully assembled focusing monochromator.
faint
The
visible.
are
of
graphite
layers
and
strips
several
curvature (R = 20.8") of the holder (lower surface) is
difficult to make out.
Figure 2.14: Schematic depicting the experimental arrangement used
to determine that the optical surface and the
crystallographic planes of the graphite crystals are
102
parallel to each other.
Figure 2.15:
Photograph of the alignment apparatus for positioning
the many pieces of graphite crystal. The bottom of the
photograph shows a layer of crystals with the laser
reflecting from one of the pieces. The top right portion of
the photograph shows the reflection viewing screen with a
103
ruler scale.
Figure 2.16: Schematic of experimental setup for measurements on the
107
sapphire crystal.
Figure 2.17: Transmitted beam count rate vs. Nominal position in the
The count rate does not
sapphire single crystal.
significantly change regardless of where the beam is
109
positioned, indicating a very uniform crystal.
Figure 2.18: Schematic showing the locations at which the uniformity
110
of the crystal was tested.
Figure 2.19: Transmitted neutron count rate plotted versus rotational
position of the sapphire single crystal.
112
Figure 2.20: Neutron time of flight energy spectrum for an unfiltered
neutron beam (top) and a beam filtered with 15 cm of
sapphire single crystal (bottom).
116
Figure 2.21: Fraction of transmitted neutrons plotted versus neutron
energy for a 15cm long sapphire single crystal filter.
Transmission is greatest at approximately 0.02 eV, in
rough agreement with Figure 2.10.
118
Figure 2.22: Comparison of measured attenuation coefficients versus
data published in the literature. Published values from
(13).
120
Figure 2.23:
Schematic depicting locations at which the dose rates
shown in Table 2.3 were measured.
126
Figure 2.24: Rocking curve for focusing monochromator, measured at
the 4DH3 beamport.
128
Figure 2.25: Tilt curve for the focusing monochromator, measured at
the 4DH3 beamport.
130
Figure 2.26: Schematic depicting the layout of the PGNAA after the
modifications described in this thesis. The significant
changes are the removal of the collimator shims, the
addition of the focusing monochromator, and the addition
of the lithium cage around the sample position.
133
Figure 2.27:
Estimated required counting times to reach 10%
statistical uncertainty in the net area under the boron
peak for various concentrations and sample sizes.
140
Figure 2.28:
Comparison of counting time required to reach 10%
statistical uncertainty in the boron peak. The dashed
curve represents the facility after modification and solid
lines represent the facility before modification.
Each
curve is for a sample of 0.1 ml.
141
CHAPTER THREE
147
Figure 3.1: Calibration curves for the PGNAA (closed circles) and the
ICP-AES (closed squares) facilities.
160
Figure 3.2:
Schematic depicting the High Efficiency Nebulizer
purchased from JE Meinhard Associates Inc. () Image
courtesy of JE MeinhardAssociates Inc.
164
Figure 3.3: Calibration curve for the ICP-AES facility using the High
168
Efficiency Nebulizer.
CHAPTER FOUR
Figure
4.1:
173
A plot showing the range of sample sizes and
concentrations that can be efficiently analyzed with each
technique. A sample falls within the analytical range of
the technique if its coordinates on the above plot fall
184
above and to the right of the appropriate line.
APPENDIX A
197
Figure A- 1: Composite drawing of the port plug constructed for 4DH3. 199
Figure A- 2: Composite drawing showing the final configuration of the
port plug at 4DH3 with the two 6" sections of sapphire
crystal. The inlet and outlet lines for the water shutter
have not been included since the water shutter is no
200
longer operable.
Figure A- 3: Front view of lead and borated paraffin inserts.
201
Figure A- 4: Side view of lead and borated paraffin inserts.
202
Figure A- 5: Front view of the water shutter insert.
203
Figure A- 6: Front view of the end cap (for end of plug nearest reactor). 204
Figure A- 7: Side view of end cap.
205
Figure A- 8: Front view of water shutter seal plate.
206
Figure A- 9: Side view of water shutter seal plate.
207
Figure A- 10: Front and side views of stainless steel flange for the
208
concrete and lead filled port plug.
Figure A- 11: Side view of lead and concrete filled port plug.
209
Figure A- 12: Top, side and front view of the insertion jig that was
used to insert the two 15 cm sections of single crystal
sapphire into the beam port at 4DH3. This drawing
210
shows the jig that was inserted nearest the reactor core.
Figure A- 13: Detail drawing for the jig that was inserted nearest the
211
reactor core.
Figure A- 14: Detail drawing for the insertion jig that was inserted
212
furthest from the reactor core.
APPENDIX B
213
Figure B- 1: Biodistribution test dose curve for subject 94-1.
217
Figure B-
Biodistribution test dose curve for Subject 94-2.
218
Figure B-
Biodistribution test dose curve from Subject 94-3.
219
Figure B-
Biodistribution test dose curve for Subject 95-1.
220
Figure B-
Biodistribution test dose curve from Subject 96-1.
221
Figure B-
Biodistribution test dose curve for Subject 96-2.
222
Figure B-
Biodistribution curve for Subject 96-3, taken on the day of
irradiation.
223
Figure B- 8: Biodistribution curve for Subject 96-4, taken on the day of
irradiation.
224
Figure B- 9: Biodistribution curve for Subject 97-1, taken on the day of
irradiation.
225
Figure B- 10: Biodistribution curve for Subject 97-2, taken on the day
of irradiation.
226
Figure B- 11: Biodistribution curve for Subject 97-3, taken on the day
of irradiation.
227
Figure B- 12: Biodistribution curve for Subject 97-4, taken on the day
of irradiation.
228
Figure B- 13: Biodistribution curve for Subject 97-5, taken on the day
of irradiation
229
Figure B-
14: Data from all subjects receiving 250 mg/kg IV
administration of BPA-f, except Subject 96-4.
The
infusion period is fit with a rising exponential (table on
the right), and the washout period is fit with a triple
exponential (table on the left). The parameters represent
a least squares fit to all the data points on the curve.
230
List of Tables
19
CHAPTER ONE
Table 1.1:
10B
sensitivities for several prompt gamma facilities. The
results for MIT are for the prompt gamma facility before
and after (in bold) its modification as described in this
31
thesis.
41
CHAPTER TWO
Table 2.1: Summary of Background Components seen by Detector at
47
4DH3
Table 2.2:
Table 2.3:
Summary of measured and calculated
percentages for single crystal sapphire.
transmission
115
Comparison of dose rates and neutron count rates
surrounding the masonite shielding at 4DH3. Bold values
indicate measurements taken after the removal of the
collimator shims and insertion of the sapphire crystals.
N/A indicates that no significant count rate or dose rate
125
could be measured.
Table 2.4: Contribution factors to the slow neutron flux at the sample
position at 4DH3. The net flux increase is due to both the
removal of the collimator shims and the effect of the
132
focusing monochromator.
Table 2.5: Summary of background components seen by the detector at
4DH3 before and after the modifications described in this
chapter. A 0.5 ml deionized water sample was used to
134
determine the sample interaction component.
Table 2.6:
Measured sensitivities for various elements (all with
natural isotopic abundances) for the PGNAA system
before and after the modifications described in this thesis.
The sensitivity has been increased on average by a factor
138
of about 2.6.
CHAPTER THREE
147
Table 3.1: Results of analyses for boron in human blood samples, using
161
both the PGNAA and ICP-AES techniques.
CHAPTER FOUR
173
Table 4.1: Performance summary for the PGNAA facility and the ICPAES facility, using the cross flow and high efficiency
nebulizers.
174
Table 4.2:
10B
sensitivities for several prompt gamma facilities (). The
results for MIT are for the prompt gamma facility after its
modification as described in this thesis.
176
APPENDIX A
197
APPENDIX B
213
CHAPTER ONE
Introduction
1. 0 Background
Boron Neutron Capture Therapy (BNCT) is a binary form of radiation
therapy that has the potential to selectively destroy cancerous lesions, while
leaving normal tissue intact. BNCT was first suggested by Gordon Locher in
1936, only 4 years after the discovery of the neutron (1). BNCT makes use of
the (n,a) reaction that the stable isotope 10B undergoes. The cross section for
this reaction is quite high (3800 barns) for slow neutrons, thus reasonable
reaction rates can be obtained at fairly low concentrations of 10B (several
parts per million). The reaction products are heavy charged particles, which
have ranges on the order of gm in tissue. By selectively loading the tumor
with 10B, via special boron compounds, the reaction products will deposit
nearly all of their energy in tumor cells and spare neighboring healthy tissue.
19
Improved Boron 10 Quantification via PGNAA and ICP-AES
BNCT was first tested on human subjects in the 1950's at the MIT
Research Reactor and at Brookhaven National Laboratories. These early
trials did not demonstrate encouraging results and were eventually
terminated. Since then, boron compounds have been developed with better
selective uptake properties (2), and more penetrating epithermal neutron
beams have been developed (3). These advances, combined with encouraging
results from preliminary animal studies (4), have led to a resurgence in
BNCT research. BNCT is currently under Phase I clinical investigation by
the MIT Nuclear Reactor Laboratory - Beth Israel Deaconess Medical Center
(MIT/BIDMC) project. Brookhaven National Laboratory (BNL) is also
currently involved in a Phase I/ Phase II clinical trial at the Brookhaven
Medical Research Reactor. Several other groups around the world are also
performing research to prepare for or to support clinical trial investigations of
BNCT.
1.1 The Role of 10B Quantification
Boron-10 quantification is a vital part of the MIT/BIDMC BNCT
research program. The MIT-BIDMC group performs Boron quantification at
both the microscopic (- 2.0 gm) level of spatial resolution , (5) and the
macroscopic level (sample sizes 0.5 ml and smaller). Microscopic
quantification with imaging can be used to perform microdosimetric analyses,
20
Chapter 1: Introduction
Kent J. Riley
and can be used to verify and/or correlate macroscopic measurements. This
thesis deals only with macroscopic measurement techniques.
Macroscopic measurements are typically much simpler, and therefore
much less time intensive. Our group uses macroscopic measurements to
obtain biodistribution curves (i.e. boron concentration in a subject as a
function of elapsed time after administration of the boron drug) for each
subject in our Clinical Trial Protocol (6). Figure 1.1 shows an example of a
biodistribution curve from one of the subjects in the MIT- BIDMC Phase I
clinical trial. The first shaded region under the curve shows the time during
which the drug is being infused. Subsequent shaded regions represent time
while the irradiation is taking place.
Improved Boron 10 Quantification via PGNAA and ICP-AES
Subject 96-2 Biodistribution Curve in Blood
25
0J
.i
................
...i ...... .........
. ...
........ ............... .... ..... ......... .
20
-
a
a
0
0
sioni
U
Infusion of BPA-f
- ---
15
Cd
Irradiation Periods
1
0
0
------------------------- - - -
10
0
i
5
-- ------
---- ------- -----
" 1;;
..
1
.
~~_.~
_ I~.
i~__~.
~__~~L.----------------.....
V
0
..
-e
200
400
600
800
1000
Time (minutes)
Figure 1.1: Example of a biodistribution curve. The first shaded region
represents the period during which the boron drug is infused.
Subsequent shaded regions represent irradiation times.
These biodistribution curves are typically generated via
administration of a test dose of the boron drug, followed by sampling of tumor
and normal tissues, as well as frequent blood sampling at regular intervals.
The blood samples are analyzed macroscopically for boron content to
generate the curve seen in Figure 1.1. Tumor and normal tissue samples,
from small biopsy volumes, are usually analyzed microscopically to obtain
information on how the boron distributes itself within normal and tumor
cells. On the day of irradiation, the boron drug is administered, and a blood
sample is taken and rapidly quantified. From the steep drop after the end of
infusion in Figure 1.1, it is clear that rapid analysis is crucial so that the
22
Kent J. Riley
Chapter 1: Introduction
irradiation can begin while boron concentrations are still high. This data
point, combined with the shape of the previously measured biodistribution
curve (or the curve can be measured during the irradiation via remote blood
sampling techniques) allows our group to know how the boron concentration
varies throughout the course of the irradiation. This information is essential
for treatment planning calculations, since the dose delivered will be at least
partially dependent on the concentration of 10B. Previous research has
demonstrated that the ratio of boron concentration in blood to the boron
concentration in normal tissue is near unity (7). Blood concentration values
can therefore serve as a reasonable surrogate for tissue concentrations in
treatment planning and dose calculations.
Macroscopic quantification can also be used to rapidly analyze nearly
any samples of interest for boron content. Microscopic analyses are very time
intensive (several days), yet valuable because they reveal the exact spatial
location of boron in the cell. It may also be useful to obtain only bulk boron
concentrations, but with a much more rapid turnaround time (several
minutes). Of particular interest is the analysis of small samples, with
masses on the order of 0.05 g. Brain biopsies of tumor and/or normal tissue
will likely be 0.05 g or smaller, with boron concentrations in the tens of ppm.
Furthermore, distribution studies with small animals may only permit small
volumes of blood to be drawn if many draws are to be taken. These samples
might be as small as 10 gl. Such small samples demand a very sensitive
23
Improved Boron 10 Quantification via PGNAA and ICP-AES
analytical technique; a technique that is capable of quantifying only a few
hundredths of a microgram of 10B.
1.2
10B
Quantification Facilities
The MIT - BIDMC research group uses two methods to perform
10 B
quantification; Prompt Gamma Neutron Activation Analysis (PGNAA) and
Inductively Coupled Plasma - Atomic Emission Spectroscopy (ICP-AES).
1.2.1 PGNAA
Prompt Gamma Neutron Activation Analysis (PGNAA) is a nuclear
analytical technique that detects the secondary particles that result from a
nuclear interaction (usually slow neutron absorption). When 10 B absorbs a
neutron, an excited state of the isotope
11B
is formed, which rapidly fissions
into an alpha particle and a lithium recoil nucleus. The lithium recoil
nucleus is left in an excited state 93% of the time and emits a characteristic
478 keV photon. Figure 1.2 shows a schematic of the decay scheme for the
excited state of 11B.
24
Kent J. Riley
Chapter 1: Introduction
2.31 MeV
93%
r
0.478 MeV y
Tl2- 10-13 s
Figure 1.2: Decay Scheme for 11B*
The de-excitation of the lithium recoil nucleus can then be detected
with high resolution gamma ray spectroscopy equipment. The number of
photons collected under the 478 keV peak is proportional to the number of
boron atoms present in the sample. PGNAA is a non-destructive analysis
technique because the sample remains intact after analysis.
1.2.1.1 Facilities at the MITR-II
The PGNAA facility at the MITR-II 5 MW research reactor is the
result of many years of work, performed by several graduate students. Each
student has incrementally improved upon the work of his or her predecessor,
and this thesis is another example of that process. A detailed history of the
many iterations the PGNAA facility has undergone will not be given here,
rather the reader is referred to the theses and reports of Lizzo (8),Rogus (9),
Wirdzek, Kubali (10), and Chabeuf (11).
The PGNAA system at the MITR-II is currently situated on the 4DH3
beam port of the reactor. Figure 1.3 shows the location of 4DH3 relative to
25
Improved Boron 10 Quantification via PGNAA and ICP-AES
the reactor core. The 4DH3 beamport runs 4" beneath the bottom of the
reactor core, at a vertical position that coincides with the thermal flux peak.
The beam port passes through the concrete biological shield and the graphite
region to a re-entrant thimble that penetrates the D 20 reflector tank below
the reactor core. At the face of the reactor biological shield, a right-handed
port box (meaning the port plug sits to the right side of the box as you face it)
provides water line and cable access to the face of the port plug. The 4 inch
diameter 4DH3 beam port has no lead shutters that can be lifted into place
as do the 6 inch ports. The beamport at the start of this research was fitted
with a plug and soller slit collimators that collimated the beam into three
rectangular shapes of equal area (approximately 1.85" high x 0.54" wide
each). The plug extends to the shielding step, near the bolt up rings that are
marked in Figure 1.3. The plug is filled with high density concrete (concrete
filled mixed with steel punchings) and houses a water shutter that can be
drained and filled to turn the neutron beam on and off. However, as of
January 1993, the water shutter became inoperable due to a suspected leak.
The water shutter was drained and not refilled, except for investigational
purposes.
26
Chapter 1: Introduction
Kent J. Riley
Figure 1.3: Cross sectional View of the MITR-II reactor showing the 4DH3
beamport and adjacent experimental facilities. (Drawing courtesy of
Todd Date, RRPO)
Prior to the present modifications, the PGNAA system at the MITR-II
achieved a slow neutron flux, with a nominal energy of 0.015 eV, at the
27
Improved Boron 10 Quantification via PGNAA and ICP-AES
sample position of about 6 E+06 n/cm 2 sec. This translates into a sensitivity
of 6.6 counts/jg sec for 10B. The background and the slow neutron flux of the
system prior to this modification permitted quantification of a 2.5 microgram
sample (5 ppm, 0.5 ml) in about 10 minutes with less than 10% statistical
uncertainty (1 std).
A schematic of the system prior to modification is
shown in Figure 1.4.
Sol
Tangential beam tube
from D20 reflector
Rsoanvr w-ll
r! ulll
Sol
and
Sap
alti-layered
phite crystals
nsic
Ype
tector
N2
ur
Sample location
Beam catcher
rY uli
" Lithium
carbonate
Lead
Figure 1.4: PGNAA Facility Prior to Renovation
The collimated neutron beam was diffracted by several slightly
overlapping and slightly misaligned pieces of single crystal pyrolitic graphite,
28
Kent J. Riley
Chapter 1: Introduction
oriented to use the basal (002) plane for diffraction of neutrons. The graphite
selects a particular neutron energy (in this case the Bragg angle is 210,
corresponding to neutrons of 0.015 eV) and diffracts it toward the sample,
while the remainder of the beam passes through and interacts in the
downstream shielding. The slight misalignment of the separate crystals
serves to select a broader energy range so that more of the useful slow
neutron flux is diffracted. Using several slightly misaligned crystals can also
be thought of as increasing the effective mosaic of the monochromator. A
monoenergetic neutron beam is not necessary for prompt gamma analysis,
though fast and epithermal neutrons, as well as photons, are undesirable.
After being diffracted, the neutron beam passed through a series of
collimators and a sapphire crystal. The sapphire crystal served to filter out
the undesired neutron and photon components. The properties of the
sapphire filter crystal are discussed in greater detail in Section 2.4.
A High Purity Germanium Detector is used to collect the photon
spectrum at the position shown in Figure 1.4. The crystal is positioned only 2
cm from the sample position, allowing for good solid angle efficiency. The
germanium crystal is 5.5 cm in diameter and 5.5 cm thick, with an active
volume of 122.1 ml when at the operating voltage of 3500 V. Signals from
the crystal are fed to a Canberra 2002 preamplifier, which is equipped with
voltage suppression circuitry to prevent applying voltage when the crystal is
not cold. Signals from the preamp are sent to a Canberra 2024 fast
29
Improved Boron 10 Quantification via PGNAA and ICP-AES
spectroscopy amplifier, which feeds an 800 ns fixed dead time (successive
approximation type) Canberra 8715 ADC. The amplifier and ADC have
pileup rejection and live time correction capabilities. A Canberra Series 85
MCA processes output from the ADC for spectral display and displays the
appropriate dead time.
The system described above performs very well, however, the system
was not clearly able to meet all the future needs of our Clinical Trials
involving brain tumors and animal experiments. Under the protocol for
brain tumors, it may be required to analyze very small samples (0.05 ml)
from needle biopsies with relatively low boron content (10 ppm). To
accurately quantify 0.5 micrograms of 10B would have required an hour or
more of counting time. By modifications to the PGNAA system, we estimated
that it would be possible to bring the analysis time down to around 10
minutes, suitable for bulk analyses of many samples, or perhaps even rapid
pre-irradiation analysis.
For the work described in the remainder of this thesis, the conceptual
design of the PGNAA facility will remain the same. The following chapters
will describe the measures taken to increase the flux at the sample position,
and to decrease the background seen by the HPGE crystal.
1.2.1.2 Other PGNAA Facilities
The Brookhaven National Lab BNCT research group also uses PGNAA
for
10B
quantification. The Brookhaven Medical Research Reactor (BMRR) is
30
Kent J. Riley
Chapter 1: Introduction
a 3 MW research reactor that was designed for medical purposes and is now
used almost exclusively for BNCT research. PGNAA also finds use as a
valuable tool in trace element analysis by a wide variety of research groups.
As such, the National Institute of Standards (NIST) and the Missouri
University Research Reactor (MURR) at Columbia both have developed and
maintained PGNAA facilities for a variety of research projects. All of these
facilities are, however, direct beam facilities, unlike the diffracted beam that
is employed by the MIT-NRL research group.
Facility
MITR-II
MURR
NIST
BMRR
Table 1.1:
Power (MW)
5
10
10
3
10B
Sensitivity (cps/tg)
6.6 / 18.8
3.7
2.7
3.0
10B sensitivities
for several prompt gamma facilities. The results for
MIT are for the prompt gamma facility before and after (in bold) its
modification as described in this thesis.
Table 1.1 summarizes the important parameters for each of these
facilities (12). It is interesting to note that the sensitivity for the MIT facility
is higher than any other facility, including the two 10 MW reactor facilities.
The superior performance is likely due to the high solid angle efficiency that
is achieved at the MIT diffracted beam facility. In spite of the superior
performance of MITR-II facility it was deemed desirable to make further
improvements. This thesis will outline measures that have improved the
sensitivity by another factor of 3.
31
Improved Boron 10 Quantification via PGNAA and ICP-AES
It should be pointed out that all of the facilities listed in Table 1.1 are
slow neutron PGNAA facilities. Cold neutron facilities have been developed
that employ neutron guide tubes (13). Such facilities have the potential to be
extremely sensitive because they can achieve intense cold neutron beams
that are virtually free of fast neutron and photon contamination. Other
facilities are being developed that will use curved optical fibers to focus a
slow neutron beam onto the sample (14). The focal spots for such beams may
be quite small (- mm 2 ) and allows one to investigate the spatial distribution
of trace elements with mm 2 resolution.
1.2.2 ICP-AES
Inductively Coupled Plasma - Atomic Emission Spectroscopy (ICPAES) is an atomic excitation technique. ICP-AES excites the sample via an
RF plasma and collects the subsequent atomic de-excitations. ICP-AES is
insensitive to isotopes of the same element, as it is an atomic excitation
technique. In the system used at MIT, a peristaltic pump supplies the liquid
sample to a nebulizer, where it is vaporized. To prevent clogging the tubing
or the nebulizer, samples must be in a low viscosity liquid matrix and
adequately filtered. The fine sample mist from the nebulizer is then vented
to an argon plasma where the atoms in the sample are inductively excited.
The instrument is tuned to detect the characteristic de-excitation photons
from the element of interest and the number of detected photons is
proportional to the amount of element present in the sample.
32
Chapter 1: Introduction
Kent J. Riley
plasma,
spectrometer
data
output
Ar gas
supply
p(
Figure 1.5: Schematic of the ICP-AES facility
Any excess sample that does not travel to the plasma is pumped out
through the drain on the other end of the spray chamber. Figure 1.5 shows a
schematic of the ICP-AES facility that has been used to carry out this
research. Once the sample collects in the drain, it mixes with any samples
that were previously analyzed and it is therefore impossible to recover any of
the sample intact. For this reason, and the fact that solid samples must be
dissolved in order to analyze them, ICP-AES is considered a destructive
analysis technique.
ICP-AES and other plasma analysis techniques (Directly Coupled
Plasma - AES, ICP - Mass Spectroscopy) have been used by countless
research groups to perform trace element analysis for a wide variety of
elements. This thesis will describe the implementation of ICP-AES analysis
33
Improved Boron 10 Quantification via PGNAA and ICP-AES
to serve as an alternate to PGNAA analysis for
10B
quantification.
Furthermore, this thesis will investigate the feasibility of using the ICP-AES
to perform analyses for small samples like those described in preceding
sections. AES techniques are extremely sensitive, the machine used for this
research has a
10 B
sensitivity of about 2,500 counts/sec gg. There are,
however, disadvantages of the AES system that effectively reduce its
sensitivity. Furthermore, AES techniques are destructive, which can be an
important factor when evaluating quantification schemes. Details on the
performance of the ICP-AES and refinements that were made to the system
are left to Chapter 3. A comparison of the ICP-AES and PGNAA techniques,
including an evaluation of the merits and liabilities of each technique, is
made in Chapter 5.
1.3 Research Goals
A major goal of the work described in this thesis was to improve the
speed and/or sensitivity of both the ICP-AES and the PGNAA facilities. The
details and specific goals of these improvements will be left to their
respective sections, however the motivation behind these goals will be
outlined here. By obtaining a more sensitive facility, we will be able to detect
smaller amounts of boron, permitting the use of smaller samples. Small
sample analysis may become important for the analysis of stereotactic needle
34
Kent J. Riley
Chapter 1: Introduction
biopsies of brain tumors, or repeated samples from small animal studies. A
fast technique is desirable to permit bulk analysis of many samples, and to
minimize lost time during the irradiation of a subject in our clinical protocol.
This thesis will also present and discuss the work done to improve the
performance characteristics of the PGNAA facility. The sensitivities for a
variety of interesting isotopes will be presented and compared to the
sensitivities of other PGNAA facilities.
A final purpose of this work was to evaluate and compare ICP-AES
and PGNAA in light of the needs of our research group. This thesis seeks to
outline and describe the strengths and weaknesses of each technique, and to
provide guidance for selecting a technique to perform macroscopic analysis in
a variety of situations.
35
Improved Boron 10 Quantification via PGNAA and ICP-AES
1.4 References
1. "Biologic Effects and Therapeutic Possibilities of Neutrons," G. L. Locher,
Am. J. Roentgenol., 36:1, 1936.
2. "Pharmacokinetics
and Tissue Distribution
of Boronophenylalanine
Following Interperitoneal Injection in Nude Rats with Intracerebral
Melanoma," A. E. Staubus, K. Matalka, R. F. Barth, M. Q. Bailey,
Advances in Neutron Capture Therapy - Proceedings of the Fifth
International Symposium on Neutron Capture Therapy, Sept. 14-17,
1992, Columbus, OH, pp. 495-499
3. "Performance of the Currently Available Epithermal Neutron Beam at the
Massachusetts Institute of Technology Research Reactor (MITR-II)," J.
R. Choi, R. G. Zamenhof, J. C. Yanch, R. Rogus, O. K. Harling,
Progress in Neutron Capture Therapy - Proceedings of the Fourth
International Symposium on Neutron Capture Therapy for Cancer,
Dec. 4-7, Sydney, Australia.
4. "Treatment of Intracerebral Malignant Melanoma Using a Rat Model and
L-Boronophenylalanine as the Capture Agent,"
M. Q. Bailey, K. Z.
Matalka, R. F. Barth, J. A. Coderre, A. H. Soloway, J. H. Goodman, A.
36
Kent J. Riley
Chapter 1: Introduction
E. Staubus, E. K. Rofstad, Advances in Neutron Capture Therapy Proceedings of the Fifth International Symposium on Neutron Capture
Therapy, Sept. 14-17, 1992, Columbus, OH, pp. 519-523
5. "A Novel Approach to the Microdosimetry of Neutron Capture Therapy.
Part I.
High-Resolution Quantitative Autoradiography Applied to
Microdosimetry in Neutron Capture Therapy," G. R. Solares, R.G.
Zamenhof, Radiation Research, Vol. 144, 1995, pp. 50-58.
6.
"Monte Carlo-Based Treatment Planning for Boron Neutron Capture
Therapy Using Custom Designed Models Automatically Generated
from CT Data," R. Zamenhof, E. Redmond, G. Solares, D. Katz, K.
Riley, S. Kiger, O. Harling, Int. J. Radiation Oncology Biol. Phys., Vol.
35:2, 1996, pp. 383-397.
7. Microdosimetric Studies for Neutron Capture Therapy and Techniques for
Capture Element Selection, C. S. Yam, Ph. D. Thesis, Massachusetts
Institute of Technology, 1995
8 Prompt Gamma Activation Analysis of Boron 10 in Blood and Dosimetric
Measurements Associated with Boron Neutron Capture Therapy,
Nicholas S. Lizzo, S.M. Thesis, Massachusetts Institute of Technology,
1988.
37
Improved Boron 10 Quantification via PGNAA and ICP-AES
9. Design and Dosimetry of Epithermal Neutron Beams for Clinical Trials of
Boron Neutron Capture Therapy at the MITR-II Reactor, R. D. Rogus,
Ph. D. Thesis, Massachusetts Institute of Technology, 1994
10. "Design of the PGNAA Facility at the MITR-II Reactor for Multi-Element
Analysis," S. Wirdzek, V. Kubali, Report to Prof. Harling, July 1989
11.
