Lithium-6 Filter for a Fission Converter-Based
Boron Neutron Capture Therapy Irradiation Facility Beam
by
Wei Gao
B.S, Engineering Physics
Tsinghua University, (2002)
A'1ONNOi>
-dO3.UmSNI S-iSnHovSSn
Submitted to the Department of Nuclear Engineering
in partial fulfillment of the requirements for the degree of
ARCHIVES
MASTER OF SCIENCE IN NUCLEAR ENGINEERING
at the
MASSACHUSSETTS INSTITUTE OF TECHNOLOGY
2053
Esep-rM
April 2005
Copyright ©(2005 Massachusetts Institute of Technology
All Rights reserved
of
SignatureSignature
ofAuthor:
Author:
a/^
'
- Vv
Department of Nuclear Engineering
April 15, 2005
,
X ,
Certified by:
-
,../
/
- -
,v-I
.-
-
/
-
-
_
(§Prof.
.4
i .,
Accepted by:
In
Otto K. Harling
Thesis Supervisor
Dr. Kent J. Riley
Thesis Reader
I
Prof. Jeffrey A. Coderre
Chairman, Department Committee on Graduate Students
Lithium-6 Filter for a Fission
Converter-Based Boron Neutron Capture
Therapy Irradiation Facility Beam
by
Wei Gao
Submitted to the Department of Nuclear Engineering on
in partial fulfillment of the requirements for the degree of
Master of Science in Nuclear Engineering
Abstract
The design of a lithium-6 filter to be used in Boron Neutron Capture
Therapy was developed. The lithium-6 filter increases the average energy of
the epithermal neutrons in the epithermal neutron beam. This filter allows the
beam to be used for effective BNCT treatment at greater depth in tissue.
Based on Monte Carlo calculations, 8mm thick lithium-6 filter was found
to be the optimum filter thickness for the MIT fission converter based
epithermal neutron beam (FCB). The highly reactive lithium metal filter is
sealed with aluminum covers against the humidity and surrounding air. A well
1
shielded and convenient frame was also designed to hold the lithium-6 filter.
The frame is separated into two parts. The fixed part of the frame will be
mounted into the patient collimator of the FCB and provides a slot for the
lithium-6 filter. The filter itself will be connected to the movable part of the
frame and slid in and out of the beam through a pair of roller bearing tracks
like a vertical drawer. Both parts of the frame are built with borated
polyethylene (RICORAD) and steel to insure good shielding. Many safety
issues have been considered in the design including tritium production,
nuclear heating, pressure from released gases and radiation leakage on the
side of the collimator. A storage system was designed to contain the lithium-6
filter safely when it is not in use. A mixed field dosimetry method was used to
measure the photon, thermal neutron and fast neutron dose. The measured
advantage depth is 9.3 ± 0.1cm without filter and 9.9 ± 0.1cm with 8mm
lithium-6 filter. The result is consistent with the result of Monte Carlo
calculation.
Thesis Supervisor: Otto K. Harling (Professor of Nuclear Engineering)
Thesis Reader: Kent J. Riley (Research Scientist)
2
Acknowledgments
First of all, I would like to thank my thesis advisor, Professor Otto K.
Harling, for giving me the opportunity to work on this wonderful project in
the BNCT group. Professor Harling has been giving me extensive and helpful
guidance and advice throughout the project.
I would also like to thank Dr. Kent J. Riley for sharing with me of his
broad knowledge in BNCT field and his advice on the filter design. His
excellent instruction made my research work go faster and more successfully.
I also want to thank Dr. Peter J. Binns. He gave me lots of useful suggestion
during my research work. Along with Kent, Peter helped me finish the
dosimetry measurement and data processing. Without your help, I can not
imagine how I could finish my work.
I would also like to thank Yakov Ostrovsky. He taught me lots of things
in mechanics which is essential in this project. His ideas made the final
mechanical design more doable and efficient.
In addition, I want to thank Peter Stahle, Paul Menadier and Frederick
McWilliams for their advice and assistance.
Finally I want to thank my family and friends for their support
throughout this project.
3
Table of Contents
ABSTRACT .....................................................................................................................................1
ACKNOWLEDGMENTS ..............................................................................................................3
TABLE OF CONTENTS ................................................................................................................4
LIST OF FIGURES ........................................................................................................................9
LIST OF TABLES ........................................................................................................................16
CHAPTER 1: INTRODUCTION ...............................................................................................18
1.1 OBJECTIVE ...........................................................................................................................
18
1.2 BORONNEUTRONCAPTURETHERAPY.................................................................................
18
1.3 FISSIONCONVERTERBEAM ......................
........................................................................... 20
1.4 THE LITHIUM-6FILTER.........................................................................................................
22
CHAPTER 2: BEAM CALCULATIONS USING MONTE CARLO SIMULATIONS ......... 25
2.1 INTRODUCTION
....................................................................................................................
4
25
2.2 MONTE CARLOMETHODINTRODUCTION
............................................................................
26
2.3 M CN P MODELS...................................................................................................................
27
2.3.1 Lithium-6filter M odel ................................................................................................27
2.3.2 Collimator M odel .......................................................................................................30
2.3.3 Head Phantom M odel.............................................................................................. 32
2.4 RESULTS...............................................................................................................................
34
2.4.1 WithoutFilter ..............................................................................................................34
2.4.2 With 6mm, 8mm and 10mm Thick Lithium Metal Filter .............................................39
2.4.3 Lithium Oxide Filter...................................................................................................43
2.4.4 Effects ofAluminum Clad Around the Lithium-6 filter ............................................. 54
2.5 RESULTSOFANALYSIS.........................................................................................................
58
2.5.1 Selecting the Thickness for Lithium-6 filter ................................................................59
2.5.2 Analysis of the Lithium Oxide Option.........................................................................77
2.5.3 The Aluminum Clad of the Lithium-6 Filter .............................................................. 93
5
CHAPTER 3: ENGINEERING
DESIGN OF THE LITHIUM-6 FILTER .............................
99
3.1 INTRODUCTION ....................................................................................................................
99
3.2 LITHIUM-6FILTER STRUCTURE ..........................................................................................
100
3.2.1 The Core of the Lithium-6filter ................................................................................
100
3.2.2 The Fixed Frame ......................................................................................................