Design and Construction of a Prompt Gamma Activation Analysis
Facility and Improvement of the On-Line Beam Monitor System for
the Medical Beam at the MITR-II, J-M. Chabeuf, M. S. Thesis,
Massachusetts Institute of Technology, 1993
12. "A prompt gamma neutron activation analysis facility using a diffracted
beam," 0.
Harling, J. Chabeuf, F. Lambert, G. Yasuda, Nuclear
Instruments and Methods in Physics Research B, Vol. 83, 1993, pp.
557-562.
13. "Cold Neutron Prompt Gamma Activation Analysis at NIST: A Progress
Report,"
R. L. Paul, R. M. Lindstrom, D. H. Vincent, Journal of
Radioanalytical and Nuclear Chemistry, Articles, Vol. 180:2, 1994, pp.
263-269
14.
R.M Lindstrom, H.H. Chen-Mayer, V.A. Sharov, J.K. Langland, Y.T.
Cheng, D.F.R. Mildner, "Installation of a Neutron Bender Lens for
38
Kent J. Riley
Chapter 1: Introduction
Spatially Resolved Prompt-Gamma Activation Analysis," Transactions
of the 1996 American Nuclear Society Winter Meeting, November 1014, 1996, Washington D.C., Volume 75, pp. 16.
39
10
CHAPTER TWO
Prompt Gamma Neutron
Activation Analysis
2. O0 Figures of Merit
To adequately assess the improvements that have been made to the
PGNAA facility, it is helpful to outline a few figures of merit that reflect
important performance characteristics.
2.0.1 Sensitivity
Sensitivity is expressed in units of counts per second per microgram
of the isotope being measured. For the PGNAA facility, the sensitivity for a
given isotope is dependent upon three factors; the slow neutron flux at the
sample position, the absolute efficiency of the detector, and the neutron
41
Improved Boron 10 Quantification via PGNAA and ICP-AES
absorption cross section of the isotope being measured. Sensitivity is a good
indicator of signal strength and detection efficiency, but contains no
information about the background count rate that is seen by the detector.
2.0.2 Limit of Detection
Limits of Detection (expressed in gg) are useful figures of merit
because they combine information about the sensitivity and the background
count rate of the system. To define a detection limit, one arbitrarily sets the
detection limit as the amount of material that results in a signal that has a
magnitude of twice its uncertainty, for a specified counting time.
Mathematically, this can be expressed as:
Equation 2.1
B = 2c
Where B is the net counts under the boron beak, and GB is the
uncertainty in the net counts under the boron peak. We can also write the
following for GB:
=
N+R
Equation 2.2
= B+2R
Where N is the total counts (boron plus background) and R is the
number of background counts under the
10B
42
peak. We arrived at Equation
Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
2.2 by realizing that onB is simply the quadrature sum of the uncertainty of
the total counts and the uncertainty of the background counts, and by
making use of the Poisson nature of counting statistics ( aR =
KR ).
We can
then equate the right hand side of Equation 2.1 and the left hand side of
Equation 2.2. Upon doing so, and realizing that the area under the boron
peak is simply the product of the sensitivity S (cps/gg), the amount of boron
in the sample C (gg), and the time the sample is counted for t (s) (B = CSt),
we can rearrange and solve for C. The result for the detection limit C is
given in Equation 2.3 (1):
Equation 2.3
3.29
C-_
S
C
Rb
t
Rb = background count rate under 10B peak
t = count time (sec)
S = sensitivity (cps/gg)
The above expression allows us to calculate a detection limit for a
given counting time, provided we know the sensitivity and background count
rate under the 10B peak. Lower background count rates and higher
sensitivities will improve C (make C smaller), though the background count
rate varies only as a root dependence, while the sensitivity is linearly
inversely proportional. Longer counting times also yield better detection
limits, though also only with a root dependence.
43
Improved Boron 10 Quantification via PGNAA and ICP-AES
2.0.3 Background Count Rates
The preceding section demonstrated that the background count rate
impacts the detection limit of a PGNAA facility. The background count rate
itself can serve as a figure of merit. For the work described in this thesis,
reference will be made to two different types of background count rates; the
boron background count rate, and the gross integral count rate. The boron
background count rate is the background count rate under the boron peak, as
was defined in the previous section. The gross integral count rate will refer
to the count rate in all 2048 channels of the MCA, in other words, the area
under the entire spectrum. The gross integral count rate is useful in
assessing the impact of system dead time and the effect of various facility
modifications. We need to minimize the boron background to obtain the best
detection limit possible, and we also need to minimize the gross integral
background to keep the system dead time within reasonable limits.
2.1 Design Considerations
2.1.1 Design Goals
44
Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
As mentioned earlier, we wish to lower the detection limits for the
PGNAA system. To lower the detection limits Equation 2.3 tells us that we
must accomplish one or both of the following goals:
* improve (increase) the sensitivity of the system
* reduce the background seen by the detector
The sensitivity of the PGNAA system is directly proportional to the
slow neutron flux seen at the sample position, assuming the detection
efficiency is not varied. Increasing the flux by a factor of two will increase
the sensitivity by a factor of two. The flux can be increased by removing the
collimator shims inside the water shutter (see Figure 1.4) and by improving
the graphite monochromator to improve an intense neutron beam.
Improvements to the monochromator will be discussed in Section 2.3. Shim
removal can be expected to increase the flux by as much as a factor of two,
while improvements to the graphite diffracting crystals can probably increase
the slow neutron flux at the sample position by a factor of 2-3.
The background seen in the vicinity of the
10B
peak (478 keV) is
comprised of two major components. The most dominant component of the
two is the continuum of background that arises from the Compton scattering
of high energy photons. Annihilation photons are likely the largest culprit
since the annihilation peak (511 keV) is by far the tallest peak in the
spectrum. The second component of background are photons that do actually
45
Improved Boron 10 Quantification via PGNAA and ICP-AES
come from 7Li emissions, but from places other than our sample. This is due
to Boron impurities in some of the shielding material, as well as some beam
components that have been made with Boron (collimator, upstream Boral
shields). This component may also be due to slow neutron absorption in the
lithium shielding near the detector, which can also give rise to 478 keV
photons. The cross section for this reaction is small (- 40 mb) and the
resultant peak in the spectrum would not be Doppler broadened.
From Equation 2.1 it is clear that the detection limit is affected by the
boron background count rate. By reducing the boron background, one can
improve the detection limit for the system. The high gross integral
background count rate also leads to considerable dead time (>10 %), even
with a fast spectroscopy amplifier and an 800 ns fixed dead time ADC. The
gross integral background must therefore be lowered to accommodate the
higher count rates that will be associated with the higher sensitivity from
increased slow neutron flux at the sample position.
The gross integral background count rate at the PGNAA facility prior
to the modifications described in this thesis was in excess of 6000 counts per
second. Though increasing the flux would undoubtedly improve sensitivity, it
was also clear that the background would have to be substantially reduced to
realize the full benefit of any other improvements. To attack this problem, it
was necessary to characterize the sources of the background. The following
section outlines the method used to accomplish this task and summarizes the
46
Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
major components of the background seen by the detector at the PGNAA
facility.
2.1.2 Background Assay
To effectively reduce the background, one must determine where the
vast majority of the background arises from. A series of measurements were
taken to isolate certain background events from the detector. Table 2.1 and
Figure 2.1 summarize the results of these measurements that were taken
prior to the modifications described in this thesis.
JULU 11~i
Component
per
xea or Room Backgrour
Upstream Interactions
Beam Spreading
Sample Interactions
690
1580
1110
2680
6060
TOTAL
Table 2.1: Summary of Background Components seen by Detector at 4DH3
47
Improved Boron 10 Quantification via PGNAA and ICP-AES
Components of the
Gross Integral Background Count Rate
Seen by the HPGE Detector at 4DH3
7000
[
6000
w 5000
0
Sample Interactions
00 Beam Spreading
I
Upstream Interactions
El
Area Background
4000
- 3000
r 2000
1000
PGNAA Bhckground
Figure 2.1: Summary of Background Components seen by Detector at 4DH3
The component labeled Area or Room Background was measured with
the water shutter filled, and the mechanical shutter closed. A gross count
rate of 690 counts per second was obtained. This number is indicative of
background that the detector sees from experiments that are nearby (e.g.
silicon irradiation, in core experiments on the reactor top, etc..). The Area
Background thus arises from photons that are not related to radiation
specifically from the 4DH3 beam port.
Upstream Interactions is simply the difference of the Area Background
count rate and the count rate measured with the reactor operating, the water
shutter drained, and the mechanical shutter closed. This component is due to
48
Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
radiation (mostly neutrons) interacting in the shielding near the reactor face
(i.e. upstream of the mechanical shutter - see Figure 1.4) and creating
photons that reach the detector.
The component labeled Beam Spreading is due to neutrons diverging
from the collimated beam, striking the lead that surrounds the detector and
creating high energy photons. These high energy photons will undergo pair
production, which contributes to the large annihilation peak (511 keV)
observed in the spectrum. Beam Spreading was measured by subtracting a
measurement with the mechanical shutter closed from a measurement with
the mechanical shutter open, but without a sample present. Again, the
reactor was at full power and the water shutter was open in both cases.
There are numerous interaction probabilities for a slow neutron
incident upon a sample. Blood and tissue samples can contain several
elements in a variety of abundances. For the samples we are particularly
interested in, the interaction probabilities are dominated by boron and
hydrogen. Other elements, such as sodium and chlorine can play a role, but
their interaction probabilities are usually orders of magnitude lower. It is
therefore reasonable to surmise that much of the background that arises from
sample interactions is due to interactions with hydrogen. A slow neutron
incident upon a hydrogen nucleus can either scatter from the hydrogen
nucleus, or be absorbed by it (creating a 2.2 MeV photon). The scattered
neutron can interact in the surrounding shielding (mostly lead) and create
unwanted background photons.
49
Improved Boron 10 Quantification via PGNAA and ICP-AES
Hydrogen nuclei that absorb slow neutrons will immediately form the
only stable (ground) state of the deuteron and emit the excess energy in the
form of a 2.2 MeV photon. The hydrogen absorption cross section is 0.33 b,
more than four orders of magnitude lower than the boron absorption cross
section. However, the high number density of hydrogen nuclei in a typical
sample create a significant amount of 2.2 MeV photons, some of which will
reach the detector. Not all of these photons that reach the detector will
deposit their full energy there. The vast majority will undergo compton
scattering, depositing only a portion of their energy before leaving the
detector. These partial interactions will show up with a continuum of
energies below the compton edge (corresponding to maximum energy loss in a
compton scatter event), and part of this continuum will lie under the 478 keV
boron peak.
The sample interaction background component was measured by
subtracting the count rate without a sample from the count rate with a 0.5
ml deionized water sample present. The water shutter and mechanical
shutter were open, and the reactor was at full power.
The following section outlines the modifications that were deemed
desirable to reduce each of these background components as much as
possible.
50
Chapter 2: Prompt Gamma Neutron Activation Analysis
Kent J. Riley
2.1.3 Proposed Modifications
2.1.3.1 Background Reduction
A glance at Table 2.1 shows that the majority of background activity is
due to sample and upstream interactions. These components of the
background can be at least partially controlled by lining the beam path with
a lithium 6 enriched compound. This will insure that neutrons are either
absorbed by the sample, thus generating useful photons, or are scattered by
the sample and absorbed by Lithium 6. Lithium 6 primarily undergoes an
(n,a) reaction (a = 941 b) with slow neutrons and this reaction path does not
emit a photon. Lithium liners were used on the PGNAA facility prior to
modification, but due to awkward geometry the liners did not effectively
cover all of the surrounding shielding. Thus the improved PGNAA facility
will house the collimated neutron beam with a Lithium "cage." This cage will
have access ports for the sample and for the HPGE detector.
The component of background that arises due to compton interactions
(and thus only partial energy deposition) in the HPGE is impossible to
control without implementing a compton suppression system. A compton
suppression system would reject compton events in the HPGE detector by
surrounding the HPGE detector with a high efficiency (NaI for example)
detector. If a pulse is observed in both detectors within a certain window of
Improved Boron 10 Quantification via PGNAA and ICP-AES
time, then the event in the HPGE detector is rejected and not included in the
display of the MCA.
Referring to Figure 1.4, one can see that the sapphire crystal is not in
an optimal position. By positioning the sapphire crystal upstream of the
graphite crystal, it is possible to limit the upstream interaction component of
the background seen by the detector. The sapphire filter will remove a great
deal of the unwanted fast neutrons and photons from the direct beam, before
they are able to interact in the shielding near the detector and generate
background activity. (See Section 2.4 for a discussion of the sapphire crystal
filtering properties.) The upstream interaction component should scale with
the intensity of the fast neutron and photon component of the direct beam
since slow neutrons have been adequately controlled in the unmodified
facility with slow neutron absorbers. The upgraded facility will therefore
incorporate sapphire filters that are upstream of the graphite diffracting
crystals.
The next largest component, beam spreading, arises from neutrons
that diverge from the collimated beam and interact in the surrounding lead.
This component will therefore also be controlled with the use of the Lithium
cage that has been proposed in the preceding paragraphs.
The remaining background component listed in Table 2.1, the Area
Background, is simple to control. The detector at the PGNAA facility has a
few lines of sight to surrounding experiments that are sources of photon
52
Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
radiation By making more thorough use of lead shielding surrounding the
detector, this component will be reduced.
Currently, the collimator is composed of borated plastic. The photons
generated by the boron in this plastic do not reach the detector, as long as the
detector is far enough from the beam path. With the detector only 2 cm away
from the sample, some of these photons can be seen. This component of the
boron background count rate can be reduced by replacing the borated
collimator with a lithated collimator. Doing so may also allow us to move the
detector closer to the sample without increasing the boron background .
2.1.3.2 Water Shutter
Another problem with the current system is that the water shutter in
the beam port is no longer operable. When operating, the water shutter can
be filled with water to block the beam when the beam is not in use. Likewise,
the shutter can be drained when the beam is in use. Some time ago the
existing water shutter was thought to have developed a leak. No water was
observed to have collected near the biological shield face, so it was feared that
water may be leaking from the other end, possibly into the reactor's graphite
region.
For this reason, the water shutter was completely drained and left that
way. Unfortunately, this means that whenever the reactor is operating,
activity is building in the shielding surrounding the PGNAA facility. This
activity can contribute to the background seen by the HPGE detector at
53
Improved Boron 10 Quantification via PGNAA and ICP-AES
4DH3. A water shutter will also lower the dose rates at the beam port while
the shutter is closed, permitting extended work in that area. This will be
very useful for optimizing and aligning the graphite crystals, and doing other
work on the spectrometer. The above reasons prompted the installation of a
new working water shutter. To house the new water shutter, a new port plug
was built, intended to replace the old plug at 4DH3. The design, construction
and testing of the water shutter and port plug will be discussed in Section 2.2
2.1.3.3 Flux Increase
The sensitivity of the PGNAA facility is proportional to the slow
neutron flux at the sample position, the slow neutron absorption cross section
of the isotope being measured, and the absolute efficiency of the detector
being used. To improve the sensitivity for a given isotope, we must increase
the slow neutron flux at the sample position, and/or increase the absolute
efficiency of our detector.
The latter can be accomplished by using a larger detector, using a
higher efficiency detector (with a higher atomic number, for example), or by
improving the solid angle efficiency (i.e. moving the detector closer to the
sample). To accurately quantify the area under the boron peak, we must use
a detector with an energy resolution sufficient to resolve the 478 keV boron
peak and the 511 keV annihilation peak. The only practical choice for this
type of high resolution spectroscopy is a high-purity germanium detector
(HPGE). We are therefore constrained to a detector with a fairly low atomic
54
Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
number (Z=32). By the same token, germanium crystals of sufficient purity
and regularity are quite expensive to grow, so detectors that are much larger
than a few centimeters in diameter become prohibitively expensive.
The diffracted beam employed by the PGNAA facility at MIT already
allows the detector to be positioned very close to the sample (-2 cm).
Reducing the sample to detector distance to less than a centimeter or so
becomes difficult because the detector begins to protrude into the neutron
beam, which can create activation and raise the background count rate.
Previous work has shown that even though the detector is so close to the
neutron beam, it is sufficiently shielded with 6LiCO 2 so that neutrons cannot
reach the HPGE crystal and degrade the performance of the detector (2).
From the preceding discussion it is apparent that our current facility
has nearly maximized the absolute detection efficiency of the PGNAA system.
To achieve a significant gain in the sensitivity of the system, we must then
increase the flux at the sample position. This thesis has taken several
measures to accomplish that objective.
The first, and most straightforward measure is the removal of the
upstream collimator shims that are shown in Figure 1.4. These shims serve
to limit the solid angle that neutrons in the reactor core can be emitted into,
and still travel in a straight line to reach the graphite diffracting crystals.
By increasing the solid angle, we can increase the number of neutrons
striking the diffracting crystals and therefore increase the number of
neutrons that meet the Bragg criteria to be diffracted toward the sample. An
55
Improved Boron 10 Quantification via PGNAA and ICP-AES
estimate of how much the flux will increase can be obtained by considering
the percent increase in the solid angle seen by the neutrons traveling from
the core toward the graphite crystals. This simple calculation reveals that
removing the collimator shims from the water shutter at 4DH3 should
increase the flux by a factor of slightly less than 2 (1.8). The actual increase
will be likely be lower than the factor of 1.8 due to the fact that the shims are
not black to slow neutrons and that totally reflected neutrons from the
surfaces of the collimator shims can contribute to the beam incident upon the
graphite diffracting crystals.
The second modification to improve the flux involves making more
effective use of the graphite diffracting crystals. The graphite crystals
diffract slow neutrons toward the sample position in straight lines. The
neutron beam at the sample position (when it is uncollimated) is thus a
projection of the illuminated portion of the graphite crystals. If we think of
the diffracting crystals as a mirror, and the incident neutrons as traveling
waves, we then realize that the neutrons are being reflected toward a focal
point that is infinitely far away. By curving the graphite crystals in an
appropriate fashion, we can move the focal spot of the reflected neutrons to
the sample position. The focal spot of such a neutron beam will have a slow
neutron flux that is much greater than a comparable spot in an unfocused
neutron beam. A factor of 2-3 increase in the slow neutron flux can be
reasonably expected from the implementation of a focused neutron beam (see
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Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
Section 2.3.1). The design and construction of the focusing graphite crystals
for the PGNAA facility will be discussed in Section 2.3.
2.1.4 Design Constraints
Though much of the design will be unique to the component we are
considering, there are a few overriding design constraints that must be kept
in mind. Perhaps the most important from both a safety and design goal
standpoint is to control fast neutron and gamma ray streaming. Any type of
radiation will stream through small gaps where two parts form a junction.
Streaming will occur even with tight physical tolerances. Furthermore, from
a practical viewpoint, significant gaps or tolerances are desirable to reduce
the cost of fabrication and to ease assembly. For these reasons, the shape of
many components will incorporate steps so that undesired radiation does not
have a straight path out of the beam port.
Another important consideration are the tolerances of the materials we
are using to the radiation they will be exposed to. These considerations will
be especially important for the sapphire crystal, as it is a focus of our
redesign effort. Along the same lines, one should select material that will
activate as little as possible. This is beneficial not only for the purposes of
background reduction, but perhaps more importantly for dose reduction
during future work on the beam port.
Finally, one must keep in mind that the beam port may be used for
other purposes in the future. It may be necessary, for example, to remove the
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Improved Boron 10 Quantification via PGNAA and ICP-AES
sapphire crystal at some point. We need to design the crystal such that it can
be retrieved quickly and easily since dose rates will be high during the
retrieval.
2.2 Port Plug Design and Construction
This section will discuss the design and construction of a new port plug
for 4DH3, which houses a water shutter and a sapphire filter crystal.
2.2.1 Conceptual Design
Three main objectives for this research were to remove the collimator
shims upstream of the graphite crystals, move the sapphire filter crystal to a
position upstream of the graphite crystals, and to install a working water
shutter. Originally our group had considered modifying the existing port
plug by removing the collimator shims, and inserting a new water shutter
and sapphire crystal. This plan was abandoned in favor of modifying a port
plug that was found in the storage area to accommodate a water shutter and
the sapphire crystal, and then simply swap this plug with the plug already at
4DH3. It was believed that this plan would result in less radiation exposure
to the workers involved since the two plugs could be easily exchanged. This
plan was also deemed desirable because it would result in a neutron beam of
58
Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
comparable size, instead of the reduced area beam that would result from
inserting a water shutter inside the water shutter at 4DH3.
Eventually it was determined that the port plug that was found in
storage was not suitable for our needs. At this point it was decided to
construct a new port plug altogether. The new port plug was constructed to
fit any 4DH port of the MITR-II, and was designed to accommodate a
removable water shutter, which will be described in subsequent sections.
Figure 2.2 shows a side view of the port plug, the water shutter and all
of the components of the final assembly. Engineering drawings for the entire
port plug assembly have been included in Appendix A.
Section A-A
Section B-B
Boron Carbide
-
Concrete
a
-
*
-
Lithiated Paraffin
[
-
Boral
*
-
Steel
I
-
Lead
[
-
Aluminum 6061
All steel is mild steel
unless otherwise indicated
Figure 2.2: Side view of port plug assembly
59
Improved Boron 10 Quantification via PGNAA and ICP-AES
The outer shell of the plug is constructed of 1/4" steel tubing with a 7
3/4" outer diameter. Two sections of steel tubing, with inner diameters of
4.0" and 5.6", comprise the center of the plug. The 5.6" inner diameter
section has a length of 17 1/2", while the 4.0" inner diameter section has a
length of 32". The front flange of the plug is constructed of stainless steel
and matches the bolt pattern for any 4DH port. A flange made of steel
connects the inner two inner sections of steel tubing, and a steel end cap
seals the entire assembly. The outer 17" section of the port plug was filled
with lead, followed by a 30" layer of heavy concrete mixed with steel
punchings. The remaining inch of the plug on the reactor core end was filled
with a layer of boron carbide. After construction, the entire plug was nickel
plated to help prevent corrosion. The boron carbide absorbs any slow
neutrons incident upon the plug, thereby limiting activation of the concrete
fill and steel punchings. The dense concrete and steel punchings will
attenuate the intense photon flux incident upon the core end of the plug. The
heavy concrete also contains hydrogen, and therefore serves to thermalize
fast neutrons. Finally, the remaining layer of lead will further attenuate
photons that pass through the concrete or photons that are generated there
via activation.
A water shutter, constructed entirely of Aluminum 6061, sits inside
the stepped inner diameter of the port plug. The entrance window near the
reactor source is 1/8" a sheet of aluminum that is welded to the body of the
water shutter. This thin window results in only about a 1% attenuation of
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Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
slow neutrons. The water shutter is also stepped, with inner diameters of 3
3/4" and 5 1/4". The step serves to control any streaming that may result due
to air pockets that may collect in the top of the shutter after it has been filled
with water. The water shutter bolts to the front flange of the steel port plug
to prevent the shutter from rotating or moving. The rear section of the water
shutter (with the smaller inner diameter) was designed to accommodate a
3.7" diameter sapphire single crystal. When the water shutter is closed, the
crystal is immersed in water. The crystal was seated in a cylinder of thin
(0.010") aluminum with tabs at the front and rear of the crystal (not shown in
Figure 2.2). The front tabs can be grappled to pull the cylinder forward. The
rear tabs will then meet the crystal, pushing it forward, allowing the crystal
to be removed. The front section of the water shutter (inner diameter 5 1/4")
houses a series of collimators. A rectangular channel (2 1/2" by 3") was
machined down the center of a cylinder of borated paraffin that is 12" long
and 5 1/4" in diameter. An identical channel was machined out of a lead
cylinder that is 4" long and 5 1/4" in diameter. The two sections were then
aligned and inserted in the large section of the water shutter, with the
borated paraffin nearest the neutron source.
The front of the water shutter is an aluminum plate that bolts to the
flange of the water shutter. The seal is maintained by a static compression
nitrile o-ring, which is resistant to gamma radiation damage. To provide a
rigid compression surface, the front plate is 1/2" thick at the bolt holes and oring groove. The plate was thinned to 1/16" in the beam path to prevent
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Improved Boron 10 Quantification via PGNAA and ICP-AES
attenuation of slow neutrons. The 1/16" window will attenuate less than
0.5% of the slow neutrons traveling down the beam path. Inlet and outlet
tubing were welded to aluminum plate, which connect to the drain and fill
lines of a water reservoir.
An aluminum end cap and boral plate attach to the end of the port
plug nearest the reactor. The end cap was machined to properly mate with
the step in the reactor biological shielding near the bolt-up rings (see Figure
1.3), and serves to limit streaming between the outer diameter of the water
shutter and the inner diameter of the port plug. The boral plate will
drastically reduce the slow neutron flux incident upon the end cap of the port
plug.
2.2.2 Engineering Design
2.2.2.1 Sapphire Crystal
The sapphire crystal sits inside the water shutter, at the end of tube
nearest the reactor core. We therefore need to concern ourselves with the
radiation tolerance of sapphire and the behavior of sapphire in water.
If the sapphire crystal were to absorb even a small amount of water, it
could drastically attenuate the slow neutron beam when the water shutter is
empty or open. A literature survey was performed to determine if any
adverse effects might arise from the sapphire crystal sitting in water,
particularly water absorption or corrosive reactions. Nothing in the
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Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
literature indicated that sapphire would absorb water, nor were any
corrosion problems discovered between water and sapphire.
A simple wetting test was also carried out to determine whether or not
water would bead up on the surface of the crystal as the water shutter was
opened. Severe beading could significantly degrade the slow neutron flux via
slow neutron scattering from hydrogen. To see how the water might drain
from the surface of the crystal, the crystal was immersed in a water bath and
slowly removed. As the crystal was removed, the water sheeted smoothly off
the polished surface of the crystal. No beading was observed on any of the
polished surfaces of the crystal. Slight beading was observed around the
perimeter of the cylinder (which was not as finely polished). The beading was
deemed insignificant since it was very slight and would not be in the path of
the neutron beam.
The 4DH3 beamport views a slow neutron source of about 8 x 1013
n/cm 2 sec in the D2 0 reflector tank beneath the MITR core, and the neutron
flux in the 4DH beamports have a cadmium ratio of 8 (3). It is possible to
estimate the flux that the sapphire crystal will be exposed to by considering
the portion of the reflector tank that is viewed by 4DH3 as a point source of
neutrons. If we assume the neutrons in the reflector tank are emitted
isotropically, then one can estimate the percentage of flux that reaches the
sapphire crystal by calculating the solid angle that the crystal subtends and
dividing by 4n. Such calculations have been shown to agree with flux
63
Improved Boron 10 Quantification via PGNAA and ICP-AES
measurements to within a factor of 5 or better (2). Upon performing this
calculation, we find that the slow neutron flux at the position of the sapphire
crystal is approximately 1 x 1010 n/cm 2 sec, while the epicadmium neutron
flux is approximately 1 x 109 n/cm 2 sec (from the cadmium ratio), with the
water shutter open. Data found from a literature search on single crystal
sapphire indicates that a fluence of fast and epithermal neutrons (E > 0.5 eV)
in excess of 1018 n/cm 2 would be required before an observable change in
neutron transmission properties could be observed (4). Other researchers
have reported on the swelling that radiation may induce in single crystal
materials. The increase in volume for a sapphire crystal (for a variety of
types and grades) is less than 1% for a fast neutron (E > 0.1 MeV) fluence of 1
x 1021 n/cm 2 (5). None of the reports indicated the effects that prolonged slow
neutron irradiation have on the properties of sapphire single crystals.
However, the vast majority of the literature surveyed support the use of
sapphire crystals as a filter to obtain a beam of slow neutrons, and therefore
must have exposed the crystals to considerable slow neutron fluences.
Likewise, no mention is made of the effects of photon irradiation, but the
crystals were certainly exposed to photon fluences as well, since photon
contamination is an unavoidable part of any fast or slow neutron beam.
From the data reported in the literature, it is clear that the sapphire crystal
will be able to withstand irradiation at the 4DH3 beamport for at least
several years before a fluence is reached that will cause a significant change
in its performance or physical characteristics. Since the crystal will sit at the
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Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
rear of the water shutter it will be continually exposed to the flux from the
reactor core, regardless of whether the water shutter is closed or open. For
this reason, a duty factor has not been accounted for.
2.2.2.2 Water Shutter
As mentioned earlier, the water shutter is constructed entirely of
aluminum 6061. Two sections of different sized tubing (5 1/4" ID and 3 3/4"
ID) each with 1/8" wall thickness were joined with an aluminum flange that
was welded to each piece. A thin aluminum window (1/8") was welded onto
the inner diameter of the small section of tubing. A flange of 7 1/2" in
diameter was welded onto the opposite end. The flange mates with a seal
plate of equal diameter, an appropriate o-ring and groove, and 8 equally
spaced 7/16-20 bolts.
A nitrile o-ring can reasonably endure 107 R before its compression set
will be affected and leaks can occur (6). The photon dose rate at 4DH3 port
box has not been measured while the reactor is at any significant power.
Such measurements were, however, made on the 6SH4 beamport, which has
a line of sight into the reactor vessel that is comparable to 4DH3. At a power
of 20 kW, a dose rate of 24 R/hr was measured at the port box of 6SH4. This
dose rate can be linearly scaled to 5 MW, which yields a photon dose rate of
6,000 R/hr (7). This value can be used as a conservative estimate of the
photon dose rate experienced by the o-ring at 4DH3. The actual dose rate
will be lower due to the fact that the o-ring does not sit in the direct beam,
65
Improved Boron 10 Quantification via PGNAA and ICP-AES
the beamport at 4DH3 is smaller than 6SH4, and the photon beam will also
be filtered by the sapphire crystal, which will lower the photon dose rate.
Even with these conservative assumptions, the o-ring at 4DH3 is projected to
last several tens of years.
The seal plate also contains welded tubing for water inlet and outlet.
To open and close the water shutter, a reservoir system will be used. To fill
the water shutter, the water shutter will be placed under vacuum and then
an appropriate valve will be opened to allow the water to flow into the
evacuated space. The water shutter will therefore need to withstand the
pressure exerted by the atmosphere once it is evacuated. Figure 2.3 shows a
schematic of the reservoir system used for the water shutter at 4DH3. To
close the water shutter, we would close valves A and B, leaving valves D, E,
and C open. We would then use the vacuum pump to draw a vacuum on the
water shutter, connecting the pump at position 3. Once an appropriate
vacuum was reached, valve E could be closed, and valve A could be opened,
allowing the water in the reservoir to flow into the vacuum of the water
shutter. A similar procedure can be applied to drain the water shutter by
connecting the vacuum pump at position 1. Residual water can be pumped
out by opening valves B, C, and E and connecting the vacuum pump at
position 2. If this procedure is to be performed, however, a pump suitable for
pumping water vapor must be used.
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Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
Water Trap
E
3
--~~-~~~--
Figure 2.3: Schematic of the reservoir system used to open and close the water
shutter at 4DH3. A vacuum pump can be connected at positions
1,2, and 3 depending on whether or not the water shutter is to be
filled or drained.
A simple calculation of the radial and hoop stresses that the water
shutter would endure while under vacuum demonstrated that the walls of the
aluminum tubing carried a safety factor of 60. The following formula can be
used to evaluated the hoop stress, which is the largest stress in a cylindrical
pressure vessel (8).
Equation 2.4
h
- pr / t
In the above equation, p, is the pressure exerted on the vessel, which is
generally assumed to be an internal pressure. The above equation is also
valid for external pressures, as is the case with the water shutter (9). Using
Equation 2.4, (and appropriate values for the wall thickness, t = 1/16" and
67
Improved Boron 10 Quantification via PGNAA and ICP-AES
the radius of the cylinder, r = 2.0") the maximum hoop stress is calculated to
be 4 MPa, which is well below the yield strength of 55 MPa for untreated,
fully annealed aluminum 6061.
A calculation of the stress induced in the end windows of the shutter is
less straightforward and was not carried out. However, the entire water
shutter was tested under vacuum and both end windows demonstrated
sufficient strength. Other than the loads associated with being under
vacuum, the water shutter will see no significant loading.
After the water shutter was completely welded and assembled, a
simple leak test was conducted by immersing the water shutter (full of only
air at atmospheric pressure) in a large water bath. No air bubbles were
observed to rise from the water bath due to possible leaks in the shutter. The
shutter was then subjected to a more thorough leak test using a helium leak
detection unit. To detect leaks, the water shutter was pumped down to a
suitable vacuum and connected to a mass spectrometer. Helium was then
sprayed around the outside of the chamber. Any leaks would allow the
helium to enter the chamber and be subsequently recorded in the mass
spectrometer. While under vacuum, the end windows of the water shutter
(particularly the 1/16" window at the large end) were carefully observed to
insure that they could withstand the stress of being under vacuum. Neither
window showed significant deflection or deformation while under vacuum.
Furthermore, the helium leak test revealed no significant leaks in the water
shutter, and the shutter was deemed ready for operation.
68
Chapter 2: Prompt Gamma Neutron Activation Analysis
Kent J. Riley
2.2.2.3 Port Plug
The new port plug for the 4DH3 beamport was constructed to the
engineering specifications detailed in the MITR-I drawing P17-62-4A
(Neutron Spectrometer S-2 Collimator Shield Plug), with some exceptions.
As previously mentioned, the new plug is constructed entirely of steel (except
the front flange which is stainless steel), and then nickel plated, whereas the
old plug is constructed entirely of aluminum. The new plug was also
designed to accommodate the removable water shutter. The new plug
matches the old plug in all of the critical dimensions and tolerances; namely
the outer diameter, flange thicknesses, bolt patterns, weld types and
strengths, and concrete density. Furthermore, the inner diameters of the
port plug were designed to fit the water shutter with close tolerances (±
0.05"). An engineering drawing of the new port plug is included in Appendix
A.
Figure 2.4 shows two photographs of the final water shutter and port
plug assembly. The first photograph shows the assembly with the seal plate
removed so that the inner lead and borated paraffin collimators are visible.
The rear of the seal plate is visible in this photograph, showing the o-ring
and the water inlet and outlet holes. The second photograph shows the seal
plate attached, with the water inlet and outlet tubing welded into place. The
thicker outer part of the seal plate is visible, as is the machined center that
comprises the thin aluminum neutron window.
69
Improved Boron 10 Quantification via PGNAA and ICP-AES
Figure 2.4: Photographs of the final port plug assembly. The end cap is
removed (left photo), revealing the lead collimator that sits inside
the water shutter. The rear side of the end cap (to the right of the
plug) shows the water inlet and outlet holes and the nitrile o-ring
seal. With the end cap attached (right photo) the aluminum inlet
and outlet tubing welded to the cap is visible.
2.2.3 Port Plug Installation
In March of 1996, the port plug was fully assembled and tested, and
was to be installed during an extended reactor maintenance shut down
period. All of the shielding surrounding the 4DH3 beamport was removed.
The arrangement and location of all the shielding was documented with
photographs and notes, especially the masonite layers and shielding
surrounding the port box. The drain and fill lines to the old water shutter
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Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
were disconnected and removed. With all of the shielding removed, the dose
rates in the direct beam (after 4 days of reactor shutdown) were
approximately 5 R/hr, without any auxiliary shielding in the beamport.
A chain and winch were attached to the old port plug via an eyebolt
mounted in the top and bottom of the front flange of the plug. The winch was
used to tighten the chain to try to pull the plug out of the reactor biological
shield. Unfortunately, even after tightening the winch to exert the maximum
force possible (- 2000 lb), the port plug did not move. It seemed likely that
there was a considerable amount of corrosion (since the old water shutter had
been leaking) that was causing the plug to stick. An attempt was made to try
to break and loosen the corrosion by applying an impulse. The opposite end
of the winch and chain were connected to the transporter (a hand-driven
forklift device with a capacity of several thousand pounds). The force on the
winch was reduced so that there was some slack in the chain and very little
force on the port plug. The transporter was then driven away from the plug,
to jerk on the eyebolts attached to the flange. Initial jerks performed at low
transporter speeds proved unsuccessful, and increasingly higher speeds were
attempted. Eventually, it was feared that damage may be done to the flange
or the transporter and the attempts were ceased. The winch was again
loaded to see if the plug had loosened at all, but the plug still showed no
movement.
An attempt was made to increase the force applied to the plug by using
an arrangement of jacking bolts. The 6 holes in the port plug were drilled
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Improved Boron 10 Quantification via PGNAA and ICP-AES
and re-tapped to the next largest tap size (5/8-11) to provide the jacking bolts
with a leverage surface on the face of the port box. Unfortunately, the bolt
pattern on the plug is not evenly distributed over the face of the flange, (the
bolt pattern is the same as on the flange of the plug shown in Figure 2.4) so
the force would not be uniformly distributed over the face of the flange.
Nevertheless, the loading would be greater and more uniform than with the
winch arrangement. 6 jacking bolts were screwed into the re-tapped holes
and tightened in a uniform fashion. No movement of the plug was observed
even as the bolts were tightened to an applied force of approximately 8,000 lb
(estimated by Ed Block, Reactor Operations). Upon further tightening of the
bolts, the flange was observed to bow and an audible popping noise was
heard, but no movement of the plug was observed. The load was removed
and the face of the port plug was inspected. No obvious damage was
noticeable on the face and it was suggested that the popping noise was due to
the rupture of a weld somewhere between the plug and the flange.
At this point it was clear that whatever is holding the plug in place is
stronger than any static load that the flange of the plug can support. A final
effort was made to apply a large static load while also applying a moderate
to large impulse. The plug was again loaded with the jacking bolts as
described in the previous paragraph. A slide hammer was used to repeatedly
impulse the front flange while under a static load. Increased static load and
increased impulse (both in frequency and magnitude) proved unsuccessful.
Two of the jacking bolts were removed, replaced with eyebolts, and the winch
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Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
and chain were reconnected to the plug and the transporter. Impulses were
applied with the transporter with the plug under static load. This too proved
unsuccessful, and alternate plans began to be considered.
While removing the shielding from the 4DH3 beamport, much of the
shielding, the port box, and goniometer that houses the graphite crystals
were found to be corroded. The amount of corrosion present seemed to imply
that the water shutter had been leaking from the front end (at the port box)
and could possibly be repaired. The front seal plate for the water shutter was
removed, a new gasket was made and the seal plate was replaced.
Subsequent testing of the water shutter, however, indicated that a leak was
still present. Regardless of how much water was pumped into the shutter,
the same volume was always pumped out (approximately 2 liters). This
behavior suggests that water only begins to leak out of the shutter once the
water reaches a certain level. During the testing no water was observed to
leak out of the front of the shutter, indicating that the leak is at an
inaccessible location.
Faced with an inoperable water shutter and a port plug that was
unable to be removed, the possibility of installing the new port plug in
another 4DH port was investigated. There are six such ports, one of them is
used for the Junior Physics Laboratory (4DH1), three others are obscured by
the shielding for the silicon irradiation facilities (4DH2, 4DH5, and 4DH6).
The sole remaining port, 4DH4, housed an old neutron scattering experiment
that no longer sees much use. 4DH4 is situated directly adjacent to the
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Improved Boron 10 Quantification via PGNAA and ICP-AES
silicon irradiation unload shielding, and the diffracted neutron beam would
require the HPGE detector to directly abut the silicon unload port. This plan
was deemed undesirable because of the high photon background that the
detector would be exposed to in that area, and because of space constraints.
Some consideration was given to diffracting the neutron beam away from the
silicon unload side toward an open area with comparatively low background.
This plan was also not feasible because the right edge of the port box would
interfere with the diffracted beam (see Figure 1.3), and would involve a great
deal of work to construct new shielding.
The 4DH3 beamport was the only remaining option, and it was decided
to use the 4DH3 beam port with the old plug in place. The old water shutter
housed a set of stainless steel collimator shims that vertically divided the
beam into three equal segments. The collimator shims sat in a rectangular
box frame made of stainless steel. The frame is approximately 1/2" thick
along the top and bottom, and 3/8" along the sides. The collimator shims were
easily removed from the frame and placed in the wall storage area. The
frame itself also seemed to be corroded into place because repeated attempts
to remove it were unsuccessful. Since the frame was unable to be removed,
installing a water shutter would mean further collimation of the beam,
allowing fewer neutrons to strike the graphite diffracting crystals. At this
point, the idea of a water shutter was abandoned and it was decided to insert
the sapphire crystal directly into the existing water shutter.
74
Chapter 2: Prompt Gamma Neutron Activation Analysis
Kent J. Riley
Inserting the sapphire crystal into the available aperture (1 11/16" x 1
7/8") meant that the sapphire crystal (3.7" diameter cylinder, 6" long) had to
be cut to the proper shape. A cutting layout on the crystal determined that
two bars of the proper width and height could be obtained from the already
available 3.7" diameter sapphire crystal cylinder. This would allow 12" of
sapphire to be inserted into the beam coming from the reactor core, (a
discussion of the motivation behind this decision, as well as the neutron and
photon attenuation characteristics of sapphire single crystal will be given in
Section 2.4.) which would provide a slow neutron beam, with lower photon
and fast neutron contamination.
To insert the sapphire bars into the beam aperture at 4DH3, special
insertion jigs were designed and constructed. The purpose of these jigs was
twofold: 1) to insure that the sapphire crystals would fit tightly against the
inner walls, and 2) to accommodate for any irregularities that the crystals
may encounter while being inserted along the length of the aperture. From
the many days of work at the 4DH3 beamport, it was fairly evident that the
channel narrowed in a few sections toward the reactor core. This was
discovered while machined blocks of lead and borated paraffin were being
inserted into the channel for interim shielding purposes.. The pieces were
observed to fit easily at the face of the port box, but then began to stick as
they were inserted. The insertion jigs are comprised of aluminum Lbrackets with slots to accommodate the sapphire rectangular bar.
75
Improved Boron 10 Quantification via PGNAA and ICP-AES
Figure 2.5: A photograph of the insertion jigs containing the rectangular sapphire
crystal bars. The jigs were used to insert the sapphire crystals into
the water shutter at the 4DH3 beamport.
Figure 2.5 shows the two insertion jigs containing the rectangular bars
of sapphire, and Appendix A contains engineering drawings for the two
insertion jigs. The bars are sitting on top of a spring that is made of 0.002"
sheets of stainless steel. The springs were made by folding the steel sheet
into an accordion shape, making a major fold along the steel sheet to form an
L-shape (to match the bracket), and then trimming any corners that may
stick out. The sheets then act like a spring to push the crystals up and to the
left, (in Figure 2.5) keeping them snug against the inner wall, but allowing
the crystals to deflect when a region of narrowing is approached. The springs
sit in the bottom of a groove that was machined in both surfaces of the
bracket. The crystals were cut such that the bottom edge of the crystal will
always overlap with the edge of the groove in the jig so that there is no
streaming of radiation along any small cracks. The two jigs were inserted as
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Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
mirror images of one another, i.e. one was inserted with the jig to the top and
to the left, and the other inserted with the jig to the bottom and the right.
This would help control any streaming between the jig and the wall of the
channel.
Figure 2.5 also shows slotted holes in the two jigs that contain feeds
for aircraft cables. The aircraft cable was looped and inserted through the
two feeds, then fit with an aluminum rivet on the opposite side of the feed to
prevent the cable from pulling back through the feed hole. The loop of aircraft
cable can then be used to pull the jig (and the crystal) out of the beamport,
should it need to be retrieved. This entire assembly was tested with a bench
top mock-up. An exact replica of the channel in the 4DH3 beamport was
found in a storage room for the MITR-II reactor. A plastic block was
machined to the exact shape of the sapphire crystal bar and inserted into the
mock-up with one of the jigs shown in Figure 2.5. The mock crystal and jig
could be easily inserted and removed. Furthermore, it was not possible to
slide a 0.001" stainless steel shim between the mock crystal and the wall of
the mock-up. This indicates that radiation streaming between these two
surfaces in the actual beamport will be minimal.
The bars of sapphire crystal were properly oriented on the jigs with the
springs, (see Section 2.4 for a discussion of the dependence of crystal
orientation on sapphire transmission properties) and lubricated with a
graphite powder and acetone mixture. The jig with the longest cable loop
was inserted into the beamport so that the rear face of the crystal was
77
Improved Boron 10 Quantification via PGNAA and ICP-AES
approximately 40" deep. Care was taken to insure that the crystal was not
inserted too far, thereby striking the rear of the water shutter (48" deep) and
damaging either the crystal or the shutter window. The remaining crystal
with the short cable was inserted so that the rear face was approximately 18"
deep. Section 2.4 will discuss the impact that the 12" of sapphire crystal had
on the photon and fast neutron dose rates surrounding the 4DH3 beamport,
as well as the resultant slow neutron flux incident upon the graphite crystals.
Figure 2.6: Composite drawing depicting the final configuration of the port plug
at 4DH3, including the two 15 cm sections of sapphire crystal. The
water inlet and outlet lines have been omitted since the water
shutter is no longer functional.
Figure 2.6 above shows a composite drawing of the final configuration
of the port plug at 4DH3. Since the water shutter is no longer functional, the
water inlet and outlet lines for the water shutter have been omitted. The
78
Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
figure shows the orientation of the two insertion jigs and how they press the
sapphire crystal sections toward opposite corners. Although the water
shutter is inoperable, the water shutter seal at the laboratory end of the port
plug was resealed with a new rubber gasket to prevent activation problems
with argon gas in the room atmosphere. Though not shown in the diagram
(because their detail is too fine), each insertion jig is fitted with loops of
aircraft cable that can be grappled to remove the sections of sapphire.
2.3 Focusing Monochromator Design
This section describes the design and construction of a neutron
focusing crystal diffraction arrangement for use at the PGNAA facility
2.3.1 Conceptual Design
As mentioned in Section 1.2.1.1, the PGNAA facility at the MITR-II
makes use of a diffracted slow neutron beam. Neutron diffraction (also called
Bragg diffraction) can be understood by considering the wave properties of
the neutron. A slow neutron, incident upon a periodic lattice of nuclei, can
scatter coherently from those nuclei. If we think of the incident neutron as a
wave, traveling in a straight line, then the outgoing scattered waves will
constructively interfere if the outgoing waves have the same phase. This
condition of matching the outgoing phase can be met when the distance that
79
Improved Boron 10 Quantification via PGNAA and ICP-AES
the neutron must travel between planes of nuclei is equal to integral
multiples of the wavelength of the neutron.
Figure 2.7: Schematic illustrating the concept of Bragg diffraction
Figure 2.7 shows a schematic of the concept of Bragg diffraction. The
incoming neutron waves (represented by ki) each coherently scatter from
nuclei on different planes, which are separated by a distance d. The lower
incident wave must travel a greater distance before it scatters, a distance
equal to dsin(6). The lower outgoing scattered wave (kf) must also travel an
extra dsin(0), thus the total extra distance traveled is equal to 2dsin(O). The
phase of the two outgoing waves will match when the extra distance traveled
by the lower wave is equal to the wavelength of the incident neutrons. This
condition is known as the Bragg criteria and is expressed in the equation
below.
80
Chapter 2: Prompt Gamma Neutron Activation Analysis
Equation 2.5
Kent J. Riley
nA = 2d sin(0)
The factor of n in Equation 2.5 represents contributions from neutrons
with higher order wavelengths (i.e. neutrons with wavelength equal to half,
one third, etc.... the extra distance traveled).
Equation 2.5 implies that the outgoing scattered wave will be
composed only of neutrons with the selected wavelengths. In practice, such a
pure monochrome beam is not possible to achieve. The crystal matrix that
the neutrons scatter from is imperfect in that some regions of the crystal
contain lattice planes that are slightly misaligned with respect to the
alignment of another region. This inherent misalignment is called the
mosaic of a crystal. The more perfect a crystal is, the smaller its mosaic. The
mosaic of a crystal therefore presents a range of angles about the Bragg angle
to incident neutrons, allowing neutrons in a small band of wavelengths to be
included in the diffracted beam for each of the nominal wavelengths given by
Equation 2.5. For the PGNAA facility, a large mosaic is desirable since we
are seeking to achieve a large slow neutron flux and not a purely
monochrome beam. Though the mosaic of the crystal used at the PGNAA
facility is relatively small, (the mosaic for the pieces of pyrolitic graphite were
measured - see below) the effective mosaic was increased by shimming
several layers of graphite so that a small angle offsets each layer and the
Bragg condition is met for a larger range of energies in the MaxwellBoltzmann energy distribution of the neutron source.
Improved Boron 10 Quantification via PGNAA and ICP-AES
We can further utilize the wave properties of neutrons by creating a
diffraction arrangement that directs the diffracted neutrons toward a focal
spot. In the same way that a lens or a focal mirror can focus a beam of
incident light (thereby creating a higher light intensity at the focal spot), the
wave nature of neutrons allows us to focus a beam of neutrons onto the
sample. By using a graphite crystal curved to an appropriate radius, the
outgoing neutrons will converge on the focal spot. Figure 2.8 demonstrates
this schematically.
Figure 2.8: Schematic illustrating the focusing effect of curved reflecting
surfaces.
In Figure 2.8, 0 represents the angle between the incident neutron
beam and the vertical diffracting planes of the crystal lattice, which is often
called the rock angle, or when the Bragg criteria is satisfied, the Bragg angle.
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Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
Figure 2.8 shows a surface that is curved around a horizontal axis (referred
to as the tilt axis) which results in what will be referred to as vertical
focusing. Vertical focusing is so named because the vertical extent of the
beam is focused to a (horizontal) line. Similarly, horizontal focusing would
focus the horizontal extent of a beam to a vertical line (requiring curvature
about the vertical, or rock axis), and the combination of the two would result
in a beam that is focused to a point.
Basic principles of optics can be used to arrive at an appropriate
expression to relate the Bragg angle, the distance from the focal spot to the
diffracting crystal, and the radius of curvature of the diffracting crystal (10).
A detailed derivation of the relevant formula will not be given here, rather
the important result for vertical focusing will be stated.
Equation 2.6
R
+=f sin( O)
In the above expression, R is the radius of curvature of the graphite
crystals, s is the distance from the source to the graphite crystals (source
distance), f is the distance from the crystal to the sample (focal distance), and
0 is the Bragg angle. For the MITR-II, the source distance is 106" (distance
from end of re-entrant thimble to graphite crystals), the focal distance is 40"
(though this is somewhat flexible), and the Bragg angle will be 210 (see
Section 2.3.2 for a discussion of selecting the optimum diffraction angle).
83
Improved Boron 10 Quantification via PGNAA and ICP-AES
With these parameters, we can calculate that the radius of curvature
necessary for the graphite crystals is 20.8", or about 1/3 of a degree.
Instead of using a continuous piece of graphite with the proper
curvature, the focusing monochromator is composed of several thin, flat strips
of graphite, which are arranged in a holder with the proper curvature. Using
strips of graphite is advantageous for two reasons. First, to obtain a graphite
crystal of the appropriate shape would require 6-8 weeks of lead time by the
manufacturer, and a considerable cost. A suitable focusing monochromator
was constructed in about 3 weeks, using available pieces of single crystal
graphite. Second, to focus the beam, we require a small mosaic (a large
mosaic will result in a broader diffracted beam, which will tend to blur any
focusing effects), yet we also seek a large mosaic to diffract as much of the
slow neutron energy spectrum as possible, as described in the preceding
paragraph. Fortunately, with a focusing monochromator constructed of
several pieces, both of these goals can be reached. To sample as much of the
slow neutron spectrum as possible, we want a large mosaic about the rock
axis, so that many neutron energies have a chance of meeting the Bragg
criteria. However, to focus in the vertical direction, we seek a small mosaic
about the tilt axis so that focusing effects will not be blurred. By using strips
of single crystal, relatively low mosaic graphite, we can arrange them so that
the mosaic about the rock axis is large, while the mosaic about the tilt axis is
small. It is not possible to obtain a piece of single crystal graphite with a
large mosaic in one direction, and a small mosaic in another.
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Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
After collecting all of the single crystal graphite available, it was
determined that there was enough graphite to form 5 layers in the focusing
monochromator. The mosaic of the available graphite was measured to
determine how wide the strips that form the curvature should be, and how
far the rock angle should be offset for each layer. The mosaic was measured
on the well collimated beam of the Student Spectrometer system (4DH1
beamport) at the MITR-II. The graphite crystals were positioned 1.8 m from
the 1 mm cadmium slit that collimates the neutron beam at 4DH1. The
graphite crystals diffracted neutrons toward a 3He detector, positioned 40 cm
away at an angle of 420 (corresponding to a Bragg angle of 210), and
collimated to 0.5 cm. A rock curve was obtained by slowly changing the angle
between the 0002 planes of the graphite crystal and the incident neutron
beam, while recording the count rate measured by the 3He detector
(connected to a preamplifier, amplifier, 2000 V high voltage supply, single
channel analyzer, scaler and ratemeter). The full width at half maximum
(FWHM) of a plot of count rate versus nominal angular position is related to
the mosaic of the crystal and the collimation of the neutron beam and the
detector by the following formula: (11)
Equation 2.7
MS = (FWHM)2
85
Improved Boron 10 Quantification via PGNAA and ICP-AES
The mosaic spread (MS) of the diffracting crystal is simply the FWHM
of the rocking curve, minus a correction for the collimation of the neutron
beam (Cb) and the detector (Cd). Figure 2.9 below shows a rocking curve
taken from one of the two large pieces of single crystal graphite that was
available for this work. Using the FWHM shown in Figure 2.9, and the
collimation parameters described in the preceding paragraph, a mosaic of
1.20 is calculated for graphite piece #1.
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Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
Rocking Curve for
Single Crystal Graphite #1
500
450
400
)
350
300
'
r
250
g 200
cU 150
100
50
0
15
17
19
21
23
25
27
Nominal Crystal Position (degrees)
Figure 2.9: Rocking curve used to calculate the mosaic spread of the single
crystal graphite piece #1.
Similar measurements and calculations were carried out for the other
large piece of graphite crystal that was available. Measurements showed
that crystal #2 has a mosaic spread of about 1.4'. All measurements were
carried out with the crystals oriented at a Bragg angle of approximately 210
(±t0.50). With the values for the mosaic for the crystal, the size of the strips
to form the curvature, and the angular offset could be decided upon.
87
Improved Boron 10 Quantification via PGNAA and ICP-AES
Each layer of graphite was offset approximately 1/2 FWHM from the
layer beneath it by placing a shim beneath one end (see Section 2.3.4 for
details on aligning the pieces of the focusing monochromator). This results in
an offset of about 0.60 - 0.70 between each layer, for a total spread of
approximately 3.3'.
The neutron beam that exits the port plug at the 4DH3 beamport is
1.625" high. The total angular change over the vertical extent of the focusing
monochromator is therefore; 1.625/20.8 = 0.078 radians = 4.50. The number
of vertical strips that are required is then simply the angular change of the
vertical extent divided by the mosaic of the crystal being used. For the
crystals described in the preceding paragraphs, 4 strips in the vertical
direction will adequately focus the slow neutron beam. Additional strips will
not increase the slow neutron flux at the focal point much, due to the blurring
effect of the mosaic that was described earlier.
By considering the size of the focused neutron image of the source, and
the effective size of the source without focusing, we can obtain an estimate of
the gain in flux due to neutron focusing that is to be expected. The following
formula expresses the ratio of the extent of the unfocused image to the extent
of the focused image (12). A derivation of this formula will not be provided
here, rather the reader is referred to any standard optics text.
Equation 2.8
Gain =
H M
HS (
1+
1
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Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
In the above equation, li is the distance from the source to the crystals
(106"), lo is the distance from the crystals to the focal spot (-40"), HM is the
height of the monochromator (1.5"), and Hs is the height of the source (1.33").
Using this formula, we can expect the slow neutron flux to increase by a
factor of approximately 2.6.
2.3.2 Diffraction Angle Optimization
A design optimization was carried out by analyzing how the sensitivity
of the PGNAA system varies with the neutron diffraction angle (and thus
neutron energy). As mentioned earlier, the sensitivity is proportional to the
slow neutron flux incident upon the sample, the cross section of the isotope
being measured, and the absolute efficiency of the detector being used.
Clearly, the detection efficiency will not be affected by a change in neutron
energy. The cross section, and the population of neutrons within a finite
energy band of the Maxwellian spectrum are, however, dependent on the
neutron energy being examined. Moreover, the amount of slow neutrons
transmitted by the sapphire filter crystal is dependent on the neutron energy.
The functional parameters of all these variables were investigated and
combined to give a composite functional dependence of the sensitivity of the
PGNAA system versus neutron energy and corresponding diffraction angle.
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Improved Boron 10 Quantification via PGNAA and ICP-AES
2.3.2.1 Sapphire Filter Crystal
Measurements made on the transmission properties of a sapphire
single crystal as part of this research (see Section 2.4) and data from the
literature provided the slow neutron attenuation coefficient as a function of
energy. The data from the literature was plotted and fitted to a polynomial
curve as shown in Figure 2.10 (13).
Sapphire Neutron Attenuation Coefficient
as a Function of Neutron Energy
I
0.14
I -
''~~'''''-~
'
Y = MO + Ml*x + ... M8*xq + M9*x 9
-
0.12
'^1
!
i
0.1
/"
0.08
0.06
m Reference 13
0.04
0.02
I
'''''''''''''''
0
I
.