103
3.2.3 The Movable Frame .................................................................................................
107
3.2.4 Roller Bearing Tracks...............................................................................................110
CHAPTER 4: SYSTEM INSTALLATION INSTRUCTIONS ...............................................
112
4.1 INTRODUCTION
..................................................................................................................
112
4.2 ASSEMBLYOF THE FIXEDFRAME.......................................................................................
114
4.3 ASSEMBLYOF THEMOVABLEFRAME.................................................................................
116
4.4 INSTALLATION
OF THELITHIUM-6FILTERFRAME...............................................................
118
4.5 INSTALLATION
OF THE COREOF THE LITHIUM-6FILTER.....................................................
122
4.6 REMOVALOF THECORE OFTHE LITHIUM-6 FILTER............................................................
122
6
CHAPTER 5: SAFETY AND STORAGE OF THE LITHIUM-6 FILTER ........................... 124
5.1 INTRODUCTION
..................................................................................................................
124
5.2 PROPERTIESOF LITHIUM....................................................................................................
125
5.2.1 Nuclear Properties of Lithium 6.............................................................................. 125
5.2.2 Physical Properties of Lithium .................................................................................126
5.2.3 Chemical Properties of Lithium................................................................................126
5.3 SAFETYCONSIDERATION
....................................................................................................
127
5.3.1 Tritium Production....................................................................................................127
5.3.2 N uclear Heating .......................................................................................................133
5.3.3 Pressurefrom Released Gases..................................................................................135
5.3.4 Irradiation Levels on the Side of the Collimator......................................................137
5.4 STORAGESYSTEM..............................................................................................................
CHAPTER 6: BEAM PERFORMANCE
WITH LITHIUM-6 FILTER ...............................
6.1 INTRODUCTION
..................................................................................................................
7
140
142
142
6.2 METHODS..........................................................................................................................
143
6.2.1 Thermal Neutron Flux ..............................................................................................143
6.2.2 Photon and Total Neutron Dose Rates......................................................................144
6.2.3 Thermal Neutron and Fast Neutron Dose Rates....................................................... 146
6.3 RESULT..............................................................................................................................
6.3.1 Without Filter ............................................................................................................
149
149
6.3.2 With 8mm Lithium Metal Filter ................................................................................156
6.3.3 Data Analysis ............................................................................................................159
CHAPTER 7: CONCLUSION ..................................................................................................
162
7.1 SUMMARY..........................................................................................................................
162
7.2 SUGGESTIONS
FORFUTUREWORK.....................................................................................
162
REFERENCES ...........................................................................................................................
8
164
List of Figures
Chapter 1: Introduction
FIGURE 1.1 ENERGYLEVELDIAGRAMFORTHE 10 B(N,A)7Li REACTION
...................................................
20
FIGURE1.2 ISOMETRICVIEWOF FISSIONCONVERTBNCT FACILITY
......................................................
21
FIGURE1.3 6LI(N,A)3H CROSSSECTIONVERSUSNEUTRONENERGY
........................................................
23
FIGURE1.4 TOTALNEUTRONCROSSSECTIONSVERSUSNEUTRONENERGYFOR LI ................................ 24
Chapter 2: Beam Calculations Using Monte Carlo Simulations
FIGURE2.1 LITHIUMFILTER...................................................................................................................
28
FIGURE2.2 LITHIUMFILTER............................................................
31
FIGURE2.3 HEAD PHANTOMMODEL.......................................................................................................
33
FIGURE2.4 NEUTRONFLUX(WITHOUTFILTER).....................................................................................
35
FIGURE2.5 BORONDOSERATE0-20MEV (WITHOUTFILTER)..................................................................
37
FIGURE 2.6 NEUTRONDOSERATE(WITHOUTFILTER).............................................................................
38
FIGURE2.7 NEUTRONFLUX(WITH6MM LITHIUM-6FILTER)...................................................................
40
9
FIGURE2.8 NEUTRONFLUX(WITH8MM LITHIUM-6FILTER)..................................................................
41
FIGURE 2.9 NEUTRON FLUX (WITH CM LITHIUM-6FILTER) ..................................................................
42
FIGURE 2.10 BORON DOSE RATE 0-20MEV (WITH 6MM LITHIUM-6FILTER)............................................
44
FIGURE2.11 BORONDOSERATE0-20MEV (WITH8MMLITHIUM-6FILTER).............................................
45
FIGURE2.12 BORONDOSERATE0-20MEV (WITH I CM LITHIUM-6FILTER).............................................
46
FIGURE2.13 NEUTRONDOSERATE(WITH6MM LITHIUM-6FILTER)........................................................
47
FIGURE2.14 NEUTRONDOSERATE(WITH8MMLITHIUM-6FILTER)........................................................
48
FIGURE2.15 NEUTRONDOSERATE(WITH 1CMLITHIUM-6FILTER).........................................................
49
FIGURE2.16 NEUTRONFLUX(WITH 8MMLITHIUMOXIDE).....................................................................
51
FIGURE 2.17 BORONDOSERATE0-20MEV (WITH 8MM LITHIUMOXIDE)................................................
52
FIGURE2.18 NEUTRONDOSERATE(WITH8MM LITHIUMOXIDE)............................................................
53
FIGURE2.19 NEUTRONFLUX(8MM LITHIUM-6FILTERWITHOUTALUMINUMCOVERS)........................... 55
FIGURE2.20 BORONDOSERATE0-20MEV (8MMLITHIUMFILTERWITHOUTAL COVERS)....................... 56
FIGURE2.21 NEUTRONDOSERATE(8MM LITHIUM-6FILTERWITHOUTALUMINUMCOVERS).................. 57
10
FIGURE2.22 TOTALNEUTRONFLUXFOR DIFFERENTTHICKNESSES
OF LITHIUM-6..................................
60
FIGURE2.23 NEUTRONFLUXOF 0-0.5EV FOR DIFFERENTTHICKNESSES
OF LITHIUM-6.......................... 61
FIGURE2.24 NORMALIZEDTOTALNEUTRONFLUXFORDIFFERENTTHICKNESSES
OF LITHIUM-6........... 62
FIGURE2.25 BORONDOSERATEIN TUMORTISSUE0-20MEV FORDIFFERENTTHICKNESSES
OF LITHIUM-664
FIGURE2.26 BORONDOSERATEIN NORMALTISSUE0-20MEV FOR DIFFERENT
THICKNESSES
OF LITHIUM-6
...................................