I
0.1
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.16
Neutron Energy (eV)
Figure 2.10: Slow neutron attenuation coefficient for sapphire single crystal as a
function of neutron energy, showing the fourth order polynomial
curve fit parameters. Data was taken from Reference 13.
A fourth order polynomial (as shown in the figure) fits the data quite
well. The slow neutron attenuation coefficient therefore has the following
90
Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
4
3
empirical energy dependence: g(E) = 0.04 - 1.61E + 37.1E 2 - 201E + 362E .
The factor e -"(E)t has been calculated for a range of energies, and normalized
to the maximum value over that range.
2.3.2.2 Maxwellian Thermal Flux Distribution
The population of neutrons that are in thermal equilibrium with their
surroundings is represented by the following familiar expression, which is
simply the product of the neutron speed and the Maxwellian thermal neutron
density distribution as a function of energy (14).
2
Equation 2.9
(E)
1
3
2 1 2
kT
2nC
E
exp kT
T is the temperature of the media, k is Boltzmann's constant, m is the
mass of a neutron, and E is the neutron energy. The above expression is a
differential distribution, meaning that the expression on the right is not
meaningful unless it is multiplied by differential element of energy, dE. After
doing so, the resulting expression (O(E)dE) represents the flux of neutrons
that have an energy within dE about energy E. The above expression can be
written in terms of any variable that is related to energy; neutron speed,
neutron wavelength, or for our purposes, the Bragg angle. The graphite
crystals contain reflecting planes that are within some range of angles (de)
about the nominal Bragg angle 0. Using the Bragg criteria (given in
Equation 2.5), and the DeBroglie relationship (15) which provides the
91
Improved Boron 10 Quantification via PGNAA and ICP-AES
neutron wavelength as a function of velocity, we can derive an expression for
neutron energy, E, as a function of Bragg angle, 0. This expression, along
with the Jacobian for the differential element dE (found by calculating
IdE/dO I) can be inserted into Equation 2.9 to arrive at the following
expression.
-h 2
h 4 e 2 md2kTsin 2 (0) COS()
Equation 2.10
2d 4
0(2m
sin 5 (0)
This expression provides the flux of slow neutrons that have been
diffracted within dO about the Bragg angle 0 as a function of the angle 0. It is
important to note that the shape of this curve is much different than the
shape of O(E) or O(v), as shown in Figure 2.11. This is due to the fact that
energy, velocity, wavelength, and Bragg angle are not related to each other in
a linear fashion. The corresponding differential elements (dE, dv, dX, dO) are
therefore also not linearly related and will have different functional
dependencies. The differential distribution will then have a shape that
corresponds partially to the functional dependence of its differential element.
92
Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
Figure 2.11: The Maxwell-Boltzmann thermal neutron flux distribution as a
function of energy (E), speed (v) and Bragg angle (0), plotted
versus neutron energy. The differential elements for each curve
are not linearly related and therefore change the shape of the
curve.
Equation 2.10 will be evaluated at several Bragg angles, normalized to
the maximum value in the range of evaluated angles, and multiplied by all of
the other components, each of which is evaluated at the appropriate energy.
2.3.2.3 Absorption Cross Section
The absorption cross section of several elements (including Boron)
exhibits what is called 1/v behavior in the low energy region. This
93
Improved Boron 10 Quantification via PGNAA and ICP-AES
description simply implies that as neutron energy decreases, the absorption
cross section increases, and it does so in a fashion inversely proportional to
the square root of the energy (or inversely proportional to the velocity). For
the purposes of this analysis, a 1/v cross section was normalized to a neutron
energy of 0.001 eV, and multiplied with the normalized parameters described
in the preceding and following sections.
2.3.2.4 Coherent Neutron Scattering Cross Section
Another important factor affecting the slow neutron flux at the sample
position is the coherent scattering cross section of the graphite diffracting
crystals. A higher cross section will result in more neutrons being diffracted
toward the sample. A derivation of the coherent scattering cross section is
quite involved, but the relevant result is quite simple. The coherent
scattering cross section carries a 1/E dependence (16), along with other terms
that relate to the structure of the crystal lattice and the scattering length of
the isotope. For this analysis, a 1/E factor has been normalized (again to
0.001 eV) and combined with the other normalized parameters described in
the preceding paragraphs.
2.3.2.5 Optimization Results
All of the parameters described in the previous section have been
combined into a single composite function, which represents the PGNAA
sensitivity as a function of Bragg angle, per unit of differential Bragg angle.
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Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
The composite function has been plotted versus Bragg angle and the
corresponding neutron energy, as shown in Figure 2.12. This plot was
generated by evaluating each of the described functions at the appropriate
angle (and/or corresponding neutron energy) and multiplying the results.
PGNAA Sensitivity vs.
Diffraction Angle and Neutron Energy
0.242
0.061
0.028
Neutron Energy (ev)
0.016
0.010
0.007
1.000
0.800
40
600
0 400
0.200
0 000
0000
5
15
Diffraction
(degrees)
Angle
20
25
35
Diffraction Angle (degrees)
Figure 2.12: Composite function for PGNAA sensitivity plotted versus Bragg
angle and corresponding neutron energy. The peak occurs at
approximately 190 .
The above figure shows that the maximum sensitivity occurs at a
Bragg angle of approximately 190. At a Bragg angle of 210, the sensitivity
falls to about 98% of the peak value. Therefore, only a small gain would be
realized by changing the diffraction angle to 210. Since changing the Bragg
95
Improved Boron 10 Quantification via PGNAA and ICP-AES
angle would involve a considerable amount of work to change the shielding at
4DH3, it was decided to leave the Bragg angle at 21'.
2.3.3 Construction
A frame was constructed to hold the graphite pieces in place with the
proper curvature. The frame was constructed entirely of aluminum 6061. A
slab of aluminum was first cut to an appropriate size to accommodate all of
the graphite pieces. The pieces spanned an area of 1.75" tall by 3.75" wide,
which is more than large enough to cover the beam aperture at the 4DH3
beamport (1.65" tall by 1.33" wide). The frame was therefore milled, from an
aluminum slab, to a size of 2.25" tall by 4.0" wide by 0.25" thick with a
vertical curvature with a radius of 20.8". Immediately above and below
where the graphite strips sit, 6 holes were drilled and tapped to accommodate
3-40 aluminum screws. The screws penetrate through a cover plate that
applies light compression to the graphite strips to hold them into place. The
cover plate is a sheet of 1/32" thick aluminum sheet, cut to the proper size
and drilled with 6 clearance holes to accommodate the 3-40 screws. The
cover plate is also curved slightly (to an approximate radius of 21") so that
even compression is applied over all of the graphite strips. The bottom 1/4" of
the aluminum frame was trimmed to 1/8" so that the clamp in the goniometer
assembly could easily accommodate the frame. The frame was then
machined, using a programmable milling machine, to obtain a radius of
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Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
exactly 20.8" along the inner surface, with the peak of the curvature
occurring roughly in the center of the frame.
Each of the graphite pieces were placed on the frame, care was used to
insure that each piece was properly aligned as it was placed (see Section
2.3.4). Each graphite strip was cut to a width of 7/16" by using a metal
straightedge and a sharp utility knife to repeatedly score the crystal. As each
piece was cut, the edges near the cut began to exfoliate, which could cause
the crystal to lie improperly. The edges were carefully pressed back down,
and where necessary, gently filed using jewelers files. The length of each
piece varied, depending on the size of the crystal that it originated from.
Most pieces were approximately 1 7/8" long, thus requiring 8 to form a layer,
while some were 3 3/4" long, requiring only 4 to form a layer. Each layer was
carefully tacked into place using a small amount of adhesive (rubber cement
or Duco spray adhesive). The cement was weak enough to permit a piece to
be repositioned without damaging the crystal when it was removed.
Each layer was offset by using pieces of folded aluminum foil as a shim
between the each layer of graphite. The thickness of each shim varied,
depending on the length of the piece being shimmed, however, the angular
offset of each piece was verified during the alignment procedure described in
the next section.
After all of the pieces were aligned, shimmed, and tacked into place,
the aluminum cover was attached to firmly hold all of the pieces in place.
The cover was then removed and the alignment of the top layer was re97
Improved Boron 10 Quantification via PGNAA and ICP-AES
checked to be sure that the cover did not disturb the alignment of any of the
pieces. The cover was again securely attached and the assembly was then
ready for installation into the goniometer assembly.
Figure 2.13 below shows the fully assembled focusing monochromator.
The frame is labeled in the bottom of the picture, with successive layers of
graphite stacked on top. The curvature of the frame is so faint, it is difficult
to see in the photograph.
Figure 2.13: The fully assembled focusing monochromator. The several strips
and layers of graphite are visible. The faint curvature (R = 20.8") of
the holder (lower surface) is difficult to make out.
The assembly shown in Figure 2.13 was then installed in the
goniometer assembly of the 4DH3 beamport (drawing number P-17-1-4D).
The lower edge (on the right side Figure 2.13) was seated in the clamp of the
goniometer assembly. A shim was added so that when the clamp was
tightened, the graphite crystals (when positioned at a Bragg angle of 21') sit
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Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
exactly in the center of the neutron beam emitted from the port plug at
4DH3. The controls of the goniometer assembly (which allows three
rotational directions, rock, tilt, and pitch) were lubricated and tested to be
sure that they were in working order.
The goniometer and all of the shielding at 4DH3 were then reinstalled.
The performance of the focusing monochromator was then tested; the results
and discussion of the testing will be presented in Section 2.5
2.3.4 Crystal Alignment
In order for the several strips of single crystal graphite to accurately
focus the beam of slow neutrons, each piece of graphite had to be carefully
aligned. The alignment procedure was carried out using a laser, the
aluminum frame to hold the graphite pieces, a jig to hold the frame, and a
screen to view the reflected laser beam.
In order to use the laser to align the pieces, it was first necessary to
verify that the optical surface of the single crystal graphite is parallel to the
0002 crystallographic planes of the graphite. Neutrons diffract from the
crystallographic planes, while a laser beam will reflect from the surface of the
graphite. It is necessary to insure that the two surfaces are parallel so that a
beam of neutrons will respond in the same fashion that the laser beam does
during alignment. This verification was carried out with two simple
experiments at the Student Spectrometer facility at the MITR-II.
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Improved Boron 10 Quantification via PGNAA and ICP-AES
In the first experiment, a piece of the graphite crystal was mounted to
the goniometer assembly using a padded clamp. The surface of the graphite
crystal was kept flush against the mounting surface of the goniometer
assembly. The graphite crystal was then rotated to a Bragg angle of 21' (the
3He
detector was again positioned at 420 and connected to an appropriate
preamplifier, amplifier, voltage supply, and rate meter) and the peak of the
rocking curve was then found using the fine adjustment knobs of the
goniometer. The peak of the rock curve was observed to occur at a nominal
goniometer setting of 63.6 ± 0.050 (the uncertainty in reading the scale of the
goniometer). The neutron beam was then turned off and the graphite crystal
was carefully inverted (so that the surface that had faced the neutron beam is
now flush against the mounting surface) without disturbing any of the
goniometer or equipment settings. The peak of the rock curve was again
sought, and was found to occur at 63.5 ± 0.050 on the goniometer scale. This
measurement implies that the two surfaces of the graphite crystal are
parallel to each other and to the crystallographic planes of graphite to within
at least 0.05 ± 0.035'. The skew of the crystallographic planes with respect
to the surface (call this parameter 0) will manifest itself as a movement of 20
in the peak of the rocking curve after the crystal has been inverted. From
this experiment it is clear that the optical surface and the crystallographic
planes of the graphite are parallel to within a few tenths of a degree, much
less than the mosaic spread of the crystal (1.2' - 1.4').
100
Chapter 2: Prompt Gamma Neutron Activation Analysis
Kent J. Riley
In the second experiment, a laser beam was aligned parallel to the
incident neutron beam (± 0.10). The neutron beam was turned on and the
graphite crystal was rotated into the Bragg position by locating the peak of
the rocking curve. Figure 2.14 shows a schematic depicting the arrangement
used for this experiment. The neutron beam was then turned off and the
laser beam was turned on, so that it reflected off the graphite crystal toward
the detector. A screen was used to view the laser reflection, which appeared
as a diffuse illumination due to the mosaic of the graphite crystal. The center
of the reflection was determined (± 1 mm) and was found to coincide with the
center of the detector (positioned 41 mm away). Since the peak of the rocking
curve and the reflection of the laser were observed to coincide, (within an
experimental error of about 0.250) the optical surface and the
crystallographic planes of the graphite were determined to be parallel.
101
Improved Boron 10 Quantification via PGNAA and ICP-AES
incident neutron
beam
laser aligned
parallel to
neutron beam
allel neutron and
er beams
scree
laser
graphite crystal
'aligned so that peak
intensity strikes 3He
detector
collimated
3
He detector
itron and
Figure 2.14: Schematic depicting the experimental arrangement used to
determine that the optical surface and the crystallographic planes
of the graphite crystals are parallel to each other.
With the results of the two experiments, the laser alignment of all the
pieces was now ready to be carried out. Figure 2.15 shows a photograph of
the apparatus that was used to carry out the alignment. A laser was
mounted to allow precision translation (± 0.001") along the vertical direction
of the crystals. A jig was mounted to the table to contain the aluminum
frame, which could translate freely in the horizontal direction. The vertical
and horizontal translation would allow the laser to strike anywhere on the
frame without having to disturb the graphite crystals. This allowed the
alignment of each piece of graphite crystal to be checked.
A viewing screen was mounted 10.4" above the surface of the graphite
crystals. The laser was arranged such that the reflection was nearly
102
Chapter 2: Prompt Gamma Neutron Activation Analysis
Kent J. Riley
perpendicular (approximately 750) so that the focal spot would fall on the
viewing screen. This arrangement not identical to the situation that will
exist at 4DH3, (a beam striking a reflecting surface positioned at an angle of
21' relative to the incident beam, and the reflected beam traveling to a focal
spot 41" away) but it is an equivalent arrangement from an optics standpoint.
The alignment process is intended to insure that all of the graphite strips
follow the 20.8" radius of curvature. By using a beam perpendicular (or close
to perpendicular) to the reflecting surface, and a viewing screen that is 10.4"
away, we can insure that the pieces will follow the correct curvature.
Figure 2.15: Photograph of the alignment apparatus for positioning the many
pieces of graphite crystal. The bottom of the photograph shows a
layer of crystals with the laser reflecting from one of the pieces.
The top right portion of the photograph shows the reflection viewing
screen with a ruler scale.
103
Improved Boron 10 Quantification via PGNAA and ICP-AES
A grid on the reflection screen was then oriented so that its axes
coincided with vertical and horizontal translation. The center of the jig was
located and the corresponding reflection was marked on the viewing screen;
this would serve as the focal spot for aligning all of the pieces.
With the focal spot marked on the viewing screen, each piece was
placed in the frame and the reflection was observed on the viewing screen.
The reflection on the viewing screen was generally a diffuse spot
approximately 1/4" in diameter, though not always symmetric. The diffuse
nature and size of the reflection indicated that the small laser beam (-2 mm
diameter) was sampling the slight disorder in the surface layer of crystals,
though may not be indicative of the bulk mosaic properties of the deeper
lattice planes. The center of each reflection was observed to coincide with the
focal spot marked on the viewing screen to within 1/16" or better, which
implies that the tilt of each piece was adjusted to ± 0.340 or better
(arctan(0.0625/10.4)). Horizontal translation did not cause the reflection to
move away from the focal spot, except when the laser struck a surface
blemish on the graphite. Occasionally, a piece would not reflect toward the
focal spot, which indicated that the edges of the piece were causing it to be
improperly tilted. Firmly pressing the edges, or touch-ups with the jewelers
file were sufficient to properly orient the crystal.
Once the first layer was sufficiently aligned, the pieces were lightly
tacked into place by applying a thin layer of rubber cement to the aluminum
104
Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
frame. The rubber cement was placed only near the edge of the pieces, away
from the direct beam, and covering only a few square millimeters. The
cement was used sparingly and the thickness of the glue layer was
approximately 0.005: thick. The hydrogenous component of the glue should
therefore have very little effect on the slow neutron beam. Subsequent layers
were aligned in the same fashion, by insuring that the reflection coincided
with the focal spot. Shims were added between each layer to offset each layer
by about 0.6'. The angular offset was verified by viewing the horizontal
deviation of the focal spot for each successive layer. Each shim (made of
aluminum foil) was adjusted until the proper reflection on the viewing screen
was observed. Subsequent layers of graphite were aligned and shimmed in
exactly the same fashion, using rubber cement where necessary. A total of 5
layers were included before attaching the clamp and installing the system in
the goniometer assembly.
The preceding sections have discussed improvements that were made
to the graphite monochromator assembly for the PGNAA facility. The effect
that these improvements have had on the facility will be described and
discussed in Section 2.5.
105
Improved Boron 10 Quantification via PGNAA and ICP-AES
2.4 Sapphire Filter Crystal
Measurements
A cylindrical, single crystal sapphire, (grade CZ) 15 cm long and 9.4 cm
in diameter was purchased from Union Carbide Crystal Products in
December 1995. A sapphire crystal was chosen to act as a filter that would
heavily attenuate fast and epithermal neutrons, as well as photons, while
only slightly attenuating slow neutrons. Data found from a literature search
on the attenuation properties of single crystal sapphire indicated that
sapphire performed better than any other single crystal materials such as
quartz, silicon, or bismuth. For example, a 15 cm sapphire crystal will
attenuate 99.4% of neutrons with energy 0.4 eV or higher, approximately
80% of 4.0 MeV photons, while attenuating only 30% of slow neutrons. To
achieve such a high fast neutron and photon attenuation with other crystals
would mean attenuating the slow neutron flux by 50% or more.
The literature search on the transmission properties of sapphire also
revealed that it is important to "tune" the sapphire crystal to the neutron
beam to obtain maximum slow neutron transmission. Sapphire is single
crystal aluminum oxide (A12 0 3 ), also known as corundum, with a hexagonal
lattice arrangement (17). The crystal structure of sapphire is such that the
orientation of the various planes of the hexagonal lattice can affect the
106
Chapter 2: Prompt Gamma Neutron Activation Analysis
Kent J. Riley
constructive and destructive interference patterns of the coherently scattered
(transmitted) slow neutrons. For this reason, the crystal should be properly
oriented, or tuned, to obtain the maximum constructive interference in the
transmitted neutron beam.
The sapphire crystal was therefore tuned, again using the neutron
beam at the Student Spectrometer facility at the MITR-II. Other properties
of the sapphire crystal were also investigated, such as the attenuation
properties and the uniformity of the crystal. Figure 2.16 shows a schematic
of the experimental arrangement for the measurements described in this
section.
neutron beam
III
I
Cd chopper wheel
with 1mm x 3mm slits
wIth li--
0.2 m
I
+_ sapphire crystal
T
1.1 m
He 3
,graphite crystal
(at 20.4 degrees)
Figure 2.16: Schematic of experimental setup for measurements on the
sapphire crystal.
107
Improved Boron 10 Quantification via PGNAA and ICP-AES
The neutron beam passes through a series of collimators and a
cadmium chopper wheel. Unless otherwise noted, the cadmium chopper
wheel was fixed so that the 1mm wide by 3mm high slit coincides with the
beam collimator, thereby transmitting a well collimated neutron beam. The
neutron beam then passed through the sapphire crystal toward the graphite
diffracting crystal from the focusing monochromator assembly described
earlier. The diffracting crystal was mounted to the goniometer and situated
at a Bragg angle of 210, while the 3He detector was positioned at 42'. The
goniometer was then used to finely adjust the angle of the diffracting crystal
to achieve the maximum count rate in the detector (the final adjusted Bragg
angle was 20.80). A Bragg angle of 21' was chosen so that the properties of
the crystal could be evaluated under the conditions that exist at the PGNAA
facility (e.g. Bragg angle of 210, corresponding to 0.015 eV neutrons).
The various measurements made with this experimental arrangement
will be described and discussed in the remainder of this section. These
measurements were made to compare with data published in the literature,
and to evaluate characteristics of the crystal that are important to our
application. Other measurements, taken after the crystal had been installed
in the 4DH3 beamport to insure that the crystal was performing as expected,
will be described in Section 2.5.
108
Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
2.4.1 Crystal Uniformity Test
The crystal purchased from Union Carbide is of the highest (CZ) grade,
optical quality sapphire, and the crystal is guaranteed to be extremely
uniform. To investigate the uniformity of the crystal, the well collimated
neutron beam described earlier was directed at the crystal and the
transmitted beam intensity (after diffraction) was measured in the 3 He
neutron detector. In a truly uniform crystal, the transmitted intensity should
be the same, regardless of where the beam strikes the crystal, provided that
the rotational orientation of the crystal is unchanged.
Tranmitted Beam Count Rate
vs. Position in Crystal
250
200
0
150
So
I
SI
100
-
50
-
0
1
1
3
2
Position
3
2
Nominal
~
4
4
5
Nominal Position
Figure 2.17: Transmitted beam count rate vs. Nominal position in the sapphire
single crystal. The count rate does not significantly change
109
Improved Boron 10 Quantification via PGNAA and ICP-AES
regardless of where the beam is positioned, indicating a very
uniform crystal.
The transmitted intensity was measured at several positions on the
sapphire single crystal, while keeping the crystal in a fixed rotational
orientation. The positions that were sampled are shown schematically in
Figure 2.18. Figure 2.17 shows the transmitted beam intensity plotted
versus nominal position in the sapphire single crystal. The error bars in the
figure represent the 3% statistical uncertainty of each data point. No
appreciable change in the transmitted beam is observed with any of the
positions, and the line shown in Figure 2.17 can be drawn as flat and still
remain within the error bars of the measurement. These data indicate that
the crystal is quite uniform.
FRONT VIEW OF SAPPHIRE
3.7"
Figure 2.18: Schematic showing the locations at which the uniformity of the
crystal was tested.
110
Chapter 2: Prompt Gamma Neutron Activation Analysis
Kent J. Riley
2.4.2 Tuning Measurements
The tuning measurements were carried out with the neutron beam
striking position 0, shown in Figure 2.18 (approximately along the cylindrical
axis). The crystal was marked in 11.60 increments by marking an
appropriate distance on a piece of tape and wrapping it around the crystal.
The marks on the tape were aligned with a reference marker in the neutron
beam path. After each incremental rotation, a laser beam (aligned with the
neutron beam) was used to insure that the neutron beam was striking the
same spot on the crystal. At each rotational position, the count rate from the
3He
detector was recorded. Figure 2.19 shows a plot of the transmitted
neutron count rate versus the angular position of the sapphire crystal. The
data shows no clear trend, except for some possible, erratic periodicity,
presumably associated with the Bragg scattering off crystallographic planes
that contribute as they are rotated into the proper orientation. The
difference between the minimum (18.6 cps) and maximum (26.0 cps) is
substantial, amounting to a net difference in transmission of about 30%. The
data reveals a set of 4 maxima, each of which is approximately the same
magnitude, within error bars. The last maxima (at approximately 3000)
seems to be a bit more stable than the others because the drop on either side
is not as steep. This point was chosen as the optimally tuned position so that
111
Improved Boron 10 Quantification via PGNAA and ICP-AES
in case the crystal was somehow slightly misoriented, the loss in slow
neutron flux would not be as severe. When the large cylindrical crystal was
cut into rectangular bars, the orientation was carefully marked so that the
crystals could be inserted with the proper orientation.
Rotational Position of Sapphire
Crystal vs. Transmitted Count Rate
Sapphire
Rate
Position of Count
Rotational
Transmitted
vs.
Crystal
290
AAA
270
"250
P230
2 10
190
1 7n
i
|
0
100
200
i
300
400
Rotational Position (degrees)
Figure 2.19: Transmitted neutron count rate plotted versus rotational position of
the sapphire single crystal.
112
Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
2.4.3 Transmission Measurements
Several transmission measurements were made on the sapphire
crystal to determine how slow neutrons, fast neutrons and photons would be
attenuated. The maximum count rate arising from the 0.015 eV neutrons
transmitted through the sapphire crystal is 260.2 (± 5%) counts per 10
seconds (from Figure 2.19). The count rate in the 3He detector was also
measured without the crystal present, leaving the goniometer and equipment
settings unchanged. The count rate without the sapphire filter present was
416.3 (± 5%) counts per 10 seconds, which means that a 15cm sapphire
crystal transmits 63 ± 4.5% of slow neutrons. We can compare this
transmission value with data from the literature. Nieman, Tennant, and
Dolling report a slow neutron attenuation coefficient of 0.023 cm
-1 for
sapphire single crystal that has been properly tuned at 0.015 eV (13). Using
a length of 15 cm, we calculate an expected transmission of 71% (e-0. 023 (15 )).
This agrees fairly well with the measured transmission of 63%, and is nearly
within the 7% statistical uncertainty of the measured transmission value.
Measurements were also made on the transmission properties of fast
neutrons and photons. The cadmium chopper wheel was rotated so that the
two apertures did not overlap, allowing only fast neutrons (E > 0.5 eV) to be
transmitted down the beam path. A Tissue Equivalent Proportional Counter
(TEPC) was used to measure the neutron dose rate in the beam. The TEPC
113
Improved Boron 10 Quantification via PGNAA and ICP-AES
will respond to slow as well as fast neutrons, but the cadmium chopper
should sufficiently reduce the slow neutron component of the beam so that
the TEPC slow neutron response is small. The dose rate, measured at a
distance of 1.5 m from the cadmium chopper wheel, was 310 mrem/hr. The
sapphire crystal was then placed in the neutron beam, directly in front of the
chopper, so that the distance from the end of the crystal to the TEPC was
roughly 1.3 m. The dose rate in this configuration was 3.0 mrem/hr. The
sapphire crystal therefore reduces the fast neutron dose rate by more than a
factor of 100. This is in rough agreement with calculations (using the
scattering cross section of sapphire) which indicate that a 15cm crystal
should attenuate 99.4% of epithermal and fast neutrons (13).
Similar measurements were performed using a photon exposure meter
based on a Geiger-Muller detector. The photon dose rate without the crystal
present measured 19.0 mrem/hr, while the dose rate with the crystal present
measured 2.1 mrem/hr. Correlation with calculations or cross section data is
difficult since the photon energy spectrum at the Student Spectrometer
facility is unknown. Nevertheless, the sapphire crystal reduces the photon
dose rate by nearly a factor of 10 and a similar attenuation can be expected
when the crystal is installed in a similar beam used for the PGNAA facility.
Table 2.2 summarizes the results of the transmission measurements,
and compares the measured results to values calculated from data in the
literature.
114
Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
Slow Neutron (E
Measured % Calculated %
Unfiltered
Filtered
416.3 cps
260.2 cps
63 + 4.4%
71 ± 5.0%
310 mrem
3.0 mrem
1.0 + 0.14%
0.6 + 0.01%
19.0 mrem
2.1 mrem
11.1 ± 1.6%
16.5 + 0.3%*
at 4.0 MeV
= 0.015 eV)
Fast Neutron
(E > 0.5 eV)
Photon
* - Assuming good
geometry
Table 2.2: Summary of measured and calculated transmission percentages for
single crystal sapphire.
Another experiment was performed at the Student Spectrometer
facility to measure the transmission of slow neutrons as a function of neutron
energy. In this experiment, the graphite diffracting crystal was removed,
and a BF 3 neutron detector was placed in the direct path of the neutron beam
at a distance of 1.38 m from the cadmium chopper wheel. The chopper wheel
was set in motion, and was gated with a multichannel analyzer (MCA) so
that the time it takes for a neutron travel from the chopper wheel to the
detector can be measured and recorded. The MCA then sorts events into
arrival time bins, according to the time that the pulse is received from the
detector. The MCA therefore collects a histogram of arrival times, which can
be used to calculate neutron velocities (knowing the distance traveled) and
neutron energies. By recording a spectrum with and without the sapphire
115
Improved Boron 10 Quantification via PGNAA and ICP-AES
crystal present, we can see which energies of neutrons are most heavily
attenuated.
Figure 2.20: Neutron time of flight energy spectrum for an unfiltered neutron
beam (top) and a beam filtered with 15 cm of sapphire single
crystal (bottom).
Figure 2.20 shows the two spectra that were collected during this
experiment. It is clear from the two curves that the sapphire crystal
attenuates all neutron energies of the incident beam to some extent. A close
look at Figure 2.20 reveals that approximately 50% of the neutrons at 0.015
eV are attenuated, which does not agree with our earlier measurement (and
published data) of approximately 63% attenuation. This discrepancy could be
116
Chapter 2: Prompt Gamma Neutron Activation Analysis
Kent J. Riley
caused by the fact that both curves in Figure 2.20 include some sort of
background continuum that results from spurious arrival time events. These
events are due to fast neutrons that penetrate the cadmium wheel, even
when a slit and the collimator are not aligned. They may also arise from
detection events that are from the previous or subsequent neutron pulse,
instead of the neutron pulse that is currently gated. However, not all of the
area under the tails in Figure 2.20 may be associated with spurious events;
some are true events that should not be subtracted as part of the background.
It is therefore difficult to apply an accurate background correction to the
above spectra.
Nevertheless, even without a proper background correction, the data
can be evaluated to indicate useful, semi-qualitative information. The
unfiltered spectrum in Figure 2.20 peaks at an energy of approximately 0.029
eV. The MCA is collecting a differential distribution of events that are
indicative of the neutron flux as a function of speed; or the number of
neutrons per unit area and unit time within dv about speed v. However, the
detection efficiency for the BF 3 detector carries a 1/v dependence because the
thermal neutron cross section varies inversely with the speed of the neutron
being absorbed. The collected spectrum will therefore be proportional to the
number density distribution and not the flux distribution (4 = nv, so 4/v = n).
The Maxwellian distribution (for the number density, instead of flux as
shown in Equation 2.9) can be evaluated to show that the maximum will
117
Improved Boron 10 Quantification via PGNAA and ICP-AES
occur at an energy of 0.028 eV, for a system at a temperature of 50 'C
(average reflector temperature of the MITR-II). The slow neutron
distribution shown in Figure 2.20, therefore agrees with the expected slow
neutron distribution of the MITR-II reflector.
The attenuation behavior as a function of neutron energy can be
observed by plotting the ratio of the filtered spectrum to the unfiltered
spectrum versus neutron energy. The result is a plot of the transmitted
fraction of neutrons versus neutron energy, as shown in Figure 2.21.
Figure 2.21: Fraction of transmitted neutrons plotted versus neutron energy for a
15cm long sapphire single crystal filter. Transmission is greatest at
approximately 0.02 eV, in rough agreement with Figure 2.10.
118
Chapter 2: Prompt Gamma Neutron Activation Analysis
Kent J. Riley
Transmission clearly reaches a peak at approximately 0.02 eV, which
is in rough agreement with the attenuation coefficient for sapphire plotted in
Figure 2.10. The attenuation of neutrons at high energies shown in Figure
2.21 is not as great as predicted from calculations (Figure 2.10) and the dose
rate measurements performed earlier. This discrepancy may be due to errors
associated with poor counting statistics in channels away from the peak of
the spectrum (especially with the filtered spectrum), and due to the problem
with background correction mentioned earlier. Nevertheless, the trend in the
data clearly shows that neutrons of a particular energy (- 0.02 eV) are
preferentially transmitted.
119
Improved Boron 10 Quantification via PGNAA and ICP-AES
Figure 2.22: Comparison of measured attenuation coefficients versus data
published in the literature. Published values from (13).
Figure 2.22 shows a comparison of the measured attenuation
coefficients at each energy and the attenuation coefficients published in the
literature (13). In spite of the shortcomings of this analysis, the agreement is
quite reasonable. The measured data shows lower attenuation than expected
at high neutron energies, and an overestimate of the attenuation coefficient
in the region of 0.02 eV. Again, these discrepancies are likely due to the
background correction problem mentioned earlier. In spite of these
120
Chapter 2: Prompt Gamma Neutron Activation Analysis
Kent J. Riley
discrepancies, the general trend of the measured data is in agreement with
published data.
2.4.4 Conclusions
From the measurements described in this section, it is clear that the
15 cm long sapphire single crystal purchased from Union Carbide will
attenuate fast and epithermal neutrons by a factor of 100, and will attenuate
the photon dose rate by at least a factor of 10. The 15 cm crystal will result
in a loss of no more than about 37% of the incident slow neutron flux.
Due to the excellent performance of the sapphire single crystal, and
the fact that two rectangular bars could be cut from the large cylinder of
sapphire single crystal, it was deemed worthwhile to insert a total of 30 cm of
sapphire into the beamport at 4DH3 (as described in Section 2.2.3). While
the additional 15 cm of crystal would result in the loss of another 30-37% of
slow neutron flux, this loss would be more than made up for by the removal
of the collimator shims (which should result in an 80% increase in the slow
neutron flux). Meanwhile, the fast neutron flux would be reduced by another
factor of 100, and the photon dose rates would be reduced by another factor of
10. This would provide for a very clean neutron beam incident upon the
graphite diffraction crystals, relatively free of fast and epithermal neutron
and photon contamination. Several measurements were taken at 4DH3 after
the sapphire crystals had been installed to evaluate the performance of the
121
Improved Boron 10 Quantification via PGNAA and ICP-AES
30 cm sapphire filter. These measurements will be described and discussed
in Section 2.5.
2.5 Final Configuration and Performance
of Upgraded Facility
2.5.1 Sapphire Filter Crystal Measurements
Several measurements were taken after the two sapphire bars had
been inserted in the 4DH3 beamport. The photon dose rate in the beam
centerline at the port box was 10 R/hr (after only 1 day of reactor shutdown)
prior to inserting the sapphire crystal. After inserting one 15 cm section of
sapphire, the photon dose rate dropped to 150 mR/hr, a reduction in the
photon dose rate by nearly a factor of 70. The drastic reduction in the photon
dose rate indicated that the photon spectrum during reactor shutdown is
considerably softer than if they were 4 MeV photons, which would result in
only a factor of 10 reduction (see Table 2.2). Insertion of the second section of
sapphire crystal only reduced the photon dose rate to 100 mR/hr. The lower
photon attenuation of the second crystal indicates that the photon spectrum
seen by the second crystal is considerably harder than the photon spectrum
originating from the core, or that the remaining dose rate is comprised mostly
122
Chapter 2: Prompt Gamma Neutron Activation Analysis
Kent J. Riley
of activation gammas that do not travel in the path of the sapphire crystal.
Nevertheless, a net reduction of a factor of 100 in the photon dose rate is
reasonable and will considerably reduce the background dose rates in the
area.
Before restoring the shielding at 4DH3, the slow neutron flux was
measured at the face of the port box, using gold foils and the cadmium
difference technique (18. A similar measurement had been performed several
years ago by Frederique Lambert, while the collimator shims were still in
place (and without any sapphire filters in the beam) (19). Frederique reports
a peak flux in the beam of 8.4E+08 n/cm 2 sec, normalized to a reactor power
of 5.0 MW. Frederique also reports a cadmium ratio of approximately 7.7 for
the 4DH3 beamport. The measurement performed after the removal of the
collimator shims and the insertion of 30 cm of sapphire single crystal yielded
a flux of 9.8E+08 n/cm 2 sec, again normalized to 5.0 MW. It is worthwhile to
mention that the cadmium correction for the latter measurement proved to be
negligible. This is reasonable since the sapphire crystals should reduce the
epicadmium component of the beam by several decades. Unfortunately, the
cadmium ratio was not able to be accurately quantified, but it is believed to
be in excess of several thousand.
Recall that the removal of the collimator shims was expected to
increase the slow neutron flux by a factor of 1.8, while the 30 cm of sapphire
crystal is expected to attenuate approximately 50% of slow neutrons. We
therefore expect the latter measurement to yield a flux slightly lower than
123
Improved Boron 10 Quantification via PGNAA and ICP-AES
8.4 n/cm 2 sec, rather than the measured value, which is 16% higher. This
small discrepancy may arise from several factors. First, the removal of the
collimator shims may have increased the flux more than expected, or the
sapphire crystal may be attenuating fewer slow neutrons than expected.
Second, there may be systematic error in the measurement performed by
Frederique. She used a series of foils, arranged in a cross over the beam
aperture, some of which were cadmium covered. Her measurements do not
show the slow neutron flux to peak at the beam centerline, as one would
expect, but rather slightly below the centerline. This anomaly may be due to
slow neutron flux suppression if the cadmium covered foils were too closely
spaced, or improper cadmium correction, since the cadmium foils had a
different spatial location than the bare foils. The measurements performed
after the collimator removal and sapphire insertion were taken along the
beam centerline, with the bare and cadmium covered foils in the same
location, during two separate irradiations. Frederique's, measurement may
therefore indicate a flux that is lower than the actual flux value, explaining
the discrepancy mentioned earlier.
In spite of the small inconsistency of the slow neutron flux
measurement, it is clear that the removal of the collimator shims resulted in
an increase in the slow neutron flux by at least a factor of 1.8. Furthermore,
it is clear that the sapphire crystal has not reduced the slow neutron flux by
more than 50%. Measurements made during reactor shutdown indicate a
factor of 100 attenuation in the photon dose rate.
124
Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
The goniometer assembly (including the focusing monochromator) was
installed and the shielding surrounding 4DH3 was reassembled. The dose
rates surrounding the masonite shielding were then measured to compare
with the dose rates measured in that area prior to the insertion of the
sapphire crystals. Measurements of the photon dose rate, neutron dose rate
(using the TEPC), and fast and slow neutron count rates (using a lithium
scintillation detector and moderating sphere) were taken at several places
around the masonite shielding. Similar measurements were performed by
the Reactor Radiation Protection Office (RRPO) in 1991 (20).
Location
D
B
G
Thermal
Fast
Photon
Dose Rate Neutron CR Neutron CR
(cpm)
(com)
(mR/hr)
Fast Neutron
Dose Rate
(mR/hr)
130
7,000
30,000
350
9
100
7.5
100
8.5
N/A
3,500
N/A
5,000
N/A
4,500
8,500
50
10,000
1,000
N/A
70
N/A
95
N/A
Table 2.3: Comparison of dose rates and neutron count rates surrounding the
masonite shielding at 4DH3. Bold values indicate measurements
taken after the removal of the collimator shims and insertion of the
sapphire crystals. N/A indicates that no significant count rate or
dose rate could be measured.
Table 2.3 compares the dose rate measurements at a few locations,
before and after insertion of the sapphire crystals (values in bold). An entry
of N/A indicates that no significant dose rate or count rate could be
measured. Figure 2.23 shows the locations at which the measurements were
125
Improved Boron 10 Quantification via PGNAA and ICP-AES
taken. Measurements at positions other than those shown in the figure and
table were taken, but all measurements yield results similar to those shown.
Reactor Biological Shield
Path of
neutron beam
IvI nQnnifTl
LJ..
rV.LLuJuJ.J..
layers
Figure 2.23: Schematic depicting locations at which the dose rates shown in
Table 2.3 were measured.
The fast neutron dose rate at all positions has been reduced to
immeasurable levels. This is to be expected, since the 30 cm of sapphire
crystal should reduce the fast neutron flux by four decades or more. The
photon dose rate is also lower, by about a factor of 10, at nearly all of the
positions. This is consistent with the earlier measurements that were made
while the reactor was shut down. It is also interesting to note that the slow
neutron count rates are also lower, in spite of the fact that the slow neutron
flux incident upon the graphite crystals is essentially unchanged. The lower
count rates are likely due to the fact that boral plates were added to the
inside of the goniometer assembly to absorb slow neutrons that are not
126
Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
diffracted toward the sample. Lithium carbonate (95% 6Li) loaded paint was
added to surfaces that are exposed to the diffracted neutron beam, such as
the collimator plug and a few lead pieces near the goniometer. These
measures have helped reduce slow neutron background, and may have also
helped to reduce the photon dose rate by limiting the amount of activation
photons that are generated.
From the measurements described in this section, it is clear that the
30 cm of sapphire single crystal has served to greatly reduce the fast neutron
and photon contamination of the beam incident upon the graphite diffracting
crystals. The reduction of these undesired components has been achieved
with no net loss of slow neutron flux, due to the removal of the collimator
shims. The lower fast neutron and photon contamination should serve to
reduce the background events seen by the HPGE detector, which will be
discussed in Section 2.5.3.
2.5.2 Focusing Monochromator Measurements
After determining that the background dose rates around the masonite
shielding had significantly improved, measurements were made to evaluate
the performance of the focusing monochromator. A tilt and rock curve were
measured, using a low efficiency, pancake type fission counter, where the
collimated beam exits the masonite shielding.
The collimator is housed in a removable plug that fits into a notch cut
from two of the masonite layers. The collimator is cone shaped, with a
127
Improved Boron 10 Quantification via PGNAA and ICP-AES
diameter of 1.875" at the end nearest the masonite, and a diameter of 0.875"
at the end nearest the sample. The collimator is composed of a Teflon box,
with a thin sheet of Teflon forming the cone shape down the center. The
entire box is filled with 95% 6Li enriched lithium carbonate.
To measure the tilt and rock curves, a sheet of cadmium, containing a
1 mm slit, was wrapped over the small end of the cone shaped collimator.
The slit was oriented vertically for measurement of the rock curve, and
horizontally for the measurement of the tilt curve.
Rock Curve for Focusing Monochromator at 4DH3
7000
6000
5000
C
w 4000
3000
o
2000
1000
0
-8
-6
-4
-2
0
2
4
6
8
Nominal Angle Setting (degrees)
Figure 2.24: Rocking curve for focusing monochromator, measured at the 4DH3
beamport.
128
Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
Figure 2.24 shows the rock curve measured at the 4DH3 beamport.
The curve is approximately symmetric about the peak, indicating that all of
the layers are relatively evenly spaced. The FWHM of the curve is indicative
of the effective mosaic spread for the crystals, since the angular spreading
effect of collimation of the detector and the neutron beam incident upon the
crystals (see Equation 2.7) is small and can be ignored. The FWHM of the
rock curve in Figure 2.24 is 2.40, less than the expected 3.00 - 3.50 (from the
alignment procedure). Nevertheless, as subsequent measurements were able
to demonstrate, the effective mosaic is large enough to achieve a significant
gain in flux via the focusing of the diffracted neutron beam.
129
Improved Boron 10 Quantification via PGNAA and ICP-AES
Tilt Curve for Focusing Monochromator at 4DH3
6000.0
5500.0
5000.0
4500.0
M 4000.0
3500.0
o
Q 3000.0
2500.0
2000.0
1500.0
-40
-20
0
20
40
60
80
100
Nominal Tilt Angle (minutes)
Figure 2.25: Tilt curve for the focusing monochromator, measured at the 4DH3
beamport.
The tilt curve measured at the 4DH3 beamport is shown in Figure
2.25. This curve is slightly asymmetric, however the curve is fairly smooth,
and demonstrates a clear maximum. This indicates that the vertical
alignment of the graphite strips is reasonably uniform. The FWHM of this
curve is approximately 80 minutes (1.330), which is consistent with the 1.20
and 1.40 mosaic of the pieces that form the focusing monochromator. This
agreement indicates that the vertical alignment of all the pieces is quite
good.
130
Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
The true test of how well the focusing monochromator performs is a
measurement of the slow neutron flux at the sample position. This
measurement was carried out via gold foil activation techniques, this time
omitting the cadmium correction since it is known to be negligible. This
measurement revealed that the flux at the sample position is approximately
1.7E+07 n/cm 2 sec, compared to a flux of 6.OE+6 n/cm 2 sec prior to the
modifications described in this chapter. The slow neutron flux at the sample
position has therefore increased by a factor of slightly less than 3. This
increase in flux is not, however, due entirely to the focusing monochromator.
Recall that the removal of the collimator shims resulted in a factor of 1.8
increase in the slow neutron flux, while the additional 15 cm of sapphire
crystal (the original configuration contained 15 cm of sapphire, the current
configuration contains 30 cm of sapphire) results in a 29% - 37% attenuation
of the slow neutron flux (depending on whether the published data or the
data measured in this thesis is used). The net flux increase that is due to the
focusing monochromator is therefore approximately a factor of 2.2 - 2.5.
Table 2.4 summarizes the components that have affected the slow neutron
flux, and shows their relative contribution.
Component
Collimator Removal
Additional Sapphire
Focusing Monochromator
Net Flux Increase
Effect on Flux
1.8
0.63 - 0.71
2.20 - 2.50
- 2.84
131
Improved Boron 10 Quantification via PGNAA and ICP-AES
Table 2.4: Contribution factors to the slow neutron flux at the sample position at
4DH3. The net flux increase is due to both the removal of the
collimator shims and the effect of the focusing monochromator.
The net flux increase due to the focusing monochromator is therefore
within the factor of 2-3 that had been estimated from Equation 2.8.
2.5.3 Background Measurements
The measurements described in the preceding sections have
demonstrated that the described modifications have had the desired impact
on the parameters of the 4DH3 neutron beam. The remainder of the PGNAA
facility was then restored to determine more direct prompt gamma neutron
activation analysis performance parameters, namely the background count
rates and the system sensitivity. The HPGE detector was positioned
perpendicular to the neutron beam, at a distance of 40" from the diffraction
crystals (the focal spot). The lithium carbonate and lead shielding around
the detector were replaced, and the lithium cage to line the neutron beam
path was added. The lithium cage is constructed of Teflon walls, the inside of
which contain 6Li enriched lithium carbonate. Several walls have been
constructed so that the collimated neutron beam enters into a "lithium box".
Any neutrons that enter the box must therefore encounter a 1 cm layer of
lithium-6 carbonate (where they are likely to be absorbed, transmission =
0.0025% for 0.015 eV neutrons in 1 cm of 6LiCO 3 ) before creating unwanted
activation in the surrounding shielding.
132
Chapter 2: Prompt Gamma Neutron Activation Analysis
Kent J. Riley
Tangential Neutron Beam
from D20 reflector
-" Masor
D
High I
ters
tal)
Lithiau
Sapph
*
Lead
"Lithium Box" \
Sample location
for vertical focusing
Intrinsic
n-type
Ge detector
Figure 2.26: Schematic depicting the layout of the PGNAA after the
modifications described in this thesis. The significant changes are
the removal of the collimator shims, the addition of the focusing
monochromator, and the addition of the lithium cage around the
sample position.
Figure 2.26 shows a schematic depicting the PGNAA facility after the
completion of all the modifications that have been described in this chapter.
The important differences are the removal of the upstream collimator shims,
the addition of the vertical focusing enhanced mosaic graphite crystals, the
addition of the lithium cage around the sample position, and the addition of
30 cm of sapphire crystal on the reactor side of the monochromator.
133
Improved Boron 10 Quantification via PGNAA and ICP-AES
The measurements described in Section 2.1.2 were then repeated to
determine the extent to which each of the background components had been
lowered. Table 2.5 summarizes these measurements and compares the
background components before and after the modifications described in this
chapter.
Component
Before (cps) After (cps)
Area Background
690
< 100
Beam Spreading
1110
1180
Upstream Interactions
1580
900
Sample Interactions
2680
6820
TOTAL
6060
9000
Table 2.5: Summary of background components seen by the detector at 4DH3
before and after the modifications described in this chapter. A 0.5
ml deionized water sample was used to determine the sample
interaction component.
The gross integral count rate seen by the detector under the current
configuration at 4DH3 is actually higher than that of the previous
configuration. But the increase is much less than the factor of 3 increase in
the flux at the sample position. In particular, the area background count
rate and the upstream interaction count rates are lower than before, while
the beam spreading component is essentially unchanged, and the sample
interaction component is considerably higher, as expected because of the
higher flux.
134
Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
The lower area background count rate is due to a more thorough lead
shielding arrangement, so that the detector no longer has lines of sight to
other sources of radiation on the reactor floor.
The lower upstream interaction count rate is due to the insertion of the
30 cm of sapphire crystal. It is, however, puzzling that the upstream
interaction rate is only a factor of 2 lower, whereas the photon dose rates
measured outside the masonite shielding are uniformly a factor of 10 lower.
This discrepancy may be due to the fact that the increased slow neutron flux
that is diffracted toward the sample position produces more activation
photons that can reach the HPGE detector. This effect may be exacerbated
by the fact that the sapphire crystal is no longer downstream of the
diffracting crystals to help eliminate activation photons.
The beam spreading component is essentially unchanged, which
indicates that the background arising from beam spreading is not due to
neutrons (because they are now absorbed by the lithium liners). Instead,
beam spreading is likely due to photon contamination of the neutron beam
(by the activation photons discussed in the previous paragraph). Without
further collimating the neutron beam or the HPGE detector (both of which
are already collimated), there is little that can be done to control this
component.
The remaining component, sample interactions, has become the
dominant background component. From Table 2.5 we can see that the
sample interaction component increased by a factor of 2.5, slightly less than
135
Improved Boron 10 Quantification via PGNAA and ICP-AES
the increase in the slow neutron flux (2.8). The fact that the lithium cage did
not entirely eliminate the background due to sample interactions indicates
that the majority of this component is not due to scattered neutrons
interacting in the surrounding shielding, but slow neutron interactions
occurring in the sample itself. The lithium cage does, however, reduce the
background count rate. The sample interaction component jumps to
approximately 9000 cps when the lithium cage is removed.
Slow neutron absorption in the sample is desirable, but it is clear from
these data that such interactions are now also the dominant source of
background in our analysis system. Photons generated in the sample can
undergo many subsequent interactions, but the only subsequent interaction
that is desirable is full energy deposition in the HPGE detector. Such an
interaction is rare because the Compton cross section for germanium is
dominant from 150 keV up to several MeV. This means that photons in the
energy range of interest (usually a few hundred keV to a few MeV) will
undergo primarily Compton scattering events. The only way that such
photons can deposit all of their energy is to undergo multiple Compton
scatters, until they scatter into an energy regime where the photoelectric
effect is dominant. The probability of multiple Compton scatters inside the
HPGE detector is low because the detector is very small (- 100 ml active
volume). The only way to reduce undesirable Compton interactions is to
implement a Compton suppression system to reject Compton scattering
events that occur in the detector. A suppression system would employ an
136
Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
additional detector (usually NaI) to surround the HPGE detector.
Coincidence gating would be employed to reject events in the HPGE detector
if an event is detected in the NaI detector within a certain window of time.
Such a system may be difficult to employ in high count rate systems and can
lead to the loss of useful signal due to chance coincidence events. A Compton
suppression system was not implemented as part of this thesis.
In spite of the increase in the gross integral count rate, two of the
background components at the PGNAA facility are lower than before the
modifications described here. Even with the higher sample interaction
background, the fast electronics employed at the PGNAA facility are able to
limit system dead time to approximately 10% for a typical 0.5 ml blood
sample with 100 ppm or less of 10B. Most importantly, the background count
rate under the boron peak has shown an increase of only about 50% (80 cps
prior to modification, 120 cps after modification). Recall from Equation 2.3
that the detection limit for a system increases with the square root of the
background count rate under the peak, but decreases linearly with the
sensitivity. The modest increase in the background count rate is more than
offset by the increase in slow neutron flux (sensitivity), resulting in a net
improvement of the detection limit by about a factor of 2.5. Furthermore, it
is now clear that the background seen by the detector at 4DH3 is due mostly
to interactions that occur in the sample itself, and not the surrounding
shielding. To significantly reduce the background, clever detection or
137
Improved Boron 10 Quantification via PGNAA and ICP-AES
shielding schemes must be employed, such as the Compton suppression
technique mentioned earlier.
2.5.4 Sensitivity and Detection Limit Measurements
The sensitivity of the PGNAA system was measured for a variety of
isotopes. Table 2.6 shows the measured sensitivities, normalized to a reactor
power of 5.0 MW, and compares them with the sensitivities measured before
the modifications described in this chapter.
H
Photon
Energy
(keV)
2223.0
Old
Sensitivity
(cps/mg)
0.184
B
478.4
1328
Gd
182.7
943.5
558.7
651.0
231.0
278.0
333.9
439.4
517.0
1165.4
162.4
273.3
1819
199
446
79
4.2
3.6
1177
546
3.1
2.2
9.1
6.9
Element
Cd
Co
Sm
Cl
In
New
Increase in
Sensitivity
Sensitivity
(cps/mg)
0.473
2.57
3750
4910
491
1160
205
11.3
9.80
2800
1320
8.40
5.70
22.8
17.6
AVERAGE
2.82
2.70
2.47
2.60
2.59
2.69
2.72
2.38
2.42
2.71
2.59
2.51
2.55
2.59
Table 2.6: Measured sensitivities for various elements (all with natural isotopic
abundances) for the PGNAA system before and after the
modifications described in this thesis. The sensitivity has been
increased on average by a factor of about 2.6.
138
Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
The sensitivities for all of the elements have increased by about a
factor of 2.4 - 2.8. This increase is reasonable since the slow neutron flux at
the sample position increased by a factor of 2.8, and the detection efficiency
for the HPGE detector is unchanged (the sample to detector distance is the
same as before the modifications). The sensitivities listed above can be
improved by moving the detector closer (up to only 1 cm away from the
sample) but only at a cost of increased background count rates and system
dead time. The current detector position of 2 cm has been chosen to keep
system dead time and background count rates at a reasonable level, while
still enjoying the benefits of the improved sensitivity.
The improved sensitivity of the PGNAA facility also means that
samples can be quantified to within a certain level of uncertainty more
rapidly. The figure below shows the estimated counting time required to
achieve 10% statistical uncertainty in the boron peak for a range of sample
concentrations and various sample sizes. The curves in the figure have been
calculated from a formula relating the sensitivity, the background count rate,
and the boron content of the sample to the time required to reach 10%
statistical uncertainty, as given in the formula below.
2B + SC
Equation 2.11
t
= 1.67 S2 C 2
In the above equation, t is the time to reach 10% statistical
uncertainty (in minutes), B is the background count rate under the boron 10
139
Improved Boron 10 Quantification via PGNAA and ICP-AES
peak (cps), S is the sensitivity of the facility (cps/gg), and C is the amount of
10
B present in the sample (gg). Equation 2.11 was arrived at by setting the
uncertainty in the area under the peak ( 2Bt + SCt ) equal to 10% of the area
under the peak (CSt), and solving the relationship for t. The curves shown
below have been calculated using the sensitivity shown in Table 2.6 and the
background count rate under the boron peak of 120 cps.
Estimated Counting Times Required to Reach 10% Uncertainty
4.1
IU
103
102
101
D 100
E
1 01
10-2
103
100
101
102
Concentration (ppm)
Figure 2.27: Estimated required counting times to reach 10% statistical
uncertainty in the net area under the boron peak for various
concentrations and sample sizes.
From Figure 2.27 it is clear that the PGNAA facility can be used to
quantify fairly large samples (0.5 ml) in a few minutes, even at
140
Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
concentrations of only a few ppm. On the other hand, samples as small as
0.02 ml can be quantified in about an hour or less, if their concentration is 10
ppm or greater.
Required Counting Times Before and After 4DH3 Modifications
A ,-3
IU
102
101
1 02
100
101
102
Concentration (ppm)
Figure 2.28: Comparison of counting time required to reach 10% statistical
uncertainty in the boron peak. The dashed curve represents the
facility after modification and solid lines represent the facility before
modification. Each curve is for a sample of 0.1 ml.
Figure 2.28 compares the estimated counting times required to reach
10% statistical uncertainty for 0.1 ml samples before and after the
modifications to the PGNAA facility. It is clear from Figure 2.28 that the
PGNAA facility is now much more rapid than before the modifications.
According to Equation 2.11, the time required to reach a certain level of
141
Improved Boron 10 Quantification via PGNAA and ICP-AES
uncertainty is dependent upon the background count rate. For the PGNAA
facility, the background count rate is not negligible, and therefore the
improvement in counting time will vary as a function of sample concentration
and sample size. For example, at high concentrations and large sample sizes,
the count rate due to boron may be quite high (-1000 cps for a 200 ppm 0.5
ml sample) compared to the background count rate (120 cps). The
improvement in count time in this region will be approximately a factor of 3
(proportional to the increase in sensitivity). In regions where the background
count rate is dominant, the improvement in counting times will behave more
like the square of the increase in sensitivity, due to the denominator in
Equation 2.11. From Figure 2.28 we see that on average, the counting times
have been reduced by about a factor of 5.
With values of the sensitivity and the background count rate, we can
use Equation 2.3 to calculate the detection limit for the PGNAA facility.
Using a count time of 60 seconds, (for purposes of comparison with the ICPAES - see Chapter 3) a sensitivity of 0.25 gg for
10B
is achieved for the
PGNAA facility. Longer counting times will result in lower detection limits,
but as shown in Equation 2.3, the detection limit varies inversely as the
square root of the count time.
The modifications to the PGNAA facility described in this chapter have
resulted in a threefold increase in the sensitivity of the PGNAA facility.