...........................................................................
.......................................65
FIGURE2.27 TOTALNEUTRONDOSERATEFOR DIFFERENTTHICKNESSES
OF LITHIUM-6......................... 66
FIGURE2.28 NEUTRONDOSERATEOF0-0.5EV FOR DIFFERENTTHICKNESSES
OFLITHIUM-6................. 67
FORDIFFERENTTHICKNESSES
OF LITHIUM-6........ 68
FIGURE2.29 NEUTRONDOSERATEOF0.5EV-I O0KEV
FIGURE2.30 PHOTONDOSERATEFOR DIFFERENT
THICKNESSOF LITHIUM-6(0-100MEV) ..................... 69
FIGURE2.31 NORMALIZEDTOTALDOSERATEIN TUMORTISSUEFOR DIFFERENT
THICKNESSES
OF LITHIUM-6
......................................................................................................................................................
70
FIGURE2.32 TOTALDOSERATEIN NORMALTISSUEFOR DIFFERENT
THICKNESSES
OF LITHIUM-6........... 71
FIGURE2.33 THERAPEUTICRATIOFORDIFFERENTTHICKNESSES
OF LITHIUM-6.....................................
11
73
FIGURE2.34 THERAPEUTICRATIOFOR DIFFERENT
THICKNESSESOF LITHIUM-6.....................................
74
FIGURE2.35 PERCENTCHANGEFROMZEROFILTERTHICKNESSIN THERAPEUTIC
RATIOVS. DEPTHFOR
DIFFERENTTHICKNESSES
OF LITHIUM-6..........................................................................................
75
FIGURE2.36 TOTALNEUTRONFLUXFORFILTERWITHDIFFERENTMATERIALS.......................................
78
FIGURE2.37 NEUTRONFLUXOF 0-05EV FORFILTERWITHDIFFERENTMATERIALS
................................
79
FIGURE2.38 NEUTRONFLUXOF 0.5EV-1OKEVFOR FILTERWITHDIFFERENTMATERIALS
....................... 80
FIGURE2.39 BORONDOSERATEIN TUMORTISSUE0-20MEV FOR FILTERWITHDIFFERENT
MATERIALS..81
FIGURE2.40 BORONDOSERATEIN NORMALTISSUE0-20MEV FOR FILTERWITHDIFFERENT
MATERIALS82
FIGURE2.41 TOTALNEUTRONDOSERATEFORFILTERWITHDIFFERENT
MATERIALS
...............................
83
FIGURE2.42 NEUTRONDOSERATEOF 0-0.5EV FOR FILTERWITHDIFFERENTMATERIALS
....................... 84
FIGURE2.43 NEUTRONDOSERAEOF 0.5EV- 10KEV FOR FILTERWITHDIFFERENTMATERIALS............... 85
FIGURE2.44 PHOTONDOSERATEFORFILTERWITHDIFFERENTMATERIALS
(0-100MEV) ....................... 86
FIGURE2.45 TOTALDOSERATEIN TUMORTISSUEFORFILTERWITHDIFFERENT
MATERIALS
................... 87
FIGURE2.46 TOTALDOSERATEIN NORMALTISSUEFOR FILTERWITHDIFFERENTMATERIALS
................ 88
12
FIGURE2.47THERAPEUTICRATIOFOR FILTERWITHDIFFERENTMATERIALS..........................................
90
FIGURE2.48 THERAPEUTICRATIOFOR FILTERWITHDIFFERENTMATERIALS
..........................................
91
FIGURE2.49 PERCENTCHANGEFROMZEROFILTERTHICKNESSIN THERAPEUTICRATIOVS. DEPTHFORFILTER
WITHDIFFERENTMATERIALS
...........................................................................................................
92
FIGURE2.50 THERAPEUTICRATIOFOR 8MM LITHIUMFILTERWITHANDWITHOUT0.01 INCHAL COVERS96
FIGURE2.51 THERAPEUTICRATIOFOR 8MM LITHIUMFILTERWITHANDWITHOUT0.01 INCHAL COVERS97
FIGURE2.52 PERCENTCHANGEFROMZEROFILTERTHICKNESSIN THERAPEUTIC
RATIOVS. DEPTHFOR8MM
LITHIUMFITLERWITHANDWITHOUT0.01 INCHAL COVERS...........................................................
98
Chapter 3: Engineering Design of the Lithium-6 filter
FIGURE3.1 MECHANICALDESIGNOF THECOREOF THELITHIUM-6FILTER
...........................................
101
FIGURE3.2 MECHANICALDESIGNOF THE FIXEDSTEELFRAME............................................................
104
FIGURE3.3 MECHANICALDESIGNOF THE FIXEDRICORAD SHIELDING
..............................................
106
FIGURE3.4 MECHANICALDESIGNOF THE MOVABLESTEELFRAME......................................................
108
FIGURE3.5 MECHANICALDESIGNOF THE MOVABLERICORAD SHIELDING........................................
109
FIGURE3.6 MECHANICALDESIGNOF THE ROLLERBEARINGTRACKS...................................................
111
13
Chapter 4: System Installation Instruction
FIGURE4.1 THREEDIMENSIONPICTUREOF LITHIUM-6FILTER..............................................................
113
FIGURE4.2 THE FIXEDFRAMEAFTERASSEMBLED
................................................................................
115
..........................................................................
FIGURE4.3 THE MOVABLEFRAMEAFTERASSEMBLED
117
DOWNSTREAM
OF THECOLLIMATOR
BASE 19
FIGURE4.4 REMOVETHECOMPONENTS
OF THECOLLIMATOR
HAVEBEENASSEMBLED.............. 120
FIGURE4.5 THE LITHIUM-6FILTERFRAMEAFTERALL COMPONENTS
FIGURE4.6 THE LITHIUM-6FILTERWHENIT IS OPENED........................................................................
121
FIGURE4.7 THE LITHIUM-6FILTERWHENIT IS CLOSED........................................................................
123
Chapter 5: Safety and storage of the Lithium-6 filter
FIGURE5.1 STORAGESYSTEMOF THELITHIUM-6FILTER......................................................................
141
Chapter 6: Beam Performance with Lithium-6 Filter
FIGURE6.1 THE MEASURED
2200 M/SNEUTRONFLUX(WITHOUTFILTER)............................................
147
FIGURE6.2 THE COMPARISON
OF THENORMALIZEDTHERMALNEUTRONFLUXBETWEENMEASUREMENT
AND
CALCULATION
(WITHOUTFILTER)..................................................................................................
FIGURE6.3 THE MEASUREDRBE DOSEPROFILE(WITHOUTFILTER).....................................................
14
148
150
FIGURE6.4 THE MEASUREDTOTALDOSEPROFILE(WITHOUTFILTER)...................................................
151
FIGURE6.5 THE COMPARISON
OF THE NORMALIZED
TOTALDOSERATEBETWEENMEASUREMENT
AND
CALCULATION
(WITHOUTFILTER)..................................................................................................
152
FIGURE6.6 THE MEASURED2200 M/S NEUTRONFLUX(WITH 8MMLITHIUM-6FILTER)........................ 153
FIGURE6.7 THE MEASUREDRBE DOSEPROFILE(WITH 8MMLITHIUM-6FILTER).................................
154
FIGURE6.8 THE MEASUREDTOTALDOSEPROFILE(WITH 8MMLITHIUM-6FILTER)...............................
155
FIGURE6.9 THE COMPARISON
OFTHE NORMALIZEDTHERMALNEUTRONFLUXBETWEENMEASUREMENT
AND
CALCULATION
(WITH8MM LITHIUM-6FILTER)..............................................................................
157
OF THE NORMALIZED
FIGURE6.10 THE COMPARISON
TOTALDOSEBETWEENMEASUREMENT
AND
CALCULATION
(WITH8MM LITHIUM-6FILTER).............................................................................
158
RATIOWITHANDWITHOUT8MMLITHIUM-6FILTER.......... 160
FIGURE6.11 THE MEASUREDTHERAPEUTIC
FIGURE6.12 THE MEASUREDTHERAPEUTICRATIOWITHANDWITHOUT8MM LITHIUM-6FILTER
.......... 161
15
List of Tables
CHAPTER 2: BEAM CALCULATIONS USING MONTE CARLO
SIMULATIONS
TABLE2.1 NEUTRONFLUX(CM-2S ) IN AIRATTHE SURFACEOFTHE 12CMAPERTURE............................ 36
2 ............................. 39
TABLE2.2 NEUTRONFLUX(CM- S ') IN AIRAT THESURFACEOFTHE 12CM APERTURE
TABLE2.3 NEUTRONFLUX(CM 2S ) IN AIRAT THESURFACEOF THE 12CMAPERTURE............................. 50
-1
TABLE2.4 NEUTRONFLUX(CM-2S
) IN AIRATTHE SURFACEOF THE 12CMAPERTURE............................. 54
TABLE2.5 NEUTRONFLUX(CM'2S-') IN AIRATTHE SURFACEOF THE 12CMAPERTURE
.............................
59
TABLE2.6 NEUTRONFLUX(CM2Ss) IN AIRATTHE SURFACEOF THE 12CMAPERTURE
.............................
77
TABLE2.7 NEUTRONFLUX(CM 2S 1) IN AIRATTHE SURFACEOFTHE 12CMAPERTURE............................ 93
CHAPTER 5: SAFETY AND STORAGE OF THE LITHIUM-6
FILTER
TABLE5.1 PHYSICALPROPERTIES
OF LITHIUM3 )...................................................................................
126
TABLE5.2 NEUTRONFLUXUSEDFOR TRITIUMPRODUCTION
CALCULATION
.........................................
128
16
TABLE5.3 CROSSSECTIONZ [ 6LI(N,A)3H] (BARNS)..............................................................................
129
)
TABLE5.4 TRITIUMPRODUCTIONRATE(1012 S .................................................................................
130
TABLE5.5 ACTIVITYOF TRITIUMWHENTHE BEAMIS CONTINUOUSLY
OPERATED.................................
131
TABLE5.6 ACTIVITYOF TRITIUMWHENTHE BEAMIS TURNEDON FOR ONEHOURPER DAY,365 DAYSPERYEAR
.....................................................................
1...............................................................
132
TABLE5.7 PRESSUREFROMTHE RELEASEDGASES................................................................................
136
TABLE5.8 DOSE RATESATTHE SIDEOF THE COLLIMATOR
.....................................................................
138
17
Chapter 1: Introduction
1.1 Objective
The Objective is to design a lithium-6 filter that can be slipped into and
out of the collimator of the fission converter-based boron epithermal neutron
beam2). Such a lithium-6 filter can be used to increase the dose delivered to
deep-seated tumors.
This thesis has four major parts:
1. Design of the lithium-6 filter based on Monte Carlo calculations.
2. Design of the frame for the lithium-6 filter that makes it easy to
quickly install and remove the filter from inside the medical room. Also it has
to satisfy the requirement obtained from shielding computational calculations.
3. Safety concerns about handling and storing the lithium-6 filter.
4. Construction and testing of the lithium-6 filter.
1.2 Boron Neutron Capture Therapy
Boron Neutron Capture Therapy (BNCT) is an experimental binary
18
therapy modality used for the treatment of some kinds of cancer. It generally
involves two steps.
First, a chemical compound that can transport Boron is injected or
infused into the patient, such as boronophenylalanine (BPA) or borocaptate
sodium (BSH), and will concentrate in the tumor tissue so that the neutron
0 in the tumor has a greater concentration (up to 3:1 or 4:1)
capture agent l°B
compared to the normal tissue.
Secondly, a few hours after the boron compound solution injection, a
neutron beam is directed into the patient in the vicinity of the tumor. Some of
the neutrons are captured by 10 B. The reaction may be written1)
'B +n
'°B + n
)'B
)'
)'Li+
7 4a
) 'Li* +2 a----
As shown in Figure 1.1, when
10B captures
3
Li+2 a +y (480keV)
a neutron, it yields an excited
state of IB* first. Then 6% of the llB*fission into the ground state of 7Li and
releases an alpha with 2.792MeV energy. Then other 94% of the llB* fission
into the excited state of 7 Li* first and release a 2.310 MeV alpha, then 7Li*
releases a 0.478 MeV gamma to the ground state. Since the releasing alpha
particles and 7 Li nucleus are highly ionizing, they have a short range of about
19
7pm for the alpha and about 4gm for the 7 Li nucleus. If the '0B is more
concentrated in the tumor tissue than in normal tissue, more energy or dose
will be delivered to the tumor than to the normal tissue.