Although the gross integral background count rate of the improved PGNAA
142
Chapter 2: Prompt Gamma Neutron Activation Analysis
Kent J. Riley
facility is higher than the preceding configuration, the background count rate
is now dominated by interactions that occur in the sample. Furthermore, the
system dead time remains reasonable at 10%, and the background count rate
under the boron peak increased by only 50%. The threefold increase in
sensitivity combined with the increase in the boron background results in a
net improvement of the detection limit by about a factor of 2.5. The improved
PGNAA facility can quantify 0.5 ml samples of nearly any concentration of
interest for BNCT very rapidly, and can quantify samples as small as 0.02 ml
with concentrations as low as 10 ppm in less than an hour. The improved
PGNAA facility at the MITR-II is the most sensitive facility of its kind,
exceeding the sensitivity of other facilities by nearly an order of magnitude
(see Table 1.1). In Chapter 4, the PGNAA facility at the MITR-II will be
evaluated and compared to the ICP-AES facility at MIT, as well as other
PGNAA facilities around the world.
143
Improved Boron 10 Quantification via PGNAA and ICP-AES
2.6 References
1.
"Use of Cyclotron-Produced Fast Neutrons in Activation Analysis," I.
Olmez, G. S. Kowalczyk, G. H. Harrison, J. Radioanal. Nucl. Chem.,
Letters, Vol. 94:6, 1985, pp. 391-398
2. Design and Construction of a Prompt Gamma Activation Analysis Facility
and Improvement of the On-Line Beam Monitor System for the
Medical
Beam
at the MITR-II,
J-M.
Chabeuf, M.
S.
Thesis,
Massachusetts Institute of Technology, 1993
3.
Operating
Manual
for
the MIT
Reactor
(MITR-II),
MITR
Staff,
Massachusetts Institute of Technology, Cambridge, MA, June 1973.
4. B. M. Rustad, J. Als-Nielsen, A. Bahnsen, C. J. Christensen, A. Nielsen,
"Single Crystal Filters for Attenuating Epithermal Neutrons and
Gamma Rays in Reactor Beams," The Review of Scientific Instruments,
Vol. 36:1, January 1965, pp. 4 8 -5 4 .
5.
W.
Dienst,
"Mechanical
Properties
of neutron-irradiated
ceramic
materials," Journalof NuclearMaterials,Vol. 211, 1994, pp. 186-193.
144
Kent J. Riley
Chapter 2: Prompt Gamma Neutron Activation Analysis
6. Parker O-ring Handbook, Parker Seal Group, O-ring Division, Lexington,
KY, 1992, pp. A2-13.
7. Reactor Radiation Protection Office (RRPO) measurements performed on
3/29/76.
Report is on file in the RRPO office, under 6SH4
measurements.
8. Mechanics of Materials, F. P. Beer, E. R. Johnston Jr., McGraw Hill, 1992,
pp. 377-379.
9. Strum, "Computing Strength of Vessels Subjected to External Pressure,"
Transactionsof the American Society of Mechanical Engineers, Volume
69, 1947.
10. Several personal communications with Prof. Anthony Nunes, University
of Rhode Island Physics Department helped clarify these relationships.
A suitable reference text on optics should also be sufficient.
See:
Optics Second Edition, E. Hecht, Addison-Wesley Publishing Co.,
Reading, MA, 1990.
11. Personal communication with Professor Anthony Nunes, 6/25/96.
12. Personal communication with Professor Anthony Nunes, 5/23/96.
145
Improved Boron 10 Quantification via PGNAA and ICP-AES
13. H. F. Nieman, D. C. Tennant, and G. Dolling, "Single Crystal Filters for
Neutron Spectrometry," Review of Scientific Instruments, Vol. 51 (10),
October 1980, pp. 1299-1303.
14. Nuclear Reactor Analysis, J. Duderstadt, L. Hamilton, John Wiley and
Sons, New York, 1976, pp. 378.
15. Introduction to Nuclear Engineering 2nd Edition, J. R. Lamarsh, AddisonWesley Publishing Co., Reading, MA, 1983, pp. 13.
16. Neutron Diffraction, G.E. Bacon, Clarendon Press, Oxford, 1975, pp. 52.
17. Properties of Sapphire, Union Carbide, Crystal Products Division, 1993.
18. Radiation Detection and Measurement, G.F. Knoll, John Wiley and Sons,
New York, 1989, pp. 703 - 711.
19. "Design and Construction of the Second Version of the Prompt Gamma
Neutron Activation Analysis Facility at the MIT Nuclear Reactor," F.
Lambert, Special
Internship
Report to Professor
Otto Harling,
September 1991.
20. RRPO radiation survey record of 4DH3, 8/21/91, on file in RRPO office.
146
CHAPTER THREE
Inductively Coupled
Plasma Atomic Emission
Spectroscopy
3. O0 Figures of Merit
To evaluate the performance of the ICP-AES system, and to compare
its performance with the PGNAA facility, it is again useful to describe
several figures of merit. These parameters will be similar to those outlined
for the PGNAA facility, but their interpretation and significance may be
different due to differences in the two techniques.
147
Improved Boron 10 Quantification via PGNAA and ICP-AES
3.0.1 Sensitivity
For the PGNAA facility, we described the sensitivity of the machine as
the number of counts per second under the 10 B peak, per microgram of 10 B
that was placed in the neutron beam. For the ICP-AES facility, this
parameter is somewhat more difficult to define because not all of the material
that is input to the machine is actually counted by the spectrometer. A
considerable amount of the sample must be used to prime the tubing and the
nebulizer before analysis can begin. Furthermore, only a small fraction of
the spray from the nebulizer is actually transported to the plasma for
counting by the spectrometer. Another complication is due to the fact that
the time to perform an analysis is dependent largely on the time it takes to
prime the system, and not the time it takes for the spectrometer to achieve
good counting statistics. For example, it may take only 3 seconds for the
spectrometer to achieve better than 5% uncertainty in the area under the
boron peak, but it may take 40 - 60 seconds to thoroughly prime the dead
volume of the tubing.
For these reasons, it is difficult to assess the sensitivity of the ICPAES machine so that it will reflect the operating characteristics that are
most important to us. This thesis will therefore refer to an effective
sensitivity, which is the number of counts collected in the spectrometer
divided by the counting time (not including the time required for priming),
and divided by the number of micrograms of boron that is input to the
148
Chapter 3: Inductively Coupled Plasma Atomic Emission Spectroscopy
Kent J. Riley
machine (not the amount that is actually present in the plasma). The true
sensitivity of the ICP-AES machine will be much higher since only a fraction
of the sample actually reaches the plasma. Unless stated otherwise in this
thesis, sensitivity will be understood to mean the effective sensitivity and not
the true sensitivity.
3.0.2 Limit of Detection
The detection limit for the ICP-AES facility can be defined in the same
fashion as the detection limit for the PGNAA facility, as given in Equation
2.3. In this thesis, the effective sensitivity will be used to calculate limits of
detection, so that sample consumption is accurately accounted for.
3.0.3 Background Count Rates
The background count rate for the ICP-AES has a definition similar to
that for the PGNAA facility. The boron background will again be defined as
the area under the boron peak, divided by the count time (not including
priming time). The notion of a gross integral count rate is not relevant for
the ICP-AES since the machine is usually programmed to examine only a
certain band of wavelength and discard information from other wavelengths.
The system dead time therefore does not become an issue unless samples of
extremely high concentrations will be counted. For the work described in
149
Improved Boron 10 Quantification via PGNAA and ICP-AES
this thesis, sample concentrations will be sufficiently low (< 100 ppm) so as
not to warrant concern.
3.1 Routine Analysis with Cross Flow
Nebulizer
The basic operating characteristics of the ICP-AES technique have
been described in Section 1.2.2. A key component in the AES analysis
technique is the sample nebulizer. The nebulizer sprays the sample into a
fine mist so that the sample can be easily incorporated by the plasma and
inductively excited. The spray characteristics of the nebulizer will be
important in determining how much of the sample actually reaches the
nebulizer, which will, in turn, determine how much sample must be
consumed to achieve a given signal. A cross flow nebulizer forms the sample
mist by injecting a high pressure (100 psi) stream of argon gas perpendicular
to the flow path of the sample. The highly turbulent flow of the argon gas
mixes with the steady, laminar flow (- 1 ml/min in -0.9 mm ID tubing) of the
sample and ejects a fine argon - sample mist into the spray chamber. The
following sections describe the operating characteristics that were achieved
with the cross flow nebulizer during this research.
150
Chapter 3: Inductively Coupled Plasma Atomic Emission Spectroscopy
Kent J. Riley
3.1.1 Sample Preparation
As mentioned in Chapter 1, the ICP-AES facility requires samples to
be in a liquid matrix so that the sample can be introduced to the nebulizer
via the peristaltic pump. The inner diameter of the input tubing to the
nebulizer is approximately 0.5 mm, so it is important to also filter the sample
if any precipitate has formed during the preparation so that the nebulizer
does not clog.
One must also use a set of carefully prepared standards to perform
accurate boron assays with the ICP-AES. The degree to which sample atoms
are excited by the plasma depends on the chemical matrix that they are
suspended in. If the standards are not prepared in a fashion identical to that
of the samples, chemical matrix effects may cause significant systematic
error. Other important considerations during sample preparation are; the
amount by which the sample will be diluted, the boron contamination of any
chemicals that are being used, and contamination via borosilicate glassware.
The degree to which each of these concerns will be important will depend
heavily on the initial concentration of the sample. It is also important to
remember that for analysis with the cross flow nebulizer, at least 3 ml of
prepared sample is necessary. The High Efficiency Nebulizer (HEN) can
reduce the amount of prepared sample that is required for analysis. The HEN
will be discussed in Section 3.2.
151
Improved Boron 10 Quantification via PGNAA and ICP-AES
Blood samples from human and animal subjects are too viscous (and
often too low in volume) to run directly through the cross flow nebulizer. To
thin the samples and to deactivate any hazardous viral components of the
blood, 1 ml of the detergent Triton-X 100 (2.5%) is added to 1 ml of blood
sample. This slurry is then diluted to a certain volume (usually 10 ml to
permit re-analysis of the sample if necessary) with deionized water. A
mother solution of 2.5% Triton-X 100 is prepared by diluting the detergent
with an appropriate amount of deionized water. The boron contamination of
Triton-X 100 is low, as no boron contamination can be detected with the ICPAES in the 2.5% mother solution. The detection limit for the ICP-AES, using
the cross flow nebulizer is approximately 15 ppb (see Section 3.1.2). Triton-X
100 does, however, cause significant matrix effects. Standards must
therefore be prepared with amounts of Triton-X that are equal to that
contained in the sample. Typically 1 ml of blood is combined with 1 ml of
Triton-X 100 (2.5%) and 8 ml of deionized water, therefore diluting the boron
in the blood sample by a factor of 10. Blood samples of 1 - 100 ppm are
therefore contained in a slurry that contains 0.1 - 10 ppm of boron. As
Section 3.1.2 will demonstrate, these concentrations are still well within the
detection limits of the machine. Sample preparation is usually performed in
polyethylene bottles, or plastic flasks to avoid any contamination from
borosilicate glass.
Tissue samples must be dissolved (digested) into a liquid matrix before
they can be analyzed. Equal amounts of tissue and nitric acid are added to
152
Chapter 3: Inductively Coupled Plasma Atomic Emission Spectroscopy
Kent J. Riley
an Erlenmeyer flask, and the mixture is heated over a burner for
approximately 30 minutes. The Erlenmeyer flask should be tall so that the
neck can remain cool to prevent fumes (and possibly boron) from escaping. A
cooling column can be attached to the flask if necessary. The mixture will
first turn black and opaque and then gradually change to a translucent
yellow color as the digestion is completed. Once the mixture is clear and
yellow, the digestion is complete. The mixture is then removed from the heat
and poured into a volumetric flask. The Erlenmeyer flask is repeatedly
rinsed with deionized water and the rinse is poured into the volumetric flask,
until the volumetric mark is reached. The rinse is intended to capture
residual liquid that is contained on the side of the flask. Deionized water can
be added directly to the volumetric flask to bring the mixture to the desired
volume. After bringing the sample to the appropriate volume, the sample
may require filtering as small precipitates occasionally form after cooling.
The formation of these precipitates does not alter the quantification results
as they contain very little or no boron.
Nitric acid typically contains boron contamination on the order of a few
tens of ppb (for high grade reagents). These contamination levels will not be
insurmountable as long as the concentration of the original sample is a few
tenths of a ppm or higher. If borosilicate glassware is used, the samples
should not be allowed to sit in the glass for extended periods of time. Once
the sample is prepared, it can be transferred to a plastic or polyethylene
container. If these measures are followed, contamination levels that result
153
Improved Boron 10 Quantification via PGNAA and ICP-AES
will typically be less than the contamination of the nitric acid. Both of these
contamination sources can be accurately eliminated during calibration as
long as the standards are prepared in the appropriate fashion. The final
volume of the prepared sample will be dependent on how thoroughly the
rinse procedure is performed. Typically 50 ml of rinse is more than sufficient
to capture all of the residual material, which will result in a dilution factor of
50 (or less if a larger sample is used). For samples with initial concentrations
of 1 - 100 ppm, the concentration of the prepared samples will range from
0.02 - 2 ppm (or higher for less dilution or larger samples). This range is still
within the operating limits of the ICP-AES, though the low end of the range
is beginning to approach the detection limit for the machine.
Sample preparation can be carried out in any convenient fashion as
long as the following conditions are met:
*
Boron contamination is insignificant.
*
Sample dilution is not too great.
*
The resultant liquid is low viscosity and particulate free.
*
Standards are prepared in a manner consistent with the sample
preparation.
The techniques described in the preceding paragraphs are those used
by the MIT/BIDMC project, but are by no means the only alternatives. One
may use a microwaveable container that can withstand the appropriate
pressure (a microwaveable bomb) instead of an Erlenmeyer flask and a
burner to heat the sample. This would reduce the heating time to only 5
154
Chapter 3: Inductively Coupled Plasma Atomic Emission Spectroscopy
Kent J. Riley
minutes and may result in lower rinsing volumes if an appropriately small
container is used.
3.1.2 Figures of Merit
This section will describe the performance of the ICP-AES in terms of
the figures of merit that were outlined in Section 3.0. From the information
in this section, it will become clear that ICP-AES is a very sensitive
instrument. There are, however, several disadvantages of using the ICP-AES
that cannot be reflected in the figures of merit presented here. These
disadvantages will be discussed and compared to the PGNAA facility in
Chapter 4.
3.1.2.1 Background Count Rate
The background count rate of the ICP-AES facility is dependent on the
operating conditions of the machine. Making sure that the system is
adequately flushed with pure deionized water and that the proper pressure of
Argon gas is being supplied to the nebulizer will help insure that the system
records the lowest possible background count rate. Adequate flushing of the
system is crucial since boron exhibits what is known as the "memory effect."
After a sample containing boron has been analyzed, a signal from boron can
persist, even after flushing the system for several minutes. The higher the
concentration of boron, the longer the signal will persist. It is therefore
155
Improved Boron 10 Quantification via PGNAA and ICP-AES
important to insure that the system has been adequately flushed between
samples. For the samples that are typically analyzed by our group, (0.1 - 10
ppm after dilution) a flush time of 60 seconds is sufficient to thoroughly rinse
the nebulizer and spray chamber.
After adequate flushing, the ICP-AES system usually records a net
background area of approximately 900 counts for a 3 second count interval
(-300 cps), using the 249.773 nm emission line of boron (all measurements
reported in thesis have been carried out using the 249.773 nm line of boron).
If the system has just been used to analyze a sample with very high boron
content (a few thousand ppm), the background count rate may measure
several tens of thousands of counts per second, even after flushing the system
for several minutes. To help alleviate this condition, the plasma torch and
the spray chamber can be removed, rinsed thoroughly with deionized water,
dried and replaced.
3.1.2.2 Sensitivity
As discussed earlier, the peristaltic pump must be primed with the
liquid sample and a steady flow of the sample must be maintained during the
measurement. A typical analysis results in the consumption of about 3 ml of
prepared sample, requiring approximately 60 seconds. Of that 60 seconds,
only 3 seconds is spent collecting photons from the sample, the remainder of
the time is spent flushing and priming the system. A sample that is 1 ppm in
concentration (after preparation) typically results in a recorded intensity of
156
Chapter 3: Inductively Coupled Plasma Atomic Emission Spectroscopy
Kent J. Riley
8100 counts over the 3 second count interval. After subtracting the
background counts from this number (900) and dividing by the count time,
we find a count rate of 2400 cps. To obtain the sensitivity, we then divide
this number by the number of micrograms of boron consumed, in this case 3
gg. Doing so results in a sensitivity of 800 counts per second per microgram.
These measurements were performed with the peristaltic pump set at speed
2, using pump tubing with an inner diameter of 0.89 mm, which corresponds
to a flow rate of approximately 2 ml/min. From this information, it is clear
that the ICP-AES is an extremely sensitive instrument, nearly a factor of 50
more sensitive than the PGNAA facility. Nevertheless, there are
disadvantages with the ICP-AES technique that make the PGNAA technique
preferable in some circumstances. These disadvantages will be discussed in
Chapter 4.
3.1.2.3 Detection Limits
The boron background count rate of the ICP-AES facility is higher
than that of the PGNAA facility by nearly a factor of 3, but the ICP-AES
facility is more sensitive than the PGNAA facility by almost a factor of 50.
We therefore expect the ICP-AES facility to have superior detection limits.
Using the formula given in Equation 2.3 , the parameters given in the
preceding sections, and assuming a count time of 3 seconds, we can calculate
a detection limit, expressed in micrograms. Doing so yields a value of 0.041
157
Improved Boron 10 Quantification via PGNAA and ICP-AES
Rg (or a minimum concentration of 0.014 ppm if the input volume is 3 ml).
This value can be compared to the 0.25 jig sensitivity of the PGNAA facility,
using a count time of 60 seconds (a count time of 60 seconds for the PGNAA
facility seems reasonable since there is no time associated with priming and
flushing the nebulizer). The ICP-AES therefore has a detection limit that is
more than a factor of 5 better than that of the PGNAA facility.
3.1.3 Cross Calibration with the PGNAA Facility
As a means of validating both the PGNAA and ICP-AES quantification
techniques, the facilities were cross calibrated against each other, and
several blood samples from human subjects were measured with each facility
to insure that accurate results were obtained.
For the PGNAA facility, standards were prepared from NIST Standard
Reference Material granular boric acid in
10 B
concentrations ranging from 5
to 50 ppm (total boron concentrations therefore range from 25 - 500 ppm).
The procedure detailing the preparation of standards for the PGNAA facility
has been presented elsewhere (1). The standards are prepared with an
appropriate amount of saline solution to accurately reflect any possible
interference of the 472.3 keV photon from sodium. In actuality, the signal
from sodium is too small to interfere even with the lowest boron
concentrations. The sodium absorption cross section is 0.4 b (2), while the
highest sodium content in the samples typically analyzed by our group is
158
Kent J. Riley
Chapter 3: Inductively Coupled Plasma Atomic Emission Spectroscopy
-2000 ppm (which is in whole blood). The boron absorption cross section at
0.015 eV approximately 4900 barns, more than four orders of magnitude
higher than sodium. Therefore, the signal from only 1 ppm of boron will be
an order of magnitude higher than the signal from sodium.
Standards for ICP-AES analysis were also prepared from the Standard
Reference Material, independently of the PGNAA standards. Standards were
prepared with total boron concentrations ranging from 5 - 50 ppm (ICP-AES
is not isotope specific and therefore quantifies both 10B and
11B).
The
standards were mixed with appropriate amounts of Triton-X 100 and diluted
by a factor of 10 with deionized water (as were all of the blood samples). The
diluted standard samples ranged from 0.5 - 5 ppm, representing undiluted
samples of 5 - 50 ppm.
Figure 3.1 below shows the calibration curves for the PGNAA and ICPAES facilities. The abscissa denotes boron concentrations (10B concentrations
for the PGNAA curve) for the undiluted standards, while the ordinate
denotes the intensity of the 249.773 nm line from boron (on the right) and the
ratio of the count rate under the boron peak to the count rate under the 2.2
MeV hydrogen photon peak (on the left). The boron to hydrogen count rate
ratio (B/H ratio) is used to normalize effects that may interfere with accurate
boron quantification, such as geometry or volumetric effects, and small
fluctuations in the neutron flux at the sample position. The B/H ratio for a
sample of a given concentration will remain constant regardless of its
volume, geometry, or the flux incident upon the sample. One must, however,
159
Improved Boron 10 Quantification via PGNAA and ICP-AES
be careful to avoid water evaporation from the sample which will decrease
the amount of hydrogen in the sample without proportionately decreasing the
boron content. Samples should not be left exposed to the room atmosphere
for more than an hour or so to avoid significant evaporation.
Calibration Curves for
PGNAA and ICP-AES
-"-4
6
810
7 104
5
4
6 10
4
4-
5104
o
c
4 104
310 4
2
2 104
1
1 104
n
0
20
40
60
80
100
10BConcentration
120
140
160
(ppm)
Figure 3.1: Calibration curves for the PGNAA (closed circles) and the ICP-AES
(closed squares) facilities.
Figure 3.1 shows that both techniques show a linear response over a
wide range of concentrations. The ICP-AES data shows less statistical
fluctuation and a correlation coefficient that is nearer to unity. This superior
160
Chapter 3: Inductively Coupled Plasma Atomic Emission Spectroscopy
Kent J. Riley
precision is owed to the tremendous sensitivity of the AES technique. The
PGNAA technique also shows a very good linear correlation over the entire
range of concentrations.
The above calibration curves were used to carry out analyses with the
PGNAA and ICP-AES technique on several blood samples taken from human
subjects in the MIT/BIDMC Phase I clinical trial of BNCT for brain tumors
and subcutaneous melanoma lesions. Table 3.1 below summarizes the
results of these analyses for both technique.
Sample
PGNAA
ICP-AES
97-4 #5 27.3 ± 1.4 27.5 ± 1.1
97-1 #5 31.5 ± 1.6 31.3 ± 1.1
97-2 #5 17.2 ± 0.9 15.9 ± 0.6
95-1 #12 4.6 ± 0.3 4.6 ± 0.2
6.3 ± 0.4 6.4 ± 0.3
95-1 #7
6.0 ± 0.4 5.8 ± 0.2
95-1.8
Difference
(ppm)
0.2 ± 1.8
0.2 ± 1.9
1.3 ± 1.1
0.0 ± 0.4
0.1 ± 0.5
0.2 ± 0.4
Table 3.1: Results of analyses for boron in human blood samples, using both
the PGNAA and ICP-AES techniques.
From Table 3.1 it is clear that both techniques provide results that are
consistent with each other to within the uncertainties of the determinations.
Each technique was calibrated with standards that were independently
prepared from NIST Standard Reference Material boric acid (3), thereby
minimizing the possibility of systematic error in the calibration.
Furthermore, commercially prepared samples of boric acid (HACH Chemical
and Mallinckrodt) were analyzed with both techniques and shown to agree
161
Improved Boron 10 Quantification via PGNAA and ICP-AES
with the expected concentrations of boron, within the predicted uncertainties.
The accuracy of both techniques has therefore been well established and is
within the expected statistical uncertainties that can be calculated for each
measurement.
The tremendous sensitivity of the ICP-AES technique means that even
samples with very low concentrations (-0.1 ppm) need only be counted for a
few seconds to obtain good counting statistics. The higher sensitivity does
not, however, directly translate into faster analysis times for the ICP-AES
technique. In fact, the time that is required to prepare the sample and to
prime the tubing for the nebulizer comprises the bulk of the time that is
required for analysis. Sample preparation times can vary considerably,
depending on the sample type; ranging from a few minutes for blood samples
to several tens of minutes for tissue samples. The required analysis times of
the PGNAA and ICP-AES techniques for several analysis scenarios will be
compared in Chapter 4, along with other relevant performance parameters.
The high sensitivity of the ICP-AES facility allows it to quantify boron
in very small samples, even in trace amounts. While using the cross flow
nebulizer, the ICP-AES consumes roughly 3 ml of sample per analysis,
meaning that a minimum of 0.045 gg of boron can be detected (using the
detection limit calculated earlier) in an analysis requiring one minute
(neglecting sample preparation). This implies that samples as small as 0.045
ml and concentrations as low as 1 ppm can be quantified with the ICP-AES
162
Chapter 3: Inductively Coupled Plasma Atomic Emission Spectroscopy
Kent J. Riley
system, using the cross flow nebulizer. The HEN will improve the
performance of the ICP-AES system by limiting the amount of sample that is
consumed during analysis. The range of sample sizes and concentrations that
can be evaluated with the PGNAA and ICP-AES (using both the cross flow
and high efficiency nebulizers) facilities will be discussed and compared in
Chapter 4.
3.2 Small Sample Analysis with High
Efficiency Nebulizer
3.2.1 Description of the High Efficiency Nebulizer
The High Efficiency Nebulizer (HEN) is able to improve the sensitivity
of the ICP-AES analysis technique by increasing the efficiency of sample
transport to the plasma. The HEN is composed of two concentric paths, the
innermost containing the liquid sample, and the outermost containing the
high pressure (150 - 200 psi) supply of argon. The sample is therefore
sprayed into a fine mist by the turbulent flow of the concentric high pressure
argon path. The operating characteristics of the HEN have been
characterized in various publications and have been compared to more
conventional nebulizers (4, 5). Figure 3.2 below shows a schematic of the
163
Improved Boron 10 Quantification via PGNAA and ICP-AES
HEN. The inner diameter of the capillary is 0.1 mm, and the separation
between the annular orifice for argon gas flow and the outer diameter of the
capillary is 20 gim.
capillary
nozzle
shell
liquid
(sample)
input
gas input
(sidearm)
o40mm
25mm
capillary
f
nozzle end surface
sample
passage
gas annulus
Type Anozzle, front view
Figure 3.2: Schematic depicting the High Efficiency Nebulizer purchased from
JE Meinhard Associates Inc. (6) Image courtesy of JE Meinhard
Associates Inc.
For the research described here, a HEN was purchased from JE
Meinhard Associates Inc. of California. The HEN is designed to operate at an
argon flow rate of 1.0 L/min. JE Meinhard calibrates each nebulizer to
determine the appropriate operating pressure to deliver the required argon
flow rate. The nebulizer used for this research requires an operating
164
Chapter 3: Inductively Coupled Plasma Atomic Emission Spectroscopy
Kent J. Riley
pressure of 159 psi. The experimental arrangement for the use of the high
efficiency nebulizer is identical to that shown in Figure 1.5, except that a
digital pressure gage was attached between the argon cylinder and the
nebulizer. With the cross flow nebulizer, the pressure gage on the regulator
is of sufficient accuracy to monitor the pressure. An adapter was purchased
to allow the HEN to attach to the same spray chamber that is used for the
cross flow nebulizer. When starting the ICP-AES machine, argon is supplied
to the HEN, but at a much lower pressure (-30 psi) than the operating
pressure. Once the ICP-AES machine has been started, the pressure to the
nebulizer can be slowly increased to the operating pressure.
3.2.2 Figures of Merit
3.2.2.1 Background Count Rates
The number of counts registered in the 249.773 nm peak with the high
efficiency nebulizer installed and operating at 159 psi was 880 for a 3 second
count interval. This measurement was recorded after flushing the system for
approximately 30 minutes with deionized water and after recording several
measurements that showed normal statistical fluctuation about the average
value of 880. These measures insured that the recorded background count
rate was due to inherent background and not residual signal from the boron
memory effect. This background count rate is comparable to the background
count rate measured with the cross flow nebulizer.
165
Improved Boron 10 Quantification via PGNAA and ICP-AES
3.2.2.2 Sensitivity
During the measurements discussed in this section, the peristaltic
pump was fitted with 0.38 mm inner diameter pump tubing, and the
peristaltic pump was set at pump speed 1. This configuration results in a
sample flow rate of approximately 180 gl/min. A sample containing 0.5 ppm
of boron was measured and recorded 4172 counts under the boron peak, and
approximately 500 p1 of the sample was consumed during the measurement.
After performing a calculation of the sensitivity, as was outlined for the cross
flow nebulizer in Section 3.1.2.2, a sensitivity of approximately 4400 cps/pg is
realized. The HEN therefore improves the sensitivity of the ICP-AES facility
by nearly a factor of 6 by limiting the amount of sample that is consumed
during analysis, without sacrificing the amount of signal that is collected
during the allotted count time.
3.2.2.3 Limits of Detection
Making use of Equation 2.3, again assuming a count time of 3 seconds,
we can calculate a detection limit for the high efficiency nebulizer. Plugging
the appropriate parameters into Equation 2.3 yields a sensitivity of 0.007 Rg
of boron (corresponding to 0.015 ppm for a 0.5 ml sample). As expected, the
HEN has reduced the detection limit by nearly a factor of 6. Note that the
sensitivity when expressed on a parts per million basis is roughly equal for
both the HEN and the cross flow nebulizer. This reflects the fact that the
detection and excitation characteristics of the system are unchanged, and the
166
Chapter 3: Inductively Coupled Plasma Atomic Emission Spectroscopy
Kent J. Riley
HEN is simply limiting the amount of sample that must be consumed,
therefore lowering the detection limit.