B*
2.3 10 MeV
ity)
7
Li*
0.478
Figure 1.1 Energy level diagram for the l'0B(n,a) 7Li reaction4 )
In theory, the tumor can be selectively damaged during Boron Neutron
Capture Therapy while the normal tissue around it will receive much less dose.
For effective BNCT a tumor targeting capture compound is needed and a
suitable neutron beam. This thesis deals with improvement to the existing
epithermal beam at the MITR.
1.3 Fission Converter Beam
As shown in Figure 1.2, a new type of epithermal neutron irradiation
facility for use in boron neutron capture therapy was designed and constructed
during the late-1990's, and put into operation at the Massachusetts Institute of
20
Technology Research Reactor (MITR). This facility, called the Fission
Converter Beam (FCB), has a converter which is driven by the MITR and
which is used as the source of neutrons to the epithermal beam2 ). The
moderated neutrons from the reactor core hit the uranium fuel of the converter
MITR fuel elements and cause fission. The fission neutrons are then
moderated and filtered to produce an epithermal neutron beam with neutrons
primarily in the range 0.5eV- 20keV.
Figure 1.2 Isometric view of Fission Convert BNCT facility3 )
21
After implementing the fission converter-based epithermal irradiation
facility, three major goals were achieved.
First, a high intensity epithermal neutron beam is achieved. When the
reactor is operating at the licensed power of 5 MW, the epithermal neutron (1
eV < E < 10 keV) flux at the entrance to the medical irradiation room is
around 101° n/cm2
s, and at the end of patient collimator, the epithermal
neutron flux is about 3 to 5 X 109 n/cm 2 s.
Second, a high degree of beam purity is achieved. A low fast neutron and
gamma dose component is achieved after moderation and filtering ( Dyfn / 0 epi
< 2 X 10- 3 Gy cm2 ). With this negligible beam contamination, the effective
therapeutic depth of penetration can be 9 to 10 cm for current capture
compounds.
Finally, a high collimation, Jepi / oepi, is achieved. The fission converter
beam without the collimator has a Jpi / Oepi of about 0.7. And with the patient
collimator, the Jepi / 0epi is improved to near 0.853).
1.4 The Lithium-6 filter
Epithermal neutrons produce a depth dose profile that is useful for
depths up to 10cm. As the epithermal neutrons travel through the tissue, they
lose their energy mostly through elastic scattering. After thermalizing, the
neutrons are more likely to be captured by the 10°B at the tumor's vicinity. The
22
higher energy epithermal neutrons penetrate deeper, on average, before they
are thermalized and captured, than the lower energy epithermal neutrons.
Figure 1.3 shows the 6Li(n,a) cross section which is the dominant partial
cross section in the energy region of interest for this thesis. And Figure 1.4
shows the total neutron cross-section of 6 Li. Though it absorbs all energies of
neutrons, the lower the neutron energy is, the higher the cross-section
is, u(n, a) oc 1.
That means, after passing through the lithium-6 filter, a larger
V
fraction of the lower energy epithermal neutron will be absorbed by 6 Li than
the higher energy epithermal neutrons. The filter acts to increase the average
energy of the epithermal neutrons.
(from MCNP cross section data"1 )
==___
_
_Xe7m
_%
___ ____7
10-1 1-11 10-10
llll10- 1
.
_
1
110-
I0-
0.0
1111
0.1
1.1 10. 100.
x
10-1210-1 10-10 10-9 111-810-? 10-6 10-51H- 0.001 0,01 0.1
energ
energy(V)
lnev)
1,
10. 100.
Figure
Li(n,az)3 H cross
Figure 1.3
1.366Li(n,a)31cross section
section versus
versus neutron
neutron energy
energy
(from MCNP cross section data ~1))
23
-
-
I 11
I tlIll
I II II
I IIIL[
I I II
I 1
11
I I I II
I III
I I
I I II
I I I II
11
I1111
I
I ll ll
I II
7
I
1
zt
-z;r
I- I1 -
I
1
2
-7
-7
,4
-
N~ :
I-I II II
I ll l
I I I II
I II I
10-12 10-1110-10 10-9
I I III
1-
I
IIII
I I II
I I
III
t I I1I
0- 6 l-S
0-5 10- 0.00
I I
ll
.1
"I
I I II
I II I
D.1
"I
Iflll
IIII|
1.
10. 100.
energ (v)
Figure 1.4 Total neutron cross sections versus neutron energy for 6 Li
(from MCNP cross section data1 1 ))
Adding a lithium-6 filter to the neutron beam is a relatively simple and
inexpensive beam line modification, which can increase the average energy of
the epithermal neutrons and the dose delivered to the deep-seated tumor tissue
during BNCT. Along with the increase in average energy of epithermal
neutrons, the lithium-6 filter also decreases the intensity of the epithermal
beam. This is the basis of the lithium-6 filter design for the FCB.
24
Chapter 2: Beam Calculations Using
Monte Carlo Simulations
2.1 Introduction
Before starting the mechanical design of the Lithium-6 filter, we need to
consider carefully how the lithium may affect the neutron beam. The final
neutron beam has to achieve the main goal which is increasing the dose
delivered to the deep-seated tumor tissue while not exceeding the tolerance of
normal tissues. Also the additional components of the filter should not
interfere with the neutron beam significantly. Furthermore it is essential that
in any situation, the beam won't be unsafe for the patients being irradiated.
For the design, three major issues have to be considered and be resolved.
What's the best thickness for the lithium-6 filter to achieve the design
goal? To solve this question, the neutron beam performance has to be
calculated for different thickness of lithium-6 filter. To compare beam
performance we use figures of merit, therapeutic ratio and the percentage
change of therapeutic ratio with depth.
Also the effect of using different structures for the filter has to be
considered. For example will a big RICORAD (borated polyethylene) ring
25
around the filter (for shielding purpose) affect the beam adversely? Or if
aluminum is used as the clad over the lithium, what will happen to the beam,
e.g. the level of gamma contamination and the neutron attenuation.
Finally if the airtight seal protecting the lithium metal fails and part of or
all the lithium is oxidized and changed into lithium oxide (Li2 0), will the
beam performance be significantly damaged? Would this be unsafe for the
patients?
All these issues have been considered with the help of the Monte Carlo
calculations.
2.2 Monte Carlo Method Introduction
Monte Carlo methods can be used to solve the transport problem. The
approach is different from deterministic methods. Monte Carlo doesn't solve
any explicit equation. It just simulates individual particles and records their
average behavior.
All individual events are governed by some kinds of
probability distributions. Running on a fast digital computer, a large number
of events can be statistically sampled. In general, the Monte Carlo method
follows each of many individual particles from its origin throughout its birth
to its death. A statistical sum of all these events describes the total transport
phenomenon.
26
The program we use in this and the next chapter to do the simulation is
called MCNP. MCNP is a general-purpose Monte Carlo N-Particle code" ). It
can be used for neutron, photon, electron or coupled particle transport. The
user only need to describe the model in arbitrary three-dimensional geometric
cells bounded by the surfaces and filled with specified materials.
With MCNP, we can solve the three questions or issues discussed above.
And using some variance reduction techniques as weight windows supplied
by MCNP, the standard deviation of the results can be control under 10%.
2.3 MCNP Models
In order to do the MCNP calculation, a model has to be set up first. The
Model must describe the geometric structure and materials that buildup the
whole system in detail.
Of course the model of the lithium-6 filter is the main part of the design.
But also a little modification has to be made to the original model of
collimator and a good model of phantom is needed. This may help to make
clear about the results of the calculation.
2.3.1 Lithium-6 filter Model
As shown in Figure 2.1, the model of lithium-6 filter consists of five parts.
27
0)
0)
Cl,
C~~~
0-
.-
FLE
L..0
w: U-LL
~:
:
K
ILL
/
c
0
.L w
Li::::,-
.) o
E
._
-4
.' '))
a)
=O
a)
a)
U)
/\
-o
0
E
0o
to
._o
u. '~
co-'v
.
0)
1:: ---
.
I--i~r
28
The main part of them is the lithium-6 filter itself. It's made of enriched
lithium which has 95% of 6Li. It locates in the middle of the model that is a
round plate. The diameter of lithium-6 filter is 13.59 inches, a little bigger
than the dimension of the hole of the collimator at the same position that is
13.24 inches. This means the entire neutron beam has to pass through the
lithium-6 filter.
The thickness of lithium plate is varied from 5mm to 12mm. By
comparing the results from different thickness of lithium, we can tell which
one is the best for our project. I will write about the detail later in this chapter.
Over the lithium is the aluminum clad which is used to hold the lithium
and seal it from the outside air. The width of the aluminum ring surrounding
the 6 Li filter is about 1.36 inch. And the thickness of aluminum clad in front of
and back of the lithium metal is about 0.01 inch. We try to use as little
aluminum in the beam as possible to decrease the gamma-ray production from
the interaction of neutrons with aluminum and to minimize attenuation of the
epithermal beam.
Since there is an air duct behind the wall of the medical room, the air will
flow from the patient side through the collimator into the air duct behind. We
made a little air clearance between the filter housing and the RICORAD
Shielding, so that the air can flow over the lithium-6 filter. If any chemical
29
reaction happens to the lithium-6 filter, the air flow will take away fumes
generated by the reaction from the patient. This design helps to protect the
patients.
The RICORAD shielding is used for shielding purposes, and contains
2.00% boron and 12.06% hydrogen. Hydrogen and boron are effective for
slowing down and capturing the neutrons. RICORAD is difficult to machine
due to the boron carbide content. So it costs a lot to make a round one. That's
why the shape of RICORAD shielding here is a polygon.
The outside part is the steel frame. It functions not only to support the
whole lithium-6 filter system, but also for shielding the gamma radiation
because of its high atomic number.
2.3.2 Collimator Model
Since we have to install the lithium-6 filter inside the existing collimator, a
small modification of original collimator model is essential. As shown in
Figure 2.2, the lithium-6 filter is inserted in between the lead ring at the wall
of the medical room and the collimator base. All the modules upstream of the
collimator base are unchanged and the lithium-6 filter model is added
immediately after the lead ring module. All the modules downstream of the
collimator base including the collimator will be moved out along the axis of
the collimator by 1.737 cm (0.684") which is the exact thickness of lithium-6
30
ax a)
= 0
l-4 C
o
c
4 -
0co
rl e
no
o-.,
-o
r
E.
o_ o
0
)
.
esI
u>
-
C~
._
u
E
C
CD
4
0
u
-c
4
0
40
_
0)
LI-l
0)I t's
0}
-0
-ccc
0)
a)a
)0 ' fl
w.4C0
-j
W )
0
1-
31
a
filter module. All the neutrons in the FCB beam go along the collimator and
pass through the lithium-6 filter.
2.3.3 Head Phantom Model
In order to test the effectiveness of neutron beam, a head phantom model
2)
is needed to be put immediately in front of the end of collimator.
The head phantom model is shown in Figure 2.3.
The shell of the phantom is made by acrylic. And inside the shell, it is full
of water that from a neutronic perspective is similar to the material inside
human skull.
We are going to calculate the neutron flux and dose rate on the surface of
and along the central axis of the head phantom. By comparing the results, we
can select the appropriate thickness of lithium-6 filter and solve the
optimization problem discussed at the beginning of this chapter.
32
>3
a)
"0
c-
.-(z--\ } 4a)=a)
a) (U Z V
U
d
-a)
(a 3 '
dL (U="Z>
e-- C/)
I-
..
tO, U)
!c
(D
-0
0
E
E
o
c
c
I
.
*0
-o
a)
I-.
-i
Lb-
L-
w~ rw &
a) -~ =
L-
dl
_ e.
a)~
~ Co
a)0 o.d
t:
- (1.~
c
4
.