3.2.3 Calibration of the High Efficiency Nebulizer
The HEN was calibrated using the same standards that were prepared
for the measurements with the cross flow nebulizer. Measurements at 1, 2, 4
and 10 ppm (corresponding to undiluted samples of 10, 20, 40 and 100 ppm)
were made in addition to the measurement at 0.5 ppm which has already
been described. The datapoints were appropriately background corrected and
plotted on the calibration curve shown in Figure 3.3 below, where the
abscissa shows the concentrations of the undiluted samples.
167
Improved Boron 10 Quantification via PGNAA and ICP-AES
ICP - AES Calibration Curve
High Efficiency Nebulizer
~-~~~~-~-~-
/10
- --------
4
6 10
5 104
S
4 104
0
310
4
3104
4
2 10
y =53.442
+
623.9x R'= 0.99999
1 104
I
V
0
40
60
80
100
120
Boron Concentration (ppm)
Figure 3.3: Calibration curve for the ICP-AES facility using the High Efficiency
Nebulizer.
From Figure 3.3 it is clear that the HEN maintains the linear
calibration properties that were observed while using the cross flow
nebulizer. To verify that the calibration for the HEN was accurate, a few of
the samples that are shown in Table 3.1 were also analyzed with the HEN.
The results showed no significant deviation from the results obtained with
the cross flow nebulizer or the PGNAA analysis technique.
The lower sample consumption (approximately 3 ml compared to 0.5
ml) that is achieved with the HEN will permit the analysis of even smaller
samples, or samples with even lower concentrations (because the HEN will
168
Chapter 3: Inductively Coupled Plasma Atomic Emission Spectroscopy
Kent J. Riley
require less sample dilution). The HEN can reasonably analyze a sample of
0.01 ml in size and an undiluted concentration of 1 ppm (using the detection
limits reported earlier), or equivalently, a sample 0.05 ml in size and an
undiluted concentration of 0.2 ppm. The improved sensitivity of the HEN
may not always be necessary, particularly for samples of sufficient size and
concentration. Chapter 4 will examine analysis scenarios where the HEN
may be useful, or perhaps the only alternative.
169
Improved Boron 10 Quantification via PGNAA and ICP-AES
3.3 References
1 Design and Construction of a Prompt Gamma Activation Analysis Facility
and Improvement of the On-Line Beam Monitor System for the
Medical
Beam
at
the MITR-II,
J-M.
Chabeuf, M.
S.
Thesis,
Massachusetts Institute of Technology, 1993
2 Nuclides and Isotopes, 14th Edition, F. W. Walker, J. R. Parrington, F.
Feiner, General Electric Co, San Jose, CA, 1989.
3
Certificate of Analysis, Standard Reference Material 951 - Boric Acid,
National Institute of Standards and Technology, Issued April 1992.
4 J. W. Olesik, J. A. Kinzer, B. Harkleroad, "Inductively Coupled Plasma
Optical
Different
Emission
Spectrometry
Using
Sample
Consumption
Rates,"
Nebulizers
with
Widely
Journal of Analytical
Chemistry, Vol. 66, 1994, pp. 2022-2030.
5 H. Liu, A. Monasier, "Phase-Doppler Diagnostic Studies of Primary and
Tertiary Aerosols Produced by a High-Efficiency Nebulizer," Journal
ofAnalytical Chemistry, Vol. 66, 1994, pp. 3233-3242.
170
Chapter 3: Inductively Coupled Plasma Atomic Emission Spectroscopy
Kent J. Riley
6 Informational Brochure, JE Meinhard Associates Inc. 1900-J East Warner
Ave., Santa Ana, CA 92705-5549.
171
(72-
CHAPTER FOUR
Comparison and
Conclusions
4. 0 Figures of Merit
To gain an idea of how each of the analytical techniques described in
this thesis perform, particularly in comparison to one another, this section
will discuss and compare the figures of merit that have been described in the
preceding chapters for each of the techniques.
173
Improved Boron 10 Quantification via PGNAA and ICP-AES
Boron Sensitivity
(cps/pg)
Detection Limit
PGNAA
ICP-AES
(CF)
ICP-AES
(HEN)
18.6
800
4400
0.25
0.041
0.007
Table 4.1: Performance summary for the PGNAA facility and the ICP-AES
facility, using the cross flow and high efficiency nebulizers.
Table 4.1 above summarizes the sensitivity and the detection limits
that were achieved with the improved version of the PGNAA facility (Chapter
2) and the cross flow (CF) and high efficiency (HEN) nebulizers with the ICPAES (Chapter 3). For the PGNAA facility a counting time of 60 seconds was
assumed to calculate the sensitivity, while for the ICP-AES, a counting time
of 3 seconds was assumed. The difference in the count times is intended to
reflect the fact that some time must be spent flushing the system for ICPAES analysis. Comparing the detection limits in this fashion seems
reasonable since the parameters used to calculate the detection limits are
those typically used during routine analysis.
From Table 4.1 it is clear that the ICP-AES system has a definite edge
in sensitivity and detection limit, even with the cross flow nebulizer. It is
interesting to note, however, that in spite of the huge difference in sensitivity
between the PGNAA and the cross flow nebulizer (more than a factor of 40),
the cross flow nebulizer has a detection limit that is less than a factor of 10
better than the PGNAA facility. This is partially due to the differences in
174
Kent J. Riley
Chapter 4: Comparisons and Conclusions
counting times mentioned in the preceding paragraph, and is partially due to
the higher background count rate of the ICP-AES. It is also important to
note that the parameters in Table 4.1 do not reflect the time that is required
to prepare the sample, a factor that will be considered in subsequent sections
of this chapter. The HEN is even more sensitive than the cross flow
nebulizer, with a sensitivity that is more than two orders of magnitude
greater than the PGNAA facility, and a detection limit that is more than 30
times lower.
It is important to remember, however, that the ICP-AES requires
samples to be in a low viscosity liquid matrix before they can be analyzed.
Sample preparation will therefore require that the sample is diluted to some
extent, even for blood samples which are already in liquid form (but of too
high viscosity). Such dilution is not necessary for the PGNAA facility. It is
not unusual for a sample to require dilution by a factor of 10 or more, and
upon doing so, the figures of merit for the ICP-AES facility lose some of the
advantage that was obtained by the high sensitivity of the ICP-AES
instrument. This fact, combined with some of the other advantages
associated with the PGNAA technique, make the PGNAA facility an
attractive option for several types of analyses.
The cross calibration data that was presented in Chapter 3
demonstrated that both the PGNAA and ICP-AES techniques provide results
that are consistent within the uncertainties predicted for each measurement.
The AES and PGNAA techniques are therefore equally accurate. Since the
175
Improved Boron 10 Quantification via PGNAA and ICP-AES
ICP-AES technique is much more sensitive, it will provide results that are
more precise than those obtained with the PGNAA facility. This precision is
reflected in the calibration curve for the ICP-AES, which has an R 2 value
from the least squares fit that is nearer to unity, and smaller error bars on
each data point.
As mentioned earlier in this thesis, there are several PGNAA facilities
in operation around the world, and it is interesting to compare the
performance of the improved facility described in this thesis to these other
facilities. Table 4.2 below (from Chapter 1) summarizes the sensitivity
figures of merit for several facilities.
10B
Facility
MITR-II
MURR
Power (MW)
5
10
NIST
10
2.7
BMRR
3
3.0
Table 4.2:
Sensitivity (cps/gg)
18.8
3.7
10B sensitivities
for several prompt gamma facilities (1). The results
for MIT are for the prompt gamma facility after its modification as
described in this thesis.
The PGNAA facility at the MITR-II has a sensitivity superior to any of
the facilities listed in Table 4.2, even though the rated power of the MITR-II
is lower than all but one of the facilities. The superior sensitivity is due at
least partially to the high solid angle efficiency that can be achieved with the
diffracted beam by moving the HPGE detector very close to the sample
position.
176
Kent J. Riley
Chapter 4: Comparisons and Conclusions
All of the facilities in Table 4.2 are thermal neutron beam facilities.
The National Institute of Standards (NIST) operates a PGNAA facility that
employs a cold neutron source, which is able to obtain much higher
sensitivities. For example, the NIST facility is able to achieve 3.97 cps/mg of
hydrogen with their facility (2), while the improved PGNAA facility at the
MITR-II can achieve 0.454 cps/mg (from Table 2.6 in Chapter 2). The NIST
facility at a 20 MW reactor is therefore nearly an order of magnitude more
sensitive than the facility described in this thesis, due to the high flux of cold
neutrons (milli-electron volt)that is achieved at the NIST facility (1.4 x 108
n/cm 2 sec thermal flux equivalent). It is interesting to note, however, that a
simple facility, such as described in this thesis, can come within an order of
magnitude of state of the art cold neutron facilities.
4.1 Other Considerations
The performance data presented and compared in Section 4.0 do not
provide enough information to allow a thorough comparison of the analytical
techniques that have been described in this thesis. This section will present
other factors that are relevant to each technique and discuss any limitations
that they may impose, or benefits that they may offer.
177
Improved Boron 10 Quantification via PGNAA and ICP-AES
4.1.1 Sample Preparation
The fact that the ICP-AES technique requires samples to be in a low
viscosity liquid matrix has been mentioned throughout this thesis. This fact
implies that (at least for the samples of interest in BNCT; namely blood or
tissue) the sample must undergo some sort of preparation process before the
sample can be analyzed. The amount of time required to prepare the sample
can vary depending on the sample type, ranging from a few minutes for a
blood sample to 30 - 40 minutes for a tissue sample (see Section 3.1.1). In
contrast, the PGNAA technique does not require any sample preparation, the
sample can be inserted into a Teflon vial and placed in the neutron beam.
When performing rapid analyses, the PGNAA technique therefore has the
advantage that the analysis time will be dependent almost entirely on the
time it takes to achieve good counting statistics under the boron peak. The
analysis time will therefore primarily be determined by the size and boron
concentration of the sample, as was described in Section 2.5. However, with
ICP-AES, the time required to count the sample is essentially independent of
the concentration or initial size of the sample (as long as there is enough
prepared sample for the nebulizer that is being used) due to the tremendous
sensitivity of the AES technique. The time required to perform an analysis
will therefore be largely dependent upon the time required to prepare the
sample. If one is seeking an analysis technique that is fast and convenient,
the PGNAA technique holds a definite edge, unless the sample
178
Kent J. Riley
Chapter 4: Comparisons and Conclusions
concentrations are low enough to require the sensitivity of the AES
technique. Analysis scenarios will be examined in Section 4.2 to illustrate
this point.
4.1.2 Destructive vs. Non-Destructive
Perhaps the greatest strength of the PGNAA technique is that an
analysis can be performed while leaving the sample entirely intact. The
PGNAA technique is therefore non-destructive. In contrast, the AES
technique is destructive in that once the sample enters the nebulizer, the
sample is effectively lost. The sample mist emanating from the nebulizer is
either drawn into the plasma and vented out the exhaust stack as a gas, or is
collected in the spray chamber as a liquid and pumped into a waste reservoir.
Though it is possible to recover some sample from the spray chamber by
simply collecting the runoff, rather than directing it to the waste tank, the
collected sample could easily be contaminated or diluted by spray residue
from previous samples. The AES technique is also undesirable in that the
sample must be converted to a liquid state, which may be quite different from
its original form. Though this process does not truly destroy the sample, it
may eliminate the possibility of using other analytical techniques. For
example, High Resolution Quantitative Autoradiography (HR-QAR) is an
analytical technique that superimposes the location of the boron nuclei (via
track-etch autoradiography) over the actual cell morphology (3). This
179
Improved Boron 10 Quantification via PGNAA and ICP-AES
technique requires solid tissue samples that have been kept frozen so that
the in-vivo boron distribution is preserved. One could, in principle, analyze
the solid tissue sample with PGNAA (with the sample frozen) and then
section the sample for HR-QAR analysis. Such flexibility is clearly
impossible with the AES technique. It may also be possible to slice the
sample for HR-QAR analysis, for which only a very thin tissue section is
needed (- pm), and then analyze the remainder of the sample with PGNAA
or ICP-AES. Such a procedure would allow both HR-QAR and ICP-AES
analysis.
A non-destructive technique can be of immeasurable value when
samples are difficult to obtain. With a destructive technique, if something
goes wrong during the measurement, the sample will be lost forever. With a
non-destructive technique, the analysis can be easily repeated. Brain tumor
biopsies can be very small when obtained through a stereotactic biopsy.
Usually, only one sample can be taken (if a sample can be taken at all), and
such samples may contain valuable information regarding the uptake of
boron in tumor or normal tissue. It would therefore be worthwhile to analyze
such a sample via PGNAA so that results would surely be obtained.
It is difficult to imagine all of the scenarios for which a non-destructive
technique might be valuable, but the benefit of such a technique is obvious.
Under circumstances where the sample must be left intact, PGNAA analysis
is the only option. It is, however, worthwhile to point out that extremely
180
Kent J. Riley
Chapter 4: Comparisons and Conclusions
sensitive techniques like ICP-AES using the HEN, or ICP-MS (discussed in
Section 4.4) could use only a very small portion of a sample for analysis,
leaving the remainder of the sample for other purposes. The high sensitivity
of such techniques can therefore partially offset the disadvantage of
destroying the sample.
4.1.3 Matrix Effects
PGNAA is a nuclear analytical technique that is therefore sensitive
only to the nuclear isotopic properties of the sample. Though the relative
abundance of elements and their isotopes will be important, the chemical
form of those elements is irrelevant. It therefore makes little difference what
matrix the sample is suspended in, as long as there are no isotopes that emit
interfering photons (for example
24 Na,
an (n,y) product of 23Na, emits a
472.3keV photon (4), which interferes with the 478 keV photon from
10 B).
If
one intends to use a normalization technique to reduce geometric effects
(such as described in Section 3.1.3), then care must be taken to preserve the
proper isotopic abundance of the normalizing element. Aside from these two
concerns, the PGNAA technique is free from matrix effects.
AES techniques rely on atomic emission phenomena and can therefore
be affected by the chemical form of the analyte. The number of sample atoms
that are able to be excited will depend upon the chemical environment of
those atoms. If, for example, a boron atom is suspended in a highly
electronegative matrix, the electrons surrounding the boron nucleus will be
181
Improved Boron 10 Quantification via PGNAA and ICP-AES
less likely to undergo excitation in the plasma. One must then be careful to
insure that the samples and standards that are prepared for the ICP-AES
have a chemical environment that is consistent. It is not sufficient to simply
prepare samples or standards with the correct concentration or dilution. The
presence of matrix effects means that standards must be prepared for each
set of samples and digestion technique that may have differing chemistries.
Though matrix effects can almost always be accurately accounted for, such
effects can, however, add complexity to an analysis program.
The ICP-AES technique can also suffer from interference due to
emission lines from other atoms. The Spectroflame-D ICP-AES unit is
supplied with a software database that can be used to check for emission
lines that may interfere with the principal line being measured and their
intensity relative to the principal line. For boron (at 249.773 nm) the nearest
interfering lines are tantalum at 249.777 nm, manganese at 249.778 nm,
molybdenum at 249.780 nm, and iron at 249.782 nm. Most of these elements
are not commonly found in blood or tissue samples (with the exception of iron
which is present in trace amounts, - tens of ppm in blood) and the intensity
of the interfering line is at least two orders of magnitude below the principal
boron line. A scan of the measured intensity versus wavelength for both
blood and tissue samples containing no boron revealed no peaks in the
vicinity of the 249.773 nm boron emission line. It is therefore reasonable to
assume that none of the aforementioned lines represent a significant
interference concern for the samples that this group is interested in.
182
Kent J. Riley
Chapter 4: Comparisons and Conclusions
Nevertheless, it is important to always eliminate the possibility of
interference effects before implementing any type of analysis program.
4.2 Analysis Scenarios
This section will provide information that will be useful for
determining which technique is useful for a particular application. It should
be mentioned that both the PGNAA and ICP-AES are useful over a broad
range of concentrations and sample sizes. Considerations that are unique to
a particular application will often be a determining factor in deciding which
technique is preferred.
4.2.1 Analytical Range
Of the three techniques described in this thesis, the ICP-AES facility,
using the HEN is the most sensitive and has the lowest detection limit. The
HEN is the natural choice for samples that are extremely small or extremely
low in concentration. Since the inner capillary of the HEN has such a small
inner diameter, it is especially prone to becoming clogged if samples are not
properly prepared. The HEN is also slightly more complicated to install,
operate and maintain than the cross flow nebulizer. The HEN is therefore
recommended for use only when its high sensitivity is required.
183
Improved Boron 10 Quantification via PGNAA and ICP-AES
Depending on the amount of dilution that is required during sample
preparation for the ICP-AES, the cross flow nebulizer will have an analytical
range that extends to slightly smaller and lower concentration samples than
the PGNAA facility. For the most part, however, these two facilities are
comparable in analytical range.
Estimated Analysis Range
for PGNAA. Cross Flow Nebulizer. and HEN
3
10
2
10
0oB
Conc.
(ppm)
1
10
100
-1
I
-2
-3
10
-1
10
10
0
10
Sample Volume (ml)
Figure 4.1: A plot showing the range of sample sizes and concentrations that
can be efficiently analyzed with each technique. A sample falls
within the analytical range of the technique if its coordinates on the
above plot fall above and to the right of the appropriate line.
184
Kent J. Riley
Chapter 4: Comparisons and Conclusions
Figure 4.1 above shows the range of samples that can be analyzed with
each technique. The y axis denotes sample boron concentrations, expressed
in ppm, and the x axis denotes the initial (undiluted) sample size. Points
that fall above and to the right of a curve can be analyzed by the technique
represented by that curve. The curves in the plot above were calculated
assuming the detection limits that have been calculated for each of the
techniques as listed in Table 4.1 (0.25, 0.041, 0.007 [tg for the PGNAA, cross
flow nebulizer and HEN, respectively). The above curves also assume that
samples prepared for analysis by ICP-AES must be diluted by at least a
factor of 10 (as is typical for blood samples), or more if the sample is so small
that it requires further dilution to meet the minimum volume requirements
for either the cross flow nebulizer or the HEN.
From Figure 4.1 it is clear that the HEN has the largest analytical
range, with the cross flow nebulizer and the PGNAA facility each having
progressively smaller ranges. Nevertheless, the PGNAA facility is the
simplest (and usually fastest) to use, due to the fact that no sample
preparation is required. The use of the cross flow nebulizer is somewhat
more involved than the PGNAA facility, depending on sample preparation
requirements, and the HEN typically requires even more time for
maintenance and setup. A useful method for selecting an analytical
technique is to use the technique that corresponds to the line below and to
185
Improved Boron 10 Quantification via PGNAA and ICP-AES
the left of the sample coordinates in Figure 4.1. Of course available
instrumentation may ultimately dominate the choice of the user.
4.2.2 Analysis Speed
As mentioned earlier, the time required to perform an analysis with
the PGNAA facility is dependent almost entirely on the time required to
achieve good counting statistics under the boron peak. This property of the
PGNAA facility makes it a very rapid technique for analyzing samples of
sufficient size or concentration. The PGNAA facility can analyze a 0.5 ml
blood sample with concentrations as low as 1 ppm in less than 5 minutes,
with a statistical uncertainty of approximately 10% (see Figure 2.27). The
ICP-AES facility, however, will require at least 3-4 minutes to perform an
analysis due to the 2-3 minutes that are required to prepare the sample (as
described for blood samples in Chapter 3), and the 60 seconds that are
required to flush the tubing to the nebulizer. Sample preparation times for
tissue samples can be considerably longer. The PGNAA facility is therefore
faster than the ICP-AES facility for samples of sufficient size and
concentration, depending on preparation time required for AES. It is difficult
to provide a rule of thumb for comparing the speed of the two techniques, but
Figure 2.27 can be used to estimate required counting times for various
samples at the PGNAA facility, and these counting times can be compared to
the time required for the sample preparation scheme in use for the ICP-AES.
186
Kent J. Riley
Chapter 4: Comparisons and Conclusions
4.2.3 Other Considerations
4.2.3.1 Isotopic Sensitivity
The ICP-AES technique is an atomic technique, so it is therefore
insensitive to the two major isotopes of boron (10B, and
11B).
For BNCT
research, our group is typically interested only in the concentration of 10B.
The ICP-AES facility can be used to quantify
10B
only if the isotopic
abundance ratio is known. This is typically not a problem for our BNCT
group, as we use either compounds enriched in 10B (> 99%), or compounds
with natural isotopic abundances. Nevertheless, one must always be
cognizant of the fact that the ICP-AES measures signal from both isotopes of
boron.
In contrast, the PGNAA facility measures only
10B.
While this is
useful for the MIT/BIDMC BNCT group, it may not always be an advantage.
It may be useful to do investigations of boron drugs where less expensive
natural isotopic abundances are used, instead of chemicals enriched in
10B.
Furthermore, in other applications where boron analysis is required (e.g. in
geochemistry research) the boron is not enriched. This would mean that
there is less signal provided by the PGNAA technique, which would require
longer counting times, or perhaps reduce the 10B content below the analytical
range of the PGNAA facility. Some studies involving detection of boron via
nuclear magnetic resonance may seek to use only 11B, which would mean that
PGNAA analysis is not at all possible.
187
Improved Boron 10 Quantification via PGNAA and ICP-AES
4.2.3.2 Sample Destruction
If a particular application requires that the sample be left intact after
analysis, the PGNAA technique is the only option. It may also be useful to
first analyze samples via PGNAA, and if the sample concentration proves to
be too small for PGNAA analysis, the other analytical techniques can then be
applied.
4.3 Conclusions
Several improvements were made to the PGNAA facility at the MITRII which include; removal of collimator shims in the reactor biological shield,
insertion of 30 cm of sapphire single crystal upstream of the graphite
diffraction crystals, use of a focused, diffracted neutron beam, and
improvement of the shielding surrounding the HPGE detector. These
improvements led to an increased slow neutron flux at the sample position by
nearly a factor of 3. This increase in flux came with only a moderate (-50%)
increase in the background count rate under the boron peak, and acceptable
increases in the gross integral count rates. The background count rate at the
PGNAA facility is low enough to keep the system dead time below 15% and is
now dominated by neutron interactions that occur in the sample.
The work with the ICP-AES technique employed both a cross flow
nebulizer and a High Efficiency Nebulizer. Both of these nebulizers were
188
Kent J. Riley
Chapter 4: Comparisons and Conclusions
found to provide excellent linear correlation and achieve results that are
consistent with measurements at the PGNAA facility. The AES technique
was found to be much more sensitive than the PGNAA technique, but
suffered from sometimes lengthy sample preparation requirements, and the
complexity of eliminating chemical matrix effects. The ICP-AES is also a
destructive analytical technique, which eliminates the possibility of
analyzing samples that cannot be destroyed.
The strengths and weaknesses of the two analytical techniques are
such that no single technique can serve as a universal, optimal tool for
macroscopic boron analysis. What the PGNAA may lack in sensitivity and
detection limit, is partially made up for by its non-destructive nature and
lack of sample preparation requirements, which provides for rapid analysis.
On the other hand, the sensitivity of the ICP-AES facility, using the cross
flow nebulizer, may be required for samples with very low concentrations.
The High Efficiency Nebulizer can be employed with samples that are both
small and low in boron content. The two analytical techniques can
complement each other to encompass a broad range of analytical
requirements, and can in some instances serve as an alternate means of
analysis should the other technique fail or be temporarily unavailable.
PGNAA is well suited for rapid analysis of blood samples (typically 0.5
ml) with concentrations as low as 1 ppm. The PGNAA can also be used to
analyze tissue (or blood) samples as small as 0.05 ml with concentrations as
low as 5 ppm, as long as counting times of an hour or so can be tolerated.
189
Improved Boron 10 Quantification via PGNAA and ICP-AES
Extremely small samples (0.02 ml and smaller) can be analyzed with the
PGNAA with counting times on the order of an hour or less, but only at fairly
high concentrations (10 ppm and greater).
ICP-AES is not as well suited as the PGNAA facility for rapid analysis,
due to the sometimes lengthy sample preparations that are required. Blood
samples can, however, be prepared in only a few minutes, which makes the
ICP-AES suitable for rapid analysis of blood samples. The ICP-AES is very
sensitive, and can analyze samples as small as 0.05 ml (with concentrations
greater than -1 ppm) using the cross flow nebulizer, or as small as 0.01 ml
(again, concentrations greater than -1 ppm) using the HEN. The high
sensitivity of the ICP-AES facility permits the analysis of such minute
amounts of boron. A major drawback to the ICP-AES analysis technique is
its destructive nature, which means that samples will be sacrificed during
analysis.
The work presented in this thesis has demonstrated the accuracy and
consistency of the ICP-AES and PGNAA analytical techniques.
190
Kent J. Riley
Chapter 4: Comparisons and Conclusions
4.4 Possible Improvements
4.4.1 PGNAA
As mentioned earlier, the improvements to the PGNAA facility
described in Chapter 2 are the most recent in a series of improvements by
several researchers. The work presented in this thesis represents another
significant incremental improvement in the PGNAA facility at the MITR-II,
but there are still more options to pursue for improving the facility.
Little can be done to further increase the flux at the sample position,
aside from a planned increase in the MITR-II reactor power from 5 MW to 10
MW. The sensitivity of the facility can be improved, however, by increasing
the detection efficiency of the detector arrangement. The detection efficiency
can be increased by using a higher efficiency detector, or by using multiple
detectors. This would serve to collect more of the photons that are released
from the boron atoms in the sample. Though this would also increase the
background count rate, a net improvement would be realized since the
detection limit varies linearly as the inverse of the sensitivity but only as the
square root of the background (see Equation 2.3). Adding another detector
may, however, increase the system dead time beyond acceptable levels, and
this effect should be carefully considered when this option is evaluated. The
sensitivity of the PGNAA facility will increase in proportion with the
191
Improved Boron 10 Quantification via PGNAA and ICP-AES
detection efficiency, which increases approximately linearly with HPGE
detector volume.
The background count rate seen by the detector at the PGNAA facility
is dominated by Compton events that occur in the HPGE detector. The only
way to effectively control this background component is to implement a
Compton suppression system. Such a system would reject most of the
undesired events that do not deposit the full photon energy in the HPGE
detector by rejecting any HPGE events that are followed by an event in a
high efficiency detector (usually NaI(Tl)) that surrounds the HPGE detector.
This system would require coincidence circuitry to reject the undesired
events, but this circuitry may also accidentally reject desired events.
Accidental rejections are more likely with higher count rates. Compton
suppression systems are typically bulky, making them difficult to shield, and
are also expensive.
The degree to which the background count rate of the PGNAA facility
can be reduced by a Compton suppression system will depend heavily on the
timing of the coincidence circuitry. While lowering the gross integral count
rate will help reduce the system dead time, the detection limit for the
PGNAA will improve only as the inverse of the square root of the background
count rate. If any measures are taken to increase the sensitivity (as
described earlier), the system dead time may become high enough to require
the implementation of a Compton suppression system.
192
Kent J. Riley
Chapter 4: Comparisons and Conclusions
4.4.2 ICP-AES
The ICP-AES facility is an elaborate piece of hardware that was
purchased from Spectro Analytical Instruments. An analysis of ways to
improve the performance of the unit is therefore beyond the scope of this
thesis. There are, however, a few simple ideas that can be explored that may
help to improve analyses with the ICP-AES
Preparation of tissue samples for the ICP-AES is rather time
consuming. The work described in this thesis did not thoroughly explore
options for liquefying and digesting solid tissue samples. The digestion
scheme described in this thesis can certainly be improved upon, or simpler
digestion schemes could be developed.
Sample uptake with the ICP-AES is controlled by a peristaltic pump,
which pumps the sample through Teflon tubing to the nebulizer. The
amount of sample that is consumed could be minimized (especially with the
HEN) by optimizing the sample uptake arrangement. Syringe pump units
are available that provide for very low flow rates for use with the HEN. The
pump arrangement could also be optimized to minimize the amount of dead
volume between the sample reservoir and the nebulizer. These measures
would help limit sample consumption and would require less sample dilution.
These measures, however, would only reduce sample consumption by a factor
193
Improved Boron 10 Quantification via PGNAA and ICP-AES
of 2 at best. Such measures would become worthwhile only for very small or
very low concentration samples.
This thesis explored only two nebulizers, the cross flow nebulizer and
the High Efficiency Nebulizer. It is possible that there are other nebulizers
available that are superior to those described in this thesis. A more thorough
survey than was carried out for this thesis may identify nebulizers that are
more suitable to particular BNCT applications. The nebulizers described in
this thesis, however, are believed to have a performance comparable to any
of the nebulizers currently available.