I-
a
LiI~-0
o
33
3
C
-1
-. mr_
0) -0
W a)
a) -0
= WIM
-
M TE
2.4 Results
While doing all the Monte Carlo simulations, we suppose that the reactor
operates at 5 MW. The details of the results of all the calculations are shown
below.
2.4.1 Without Filter
First let's see the result without using the lithium-6 filter. Figure 2.4 shows
the neutron flux along the central axis of the head phantom. The neutron
spectrum is divided into three regions: thermal neutrons with energy between
0 and 0.5eV, epithermal neutrons with energy between 0.5eV and lOkeV, and
fast neutrons with energy between 1OkeVand 20MeV. Also the total neutron
flux is plotted. From the Figure we can see that the thermal neutrons increase
monotonically at the beginning, then reach a peak and finally decrease
exponentially. The thermal neutron curve reaches its apex at a depth around
2.7cm.
34
0
0
I
Lo -
0 c
s
1
oc
Z
1
to-
0-
©
Eu
O
Q0
So
vs
a)
So
-1i
-C
0
4-0
za)
0
a)
,i
>
C)
0
0
o~~~~~~l
ff
00
0_0 -
++l
IC-
2r
1Lj
O Li2
80C0 082
8'
C
0r
02r4
80i
L
0i
4~ r42
0CD 0[-
m
00
t-S
00+° +
s6
Lr
t'
C[)
-4
X6
3[uO/
T)x US
35
CO/
d
Table 2.1 Neutron flux (cm-2s- 1) in air at the surface of the 12cm aperture
(Without filter)
0 -
0.5eV
0.5eV -
0keV
10keV - 20MeV
Flux
Error
Flux
Error
Flux
Error
5.52E+07
0.025
3.01E+09
0.009
1.15E+08
0.033
Total
Flux
Error
3.18E+09 0.009
From table 2.1, it shows clearly the neutron flux in-air for each energy
spectrum at the end of the collimator and before entering the head phantom.
Also in Figure 2.5 and Figure 2.6, we can see the dose rate along the
central axis.
The Figure 2.5 shows the boron dose rate. Here it supposes that with BPA
the l°B in normal tissue is 18ppm, and in tumor tissue is 65ppm. The RBE
(Relative Biological Effectiveness)6 ) for boron in regular tissue is 1.3, and for
boron in tumor tissue is 3.8. Figure 2.6 shows the neutron dose rate. The RBE
selected for neutrons is 3.2. Also the neutrons have been divided into three
energy regions and shown respectively.
36
4
01
0~
CY)
')
M
m
0
o
U)
LO
(D
-
D C)
o
c
o~ o .
0,
E
E
"
a)
0
i
i
o
-
(:::O°
un
i
.
0i
ECl)
I
i
i
O
o0
Ern
CI~~ 0 0
C~j
m
0
0n
0
El
I
0
--
&
(CS
CQ
9CS
(U.LuI/xq)
C LL
llt
C~i
c'~
oc'PuJ socJ ~Jg
37
0
O
o co
E
0
0D
0
. o
LO
0CSl
O
E-
0
i
a)
L.
o
X
4-
i
- m0
cCU)..
U
00
i
I 0
i
+0 >
i
I Xc
C,
GO
o
1-4
t.
0
z
aa
I0
I
a
aa
0
0-
-
S
-.
e5
cd(
i
S
zi
Ce
e4
(UlWn/XO)a1d
S
4
aS(I
38
AM
S
u
S
do
Z.
2.4.2 With 6mm, 8mm and 10mm Thick Lithium Metal
Filter
In order to select the appropriate thickness of lithium-6 filter, we change
the thickness from 6mm to 1cm and calculate the beam performance.
Table 2.2 shows the neutron flux in-air through the lithium-6 filter for
different thickness determined at the end of the collimator. It can be seen
clearly that when the thickness of lithium-6 filter increases, the neutron flux at
the end of the collimator or at the surface of the head phantom drops all over
the energy spectrum. Compared with the epithermal neutrons, the flux of
thermal and fast neutron is pretty small and is not the emphasis of our study.
And it also won't affect our selection very much.
Table 2.2 neutron flux (cm-2 s- 1) in air at the surface of the 12cm aperture
(With Lithium-6 filter of Different Thickness)
Energy
Thickness
0 -
0.5eV
Flux
Error
6mm
3.14E+06
8mm
2.21E+06
10mm
1.85E+06
Energy
Thickness
0.5eV Flux
Error
0.077
1.67E+09
0.014
0.104
1.52E+09
0.014
0.088
1.36E+09
0.015
10keV - 20MeV
Flux
0keV
Total
Error
Error
Flux
6mm
9.60E+07
0.041
1.76E+09
0.013
8mm
9.41E+07
0.041
1.61E+09
0.014
10mm
9.79E+07
0.040
1.46E+09
0.014
39
H
0
4~)
r-4
O0
C
N
C)
o
-Ic
4-1
0
0
E
-
-
S
CD
u
z
I 41
N-
Lo
6,
0
i
0H
CD1
N
0
0
0
LCD
I
6
0
z
+} 6rs
S
S
8
s /i
(
(sgUi/j)
xnlu
40
A
8
6
0
~T0
Cl
IE
:0
CT
!
VX I.?
.E
oc
-c
.
71
0
i
)
7Lo
cl
C
0
C£
S
m
,~~4 t4
C',
C?~
Cj
C-j
(sgUwD/T) xnlTA
41
O
X
0
T
v
T
v
T
LS
T
X
o
a
o
o
o
o
D
D
.
o
.
.
.
t
I-H
©
Cl
0C
4>
o
;>
O
C)
o0
CD
l
E-I-
XE
C1)
E
4~)
-a
.
-a
aC)
O
:Iz.
s
QD
c"I
X
=
0
LC),
o)
z
i
't,
..
4
z
C)
Cl
Ln
o
-I-
C
Q
0
ZLO m
CS >
It
UD
*
*
clS
s
OL
(sun/T)
v
>
cli
CA
O
xnlA
42
>
>
S
s
From Figure 2.7 to Figure 2.9, we can see the neutron flux along the
central axis of head phantom after using the lithium-6 filter with different
thickness. Figure 2.10 to Figure 2.12 show the boron dose rate, and Figure
2.13 to Figure 2.15 show the neutron dose rate. All the BPA content and RBEs
used here are the same as in the case without filter.