Another interesting analytical option has recently become
commercially available; Inductively Coupled Plasma Mass Spectroscopy (ICPMS). ICP-MS techniques are typically several orders of magnitude more
sensitive than AES techniques, with detection limits typically in the parts
per trillion (ppt), and as low as parts per quadrillion (ppq) for some elements
(5),. ICP-MS systems employ a plasma to ionize the atoms in the sample,
which are then directed through a mass spectrometer, which sorts and
collects the ions based on the ratio of their mass to their carried charge. ICPMS systems can, therefore, be used to distinguish between different isotopes
of interest, barring interference problems (see below).
ICP-MS facilities employ sample introduction systems that do not
significantly differ from those used for ICP-AES. Sample consumption and
sample preparation requirements are therefore likely to be similar to those
described in this thesis. An ICP-MS facility would provide a tremendous
194
Kent J. Riley
Chapter 4: Comparisons and Conclusions
improvement in the ability to analyze extremely small samples, or samples
with extremely low concentrations.
The ICP-MS technique does suffer from some disadvantages, notably
interference and matrix effects. Isobars (isotopes of different elements that
have the same atomic mass) will be indistinguishable for the ICP-MS system,
as will polyatomic interference like 40Ar 16 O vs.
56 Fe.
Chemical matrix effects
will manifest themselves in a fashion much like that already described for
ICP-AES. It is also worthwhile to note that to take full advantage of the
sensitivity of a system like the ICP-MS, sample preparation can become
considerably more complex. All chemicals, glassware, plasticware, and tools
will have to be free of contamination, and sample preparation areas will have
to approach clean room conditions. Such considerations may add
considerable complexity and time to sample preparation.
In spite of its possible shortcomings, an ICP-MS facility would
represent a significant leap forward in sensitivity for trace element analysis.
195
Improved Boron 10 Quantification via PGNAA and ICP-AES
4.5 References
1 "A prompt gamma neutron activation analysis facility using a diffracted
beam," 0.
Harling, J. Chabeuf, F. Lambert, G. Yasuda, Nuclear
Instruments and Methods in Physics Research B, Vol. 83, 1993, pp.
557-562.
2 "Cold Neutron Prompt Gamma Activation Analysis at NIST: A Progress
Report,"
R. L. Paul, R. M. Lindstrom, D. H. Vincent, Journal of
Radioanalytical and Nuclear Chemistry, Articles, Vol. 180:2, 1994, pp.
263-269.
3 High Resolution Alpha-Track Autoradiography and Biological Studies of
Boron Neutron Capture Therapy, G. R. Solares, Ph. D. Thesis,
Massachusetts Institute of Technology, 1991.
4 Nuclides and Isotopes, 14th
Edition, F. W. Walker, J. R. Parrington, F.
Feiner, General Electric Co, San Jose, CA, 1989.
5 G. Tyler, "ICP-MS, or ICP-AES and AAS? - a comparison,"
Instruments at Work, ICP-MS-1, Varian Inc., April 1994,
196
ICP-MS
APPENDIX A
Engineering Drawings of
the Port Plug
Constructed for 4DH3
197
Improved Boron 10 Quantification via PGNAA and ICP-AES
This page intentionally left blank
198
Appendix A: Engineering Drawings of the Port Plug Constructed for 4DH3
Section B-B
Section A-A
"
w
-
Concrete
-
Lithiated Paraffin
-
Steel
-
Aluminurm
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]
]
-
Boron Carbide
-
Boral
-
Lead
B4C
Plate
nd Cap
ent Line
Seal Plate
Lithiated Paraffi Insert
Lead Insert
Section A-A
Section B-B
[Lead
All steel is mild steel
unless otherwise indicated
Crystal
Steel Flange
rYr
cL
C
IIILCL./
;,/U
L,LIL,
LJIIIL,
Cap
vable Water Shutter
Figure A- 1: Composite drawing of the port plug constructed for 4DH3.
199
Improved Boron 10 Quantification via PGNAA and ICP-AES
Water Shutter Seal Plate
with Rubber Gasklet
-
Aluminum
Stainless Steel
-
High Density Concrete
Sapphire
High Density Concrete
Stainless Steel Collimator
Housing
Aluminum Flange
4
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Figure A- 2: Composite drawing showing the final configuration of the port plug at 4DH3 with the two 6"sections of
sapphire crystal. The inlet and outlet lines for the water shutter have not been included since the water
shutter isno longer operable.
200
Kent. J. Riley
Appendix A: Engineering Drawings of the Port Plug Constructed for 4DH3
.------------.------------------..............................
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201
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Improved Boron 10 Quantification via PGNAA and ICP-AES
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202
7/5
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Water Shutter Insert
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a
Kent. J. Riley
Appendix A: Engineering Drawings of the Port Plug Constructed for 4DH3
.•
2
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INST.JTE, OF
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4DH3 Port Renovation
Shutter
Wa-•er
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Figure A- 5: Front view of the water shutter insert.
203
Improved Boron 10 Quantification via PGNAA and ICP-AES
V
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'NSTITTE 0
V
J CIrHNOIOC REFACTC-O
Port Renovation
4D.13
K
A
+
37
Figure A- 6: Front view of the end cap (for end of plug nearest reactor).
204
s.........r.a
...
S.----::-•
-l--
P,
4
t-nd
-lnotPi
.g
I
Kent. J. Riley
Appendix A: Engineering Drawings of the Port Plug Constructed for 4DH3
Y
P:Cr
ter
4
Ref.
[)w 9
..
400-AB
6)61 Au7niu
a
Co
.
'r e n
~i
----------
s
--------------------------
[ P..
I 0.50
•0•8
0,38
3.5
441
....--------------...........
------------,-- A :
- - --
, . TECHNOLOGYREACTOR
4.00
4D1H Port Renovation
Hot End Plug
U
o
..... ..... ......
.
------------------------
----------------------------
400- --3 l
400oo-.--..
--. --- --4
------I----Lý ----------------------------------------------------...
----...-......................
-. -....
. --
Figure A- 7: Side view of end cap.
205
Improved Boron 10 Quantification via PGNAA and ICP-AES
-----.
...-----------------
---------
-k
ae
'1
----------------- _
-----------------------------
------------
I-,oD rn 'fl-)t
M
-L-r--l
r-
(Nuvmlru-
-ort S&- Plate, -ngte Pece
L I:
l'c
wroit rb,,
.25
B:
+
,1234
pical
*'-
•"
MA.,'(,
AC 'ILT
N' -ITLIS
.
r'
HN.•,.LCrtC
REACTC-
•Z=
,tll
4DITfl Port Rrnovbtior
-z:,,,
,I
2
---------- ---------------_---"---- -------------- -------1---
Figure A- 8: Front view of water shutter seal plate.
206
. . - ..
· ,,,
...
Seal Plate
i . • . .. ..- Front
.. . ...
.
'-i,•
.-..
--[-TI -A 3J1__1
------ 300I
_
Kent. J. Riley
Appendix A: Engineering Drawings of the Port Plug Constructed for 4DH3
,-)
PQ1' -t
+4---I
----------
-------------
-----------------------------------------
MC tee'- Ian:
ll
e
4
-----------------------....
....
..------------------------------------'SwtýFe
-----------------------------------------------,0----310
0
16061AýLrsiru-i
30 3B--,
- ------------- -- B--1--------
--
ZZ
o-ring groove, 0.10 deep & 0.16 wide, centered at
Fit with 2-254 E740-75
Parker o-ring (Ethylene Propylene)
urtace tinish specs
R2.90
RO
B
sert thru clearance
le and weld
Finish 63
ace Finish 16
--
K0.50
B
nm';SCHLý'-S JINT,fUT- 'Ff
-0CY REACTCR
TFC-NIN0
4D113 Port Renovation
.. .
..........
......................
......
............
....
.....
.....................
.... ..
"*
a.tj [6,y
,216
o!0
Front Seal Plate
30 0--B -O31
-- -1C
10,116/96
Figure A- 9: Side view of water shutter seal plate.
207
7/ais 1
II
Improved Boron 10 Quantification via PGNAA and ICP-AES
4---- ---
2 ----------4---------.........
.- -4r
readed 5/8 - 11
-----------------------
1---- -----------
~ic
+
7.25
A02656
A
9.00
kwZSArIHL
S'7
ASTJTK OF
1L
TEC4HNUL
OCYRfAC-OR
4DHJ
Front flange for port plug (stainless steel)
Innermost circle accomodotes 5.580 ID pipe (steel)
-enovat-on
4DH13
Port -o-Renovation
----------e
Port P
~,
----------.-------------C 500A t
500-A
IKe' ffiky
"/go ý -
I I I-
Figure A- 10: Front and side views of stainless steel flange for the concrete and lead filled port plug.
208
1-1
-
Kent. J. Riley
Appendix A: Engineering Drawings of the Port Plug Constructed for 4DH3
-6
I-
vr
w
--------------I-·I
---
i ~t·-~
Concrete
-
Boron Carbide
-
Lithiated Paraffin
-
Boral
-
Steel
-
Lead
S-
Aluminum 6061
175
Front flange constructed of stainless steel
all other steel parts constructed of mild steel
1200
400
558
4
_4
775
4
.
t
threod with 3/8
4 holes (typical)
Top and bottom holes threaded 5/8 - 11
SSAHUSE75
INSTITUTE
OF
I
l84DH3 Pr5 Rno
-tUon
__
1.... ............... . ........
---+
I
.........
........
;..............................
..........
...........
3
Figure A- 11: Side view of lead and concrete filled port plug.
209
.........................--------....................
+4
1
.
.6B_
500 8-. .
...............
5
Improved Boron 10 Quantification via PGNAA and ICP-AES
i
·e
w
v
Part
ReF,
#
Dwg
2
Material
Conmments
At 6061
rr
v3n·-~
.
un
:Pnurro
16 .''-
60
185
0 0625
View
ont View
S'...''
1
,u~
' •u.
UYUn gu
' u
l,,,
iur
hi,~,
su iipp reiir
bar (1.73" x 1.54" x 5.91") and is
inserted into P17-61-3A
Stainless
steel shim stock (0.002") is cut to
fit in the notch and wrinkled
to act like a spring.
The steel
then pushes the crystal snugly
up against the wall of P17-61-3A.
The entire assembly is inserted as
shown in the front view
Top View
1/16" clearance holes
Form ~30" loop with 1/16"
7x7 strond core
aircraft
cable (400 Ib)
Feed ends thru holes
and fit with compression
stop sleeves (Al)
Ll]
-T
A-g
MASSC
IUSTT S INSTlITUTE
o0
EC~5NOLOC REACTm
rSapphire
n(werl"o
Lg )
....-..............
---...
--------................
- --------. ---------------
Figure
Aafji
:4r1
1/
1
o
A
Figure A- 12: Top, side and front view of the insertion jig that was used to insert the two 15 cm sections of single crystal
sapphire into the beam port at 4DH3. This drawing shows the jig that was inserted nearest the reactor core.
210
Appendix A: Engineering Drawings of the Port Plug Constructed for 4DH3
......
. .-........
-
...
6.0
Kent. J. Riley
.........
T
...................
.
+
4
-I----------
20
1.85
1.85
0188
1
0.0625
-- 1.68
--
Side View
End View
10.0
This jig holds a rectangular sapphire
bar (1.73" x 1.54" x 5.91") and is
inserted into P17-61-3A.
Stainless
steel shim stock (0.002") is cut to
fit in the notch and wrinkled
to act like a spring.
The steel
then pushes the crystal snugly
up against the wall of P17-61-3A.
The entire assembly is inserted as
shown in the end view.
~MSSAC
,
IUSOTrS INSTITU OF
T.C...NOL.OGY
REAC'OR
, l-~ .
(a
...............
.......................
..........................................
.......................
9)
i[
.
-
+31
-
Z-I, It.,
ur~
-.
......... .
:J~
~-
Figure A- 13: Detail drawing for the jig that was inserted nearest the reactor core.
211
-IN
s/
/;;OI
:
-:/s -
lu,
SS[apphJre nsertion lig
rear jig (two jigs total)
1""
:1i
.
4
-3
IU
a
1ItY
Improved Boron 10 Quantification via PGNAA and ICP-AES
")
i
TT
Ll
This jig holds a rectangular sapphire
bar (1.73" x 1 54" x 5.91") and is
inserted into P17-61-3A.
Stainless
steel shim stock (0.002") is cut to
fit in the notch and wrinkled
to act like a spring.
The steel
then pushes the crystal snugly
up against the wall of P17-61-3A.
The entire assembly is inserted as
shown in the end view
Side View
1 68
1
185
B
3
4
Figure A- 14: Detail drawing for the insertion jig that was inserted furthest from the reactor core.
212
APPENDIX B
Biodistribution Data
From Human Clinical
Trials
213
Improved Boron 10 Quantification via PGNAA and ICP-AES
To date the MIT/BIDMC project has irradiated a total of 13 subjects in
Phase I trials of BNCT for subcutaneous melanoma lesions and intracranial
brain tumors. A biodistribution study has been carried out for each of these
subjects, where the boron levels in blood have been measured periodically
after the administration of the boron drug boronated phenylalanine (BPA).
In some instances the BPA was administered orally, while in later
irradiations the BPA was administered intravenously (IV) with the BPA in
fructose form (BPA-f). Some of the data was collected during a "test dose" of
the boron drug, where the drug was administered without administering
neutrons. This was done so that the distribution could be studied,
preliminary treatment planning calculations could performed, and tissue
biopsies could be obtained to study blood to normal tissue and blood to tumor
boron ratios.. As both treatment planning and boron quantification
techniques improved, the test dose was abandoned and samples are now
collected and analyzed during the irradiation, and the dose delivered is
integrated as the irradiation progresses.
This appendix presents the biodistribution curve for each of the
irradiation subjects to date (5/1/97) in the MIT/BIDMC clinical trials. For
each subject the type of drug that was used (BPA or BPA-f) is documented, as
well as the administration of the drug (Oral or IV), the type of tumor, the
number of fractions that the irradiation was divided into, the number of
irradiation fields that were used, the technique that was used to analyze the
samples for boron content (PGNAA, ICP-AES or both), whether or not the
214
Appendix B: Biodistribution Data from Human Clinical Trials
Kent. J. Riley
subject received a test dose (Y or N), and the dates that the irradiation(s)
took place.
For the data representing oral administration, the plotted data has
been fit to a polynomial (typically of order 2). To perform dose calculations
during the irradiation, only the data corresponding to the time that the
irradiation occurred was fit to a curve (usually linear) and extrapolated to the
end of the irradiation. Since the polynomial fits do not represent data that is
indicative of that used for dose calculations, the polynomial curve fit
parameters are not presented here.
For the data representing IV administration, the plotted data
demonstrates a sharp rise during the time that the BPA is being infused, and
a washout curve that is well characterized with three exponentials. For all of
the data presented here, the infusion was of approximately one hour in
duration, with the exception of Subject 96-4. During the infusion for Subject
96-4, the infusion pump was inadvertently paused for approximately 30
minutes at a point roughly halfway through the infusion. There were, in
effect, two 1/2 hour infusions which were separated by a half hour pause.
Neglecting the previously mentioned exception, all of the data demonstrate a
clear peak at the end of the infusion, a precipitous drop immediately
thereafter, followed by washout that is dominated by exponentials with
slower decay. It is also interesting to note that the data from IV
administrations is much more consistent from subject to subject. This
215
Improved Boron 10 Quantification via PGNAA and ICP-AES
characteristic is presumably due to the fact that IV administration is largely
immune from the effects of digestive and metabolic rate.
The curve fit parameters for the triple exponential fit to the washout
portion of the curve have been included since these parameters are the actual
parameters used to estimate dose. The data recorded during the infusion of
the drug was fit to a rising exponential, though the parameters for this fit
have not been included. All of the irradiations commenced after the end of
the infusion, so the infusion portion of the biodistribution curve is not
relevant for dose calculations.
The last curve contained in this appendix plots the measured
concentrations in blood versus time after the start of BPA-f infusion for all of
the subjects that received BPA-f intravenously, except Subject 96-4 (due to
the aforementioned problems with administration). The data points have
been fit with a rising exponential for points taken during the infusion of
BPA-f, and a decaying triple exponential for measurements after the end of
infusion. The resulting curve fit parameters therefore represent a
biodistribution curve that is an average of all of the subjects receiving BPA-f
to date. The data shows statistically significant deviation from the average
curve and from subject to subject. This fact, combined with the
comparatively small amount of scatter observed in the individual
biodistribution curves, suggests that such an average curve is not suitable for
exact treatment planning calculations.
216
Kent. J. Riley
Appendix B: Biodistribution Data from Human Clinical Trials
Subject: 94-1, V.A.
Tumor: Subcutaneous Melanoma
Drug: BPA
Administration: Oral
Number of Fractions: 4
Number of Fields: 1
Analysis Technique: PGNAA
Test Dose: Y
Date(s) of Irradiation: 9/6/94, 9/9/94, 9/13/94, 9/16/94
B-10 Concentration in Blood
vs.
Time After BPA Administration
3
1
2.5
-- i --
E
C.
a.
2
C
0
o_
0
0
LC
-
.......... ............... ...
. . . . ....
i
'~
1.5
1
-I
0.5
# 94-1
S
Subject
Data taken on 9/9/94
n
0
I
i
2
2
4
4
Irradiation Periods
6
6
(hours)
8
Time
Time (hours)
Figure B- 1: Biodistribution test dose curve for subject 94-1.
217
10
10
12
12
Improved Boron 10 Quantification via PGNAA and ICF-AES
Subject: 94-2, G.H..
Tumor: Subcutaneous Melanoma
Drug: BPA
Administration: Oral
Number of Fractions: 4
Number of Fields: 1
Analysis Technique: PGNAA
Test Dose: Y
Date(s) of Irradiation: 10/24/94, 10/25/94, 10/26/94, 10/27/94
B-10 Concentration in Blood
vs.
Time After BPA Administration
03.5
3
2.5
i.
E
C.
C-
...
...........
2
0
1.5
!...... ............ . . . .........•..............
43
o
1
.
0
o
......
......
....
..
CD
0.5
.....
.........
... ..
0
-A R
0
200
400
600
800
Time (hours)
Figure B- 2: Biodistribution test dose curve for Subject 94-2.
218
1000
Kent. J. Riley
Appendix B: Biodistribution Data from Human Clinical Trials
Subject: 94-3, J.Y.
Tumor: Subcutaneous Melanoma
Drug: BPA
Administration: Oral
Number of Fractions: 4
Number of Fields: 1
Analysis Technique: PGNAA
Test Dose: Y
Date(s) of Irradiation: 12/5/94, 12/6/94, 12/7/94, 12/8/94
B-10 Concentration in Blood
vs.
Time After BPA Administration
in Blood
B-10 Concentration
VS.
Time After BPA Administration
5
..
.....
..
...
.....
......
...........
..
......
.
......................
......
................
.
....
.........
....
...
.....
.....
4
E
C.
3
0.
'-
2
0
0
o
1
o
0
-I
0
300
300
200
200
400
400
Time (hours)
Figure B- 3: Biodistribution test dose curve from Subject 94-3.
219
500
500
Improved Boron 10 Quantification via PGNAA and ICP-AES
Subject: 95-1, P.D.
Tumor: Subcutaneous Melanoma
Drug: BPA
Administration: Oral
Number of Fractions: 4
Number of Fields: 1
Analysis Technique: PGNAA
Test Dose: Y
Date(s) of Irradiation: 9/26/95, 9/28/95, 10/3/95, 10/5/95
B-10 Concentration in Blood
vs.
Time After BPA Administration
in Blood
B-10 Concentration
VS,
Time After BPA Administration
(
r.
6
i
.I.. .I
I
Ti
5
E
C.
CL
M
0
Ci
3
:
0
0om
O
ob
2
:
t.
Subject # 95-1
............
. . .........
. .. .: ...... ...
..
1
0
:
'
200
400
Si
600
800
1000
Time (hours)
Figure B- 4: Biodistribution test dose curve for Subject 95-1.
220
1200
1400
Kent. J. Riley
Appendix B: Biodistribution Data from Human Clinical Trials
Subject: 96-1, J.Y.
Tumor: Subcutaneous Melanoma
Drug: BPA-f
Administration: IV
Number of Fractions: 1
Number of Fields: 1
Analysis Technique: ICP-AES
Test Dose: Y
Date(s) of Irradiation: 5/9/96
B-10 Concentration in Blood
vs.
Time After Start of Infusion
40
35
30
o
a.
C
0
2a
25
o
20
o
15
10
5
0
0
200
400
600
800
1000
1200
Time (min)
Figure B- 5: Biodistribution test dose curve from Subject 96-1.
221
1400
Improved Boron 10 Quantification via PGNAA and ICP-AES
Subject: 96-2, S.J.
Tumor: Intracranial
Drug: BPA-f
Administration: IV
Number of Fractions: 1
Number of Fields: 2
Analysis Technique: PGNAA/ICP-AES
Test Dose: Y
Date(s) of Irradiation: 7/25/96
B-10 Concentration in Blood
vs.
Time After Start of Infusion
40
y = ml*exp(-m2*m0) + m3*exp(...
.
35
30
E 25
a0
0
20
Value
Error
ml
m2
m3
m4
m5
6
sq
15.382
0.0018856
698.63
0.085337
46.598
0.039865
1.7147
0.99075
0.00015156
7911.7
0.36054
911.73
0.15981
NA
2
0.99632
NA
o
o
15
Subject # 96-2
. .. ... . . . . . . . .
10
5
.............
..........................
,i1
u
0
0
. . . . . . . . . . . . . .
200
400
600
200
400
600
800
(min)
800
Time
1000
1200
1400
1000
1200
1400
Time (min)
Figure B- 6: Biodistribution test dose curve for Subject 96-2.
222
Kent. J. Riley
Appendix B: Biodistribution Data from Human Clinical Trials
Subject: 96-3, G.M.
Tumor: Intracranial
Drug: BPA-f
Administration: IV
Number of Fractions: 1
Number of Fields: 2
Analysis Technique: PGNAA/ICP-AES
Test Dose: N
Date(s) of Irradiation: 8/1/96
B-10 Concentration in Blood
vs.
Time After Start of Infusion
40
y = ml*exp(-m2*m0)
Value
13.137
ml
0.0010318
m2
189.95
m3
0.038122
m4
131.98
m5
0.031221
m6
0.42632
Chisq
35
30
a 25
C
0
20
•
r
o
0
R2
+ m3*exp(...
Error
1.5868
0.00013718
1.0585e+07
353.99
1.1215e+07
193.57
NA
NA
0.99965
15
Subject # 96-3
10
5
I
f
I
I
I
I
I
I
I
0
0
200
400
600
800
1000
1200
1400
Time (min)
Figure B- 7: Biodistribution curve for Subject 96-3, taken on the day of
irradiation.
223
Improved Boron 10 Quantification via FGNAA and ICF-AES
Subject: 96-4, J.L.
Tumor: Intracra.ial
Drug: BPA-f
Administration: IV
Number of Fractions: 1
Number of Fields: 2
Analysis Technique: PGNA-LA/ICP-AES
Test Dose:
Date(s) of Irradiation: 11/21/96
B-10 Concentration in Blood
vs.
Time After Start of Infusion
40
35
30
E
a 25
o
20
0
15
10
5
0
0
200
400
600
800
1000
1200
1400
Time (min)
Figure B- 8: Biodistribution curve for Subject 96-4, taken on the day of
irradiation.
224
Kent. J. Riley
Appendix B: Biodistribution Data from Human Clinical Trials
Subject: 97-1, N.M.
Tumor: Intracranial
Drug: BPA-f
Administration: IV
Number of Fractions: 1
Number of Fields: 2
Analysis Technique: PGNAA/ICP-AES
Test Dose: N
Date(s) of Irradiation: 1/30/97
B-10 Concentration in Blood
vs.
Time After Start of Infusion
40
35
30
E
. 25
C
0
o
20
C
r-
o
15
10
5
0
0
200
400
600
800
1000
1200
1400
Time (min)
Figure B- 9: Biodistribution curve for Subject 97-1, taken on the day of
irradiation.
225
Improved Boron 10 Quantification via FGNAA and ICF-AES
Subject: 97-2, T.T.
Tumor: Intracranial
Drug: BPA-f
Administration: IV
Number of Fractions: 1
Number of Fields: 2
Analysis Technique: PGNAA/ICP-AES
Test Dose:
Date(s) of Irradiation: 2/28/97
B-10 Concentration in Blood
vs.
Time After Start of Infusion
40
35
30
E
o 25
CC
0
M
20
o
15
o
10
5
U
0
200
400
600
800
1000
1200
1400
Time (min)
Figure B- 10: Biodistribution curve for Subject 97-2, taken on the day of
irradiation.
226
Kent. J. Riley
Appendix B: Biodistribution Data from Human Clinical Trials
Subject: 97-3, G.C.
Tumor: Intracranial
Drug: BPA-f
Administration: IV
Number of Fractions: 1
Number of Fields: 2
Analysis Technique: PGNAA/ICP-AES
Test Dose: N
Date(s) of Irradiation: 3/6/97
B-10 Concentration in Blood
vs.
Time After Start of Infusion
40
y = ml*exp(-m2*m0) + m3*exp(...
35
30
c.
C
25
ro
oM
o
20
Value
Error
ml
m2
m3
m4
m5
m6
.Chisq
12.289
0.0014739
157.29
0.045184
4.0415e+06
0.18022
2.7156
0.71734
0.00013938
656.93
0.041941
3.1492e+07
0.1235
NA
R;
0.99797
NA
15
Subject # 97-3
10
.
. ............
5
2
I
0
0
200
400
600
800
1000
1200
1400
Time (min)
Figure B- 11: Biodistribution curve for Subject 97-3, taken on the day of
irradiation.
227
Improved Boron 10 Quantification via FGNAA and ICF-AES
Subject: 97-4, G.R.
Tumor: Intracranial
Drug: BPA-f
Administration:
Number of Fractions: 1
Number of Fields:
Analysis Technique: PGNAA/ICP-AES
Test Dose:
Date(s) of Irradiation: 4/10/97
B-10 Concentration in Blood
vs.
Time After Start of Infusion
40
35
30
a
25
C
o
20
o•
20
o0
15
10
5
0
0
200
400
600
800
1000
1200
1400
Time (min)
Figure B- 12: Biodistribution curve for Subject 97-4, taken on the day of
irradiation.
228
Kent. J. Riley
Appendix B: Biodistribution Data from Human Clinical Trials
Subject: 97-5, I.R.
Tumor: Intracranial
Drug: BPA-f
Administration: IV
Number of Fractions: 1
Number of Fields:
Analysis Technique: PGNAA/ICP-AES
Test Dose: N
Date(s) of Irradiation: 4/24/97
B-10 Concentration in Blood
vs.
Time After Start of Infusion
I
40
y = ml*exp(-m2*m0) + m3*exp(...
35
30
E
CL
0.
25
20
Error
ml
15.104
0.61198
m2
0.0016268
9.0375e-05
m3
m4
m5
29755
0.13791
82.737
3.0045e+05
0.17156
80.553
m6
0.030858
0.0098608
Chisq
1.0641
NA
R2
0.99864
NA
C
0
Value
15
Subject# 97-5
10
5
A
0
tI t
I
200
I I
400
600
800
1000
1200
1400
Time (min)
Figure B- 13: Biodistribution curve for Subject 97-5, taken on the day of
irradiation
229
Improved Boron 10 Quantification via PGNAA and ICP-AES
Composite Data for IVAdministration of
250 mg/kg BPA-f
I
.
.
I
I
I
I
.
.
I
I
I
I
I
I
I
i
.
0.97415
0.019358
m2
0.0075851
0.0070735
um3
Chisq
78.237
638.76
56.284
NA
R2
0.86106
NA
.
ý
I
I
I
.
I
ý
ý
I
I
y = ml*exp(-m2*m0) + m3*exp(...
y = m3*(1 - n l*exp(-m0*m2))
Value
Error
ml
.
Value
13.503
Error
2.3585
m2
m3
0.0012672
53.629
0.00030194
2.0339e+06
m4
m5
m6
0.035463
95 496
0.033714
35.552
2.0344e+06
17.507
mni
r1
*
I
W
I
I
I
I
r
r
i
f
1
1
1
I
I
,,,,
4
i
1000
500
1
t I
~I
4
I
·
1500
Time After Infusion Start (min)
Figure B- 14: Data from all subjects receiving 250 mg/kg IV administration of
BPA-f, except Subject 96-4. The infusion period is fit with a rising
exponential (table on the right), and the washout period is fit with a
triple exponential (table on the left). The parameters represent a
least squares fit to all the data points on the curve.
230