2.4.3 Lithium Oxide Filter
Lithium metal is a very reactive material that is easy to oxidize. Though
it's sealed in the aluminum clad, we still have to consider the most serious
possible condition where it totally reacts with the oxygen in air and changes
into lithium oxide (Li2 O) completely.
Li2 O has a density as 2.013gcm -3 . But since the lithium may interact with
air gradually, the lithium oxide may not form like a single crystal. It will be
like a pile of powder. In order to compare the beam performance, the number
of lithium atoms per square centimeter for the lithium oxide filter has to be the
same as the lithium-6 filter. Also the total number of lithium atoms can not be
change inside the aluminum clad. Based on these two limitations, the density
of Li20 in powder can be calculated as below.
Density(Li)/MLi=2*Density(Li2 0)/MLi2o
43
O
k.N
I
CD
cJ
CH
0
E
E
.0~
r-M
C)
0
0)
H
~a
00
_
-~
3=
(*
O
Q
0.
©
~
-
o _
o
oo
0
0;
0
O
0
i
cIt
0
i0
i
40
i
(1
F~0
LO
,t
e
Lo
t4
°
i
C
i
9
CiCSj
-4
o
(uiI/&) awstaSO(tfd
44
9o
;
L'
o
Cl
UL)
U)
m
0
0
E-
0
0
U)
U)
as
E
00
v]
90
- 000
.
3
o
-
0
Csl
Q)
0
0/
Cl
{/
C15 c
cli
Cij
-
(uiuI/x!) G4P'emGSOQI 3Rp
45
L6
6
>
W)
00
0
.
00 3
c
Cl
0- l
C1)
m
(n
*H
Ed-
z
0a)
cq~~~~~j
SCr
X
A8
0
+
C ).
S
S l cs O S
(UlUI/AD)) Gca asoU
46
AmJ
SL
0
Cl
02
a*
cl~
0
0
0
4
4+) 4+)
,T,
E
0
)0
x0
-'
(0
v 0= .
U
d) )
4;
cn I C.
z
0
~
=
Z
0
.-
>~C2
0) a
4
10
a* 0
0
0
0
09
0
0
Cl
)
O
C~
-
C)
LI
L)
s
t
N
)
;i )
CD
m, oGSOQ
so(
.(UTW/AD)
(u / -x) OWjP
47
)
)
CO
tf
ZNŽ
C\
o
w
-
t
--- 4
-4
-4
a+
+a) +
a
0)
00 u
o
c9 W
O
0
al
a--
II
s
Q
E
00
._
.~_
.
oo
Iz
i
m~~ei°
4-
a-a
a-a
i
Cl
0z
+L;
C
+
8.:
I
,-
_
_
z
, ,
(u.Wu/Xo)
.,
.
O
8
C 8C C8
C
8
CSl
o
X It,
_
_~~~~~~~~~~~~~~~~~.
GaPŽIOSOG
48
]9L?
O
ua
0
~O
cl
C) t
0
Ln
4-~
4-2
O
I-
a)
o3
eth
I
Z
,2
E
00X
c~0
e ei_C
C
.:
C)
o;
cn t
0=;
"t
c1O
z
es
4-a
4a)
U0
C
o
o)
Cl
00
o~
00+
S4
4
oo~~~~~~~4-
8
8
C8T
C\
88C8
CS
Cd
lt
CA1
(Ui/AD) oqBN asoa ]qg
49
M
MLi=M(6Li)*0.95+M(7Li)*O.05=6.015*0.95+7.016*0.05=6.065 amu
MLi2 o=MLi*2+Mo=6.065*2+16.000=28.130 amu
Density(Li2 0)=MLi 2 o*Density(Li)/2*MLi
=28.130*0.534/2*6.065=1.238
gcm -3
This is the density of lithium oxide we use in the MCNP calculation. And
the result is shown below.
Table 2.3 neutron flux (cm 2 s-1) in air at the surface of the 12cm aperture
(With 8mm Lithium Oxide)
0 - 0.5eV
Flux
Error
2.26E+06 0.087
0.5eV -
0lkeV
10keV - 20MeV
Flux
Error
Flux
Error
1.41E+09
0.014
9.22E+07
0.040
Total
Flux
Error
1.51E+09 0.014
Table 2.3 shows the neutron flux in air at the end of the collimator. Figure
2.16 to Figure 2.18 shows the neutron flux, boron dose rate and neutron dose
rate along the central axis of head phantom.
50
-7
C~
+
0
C
c4'
0
znl
0
Cl
-o
-0
I
E
'
0
=
C).
t
E
00
00
()
Q0
c5
+
3
0
a)
0I
0
L0
10
'00
t
3
Cl
W
S:
a3
W6
Z"
0
C)
L~z
5 O
Z
m
_
8
X
4
5
c
c-,i
c,,ixn ,-4
(s~~wo/[) xnjd
51
LO
8LSW
s
,...4
oo
oo
OC0
..
laH
cI
0
.s0
.,
I
aC)
0
-4
o
s
1
s-
-)
0V)
0
0
10
,cIO
I0
0
~
{s
O4j
04i
0
C\)
oS5
CS
C
('i
cs
-
(uWl/ADXo)
52
-G
LS
S
0
0
0
C\
0
L52
14
0
U
a;
d
10
I-
0V1)
as
0
0
0
4H
©
-p
.. _
-0
0
E
2
c~
.z
00
.-
0X
Cd
0E
C-4
ci2
0
0a
a)
0
ct
So
0
0
0
0
z
00
In
wo
0) 0
00a)
a
U)
mU
,S
1S
Ao- lo
4+1
---
l
O0
l
-4
4 --4
8
l-4
1-4
cI
8
4
-
-4
Cl
8 8
06
(uuu/Ao)alleJ
OSO(J
53
Cl
Can
8
8
8
C-i
d
m
0e.
2.4.4 Effects of Aluminum Clad Around the Lithium-6
filter
As introduced in the discussion of the lithium-6 filter model, in order to
protect the lithium from reacting with humidity, thin sheets of aluminum are
used to isolate the lithium metal from the air. But neutrons will interact with
aluminum and emit gamma-ray that contaminates the neutron beam. The
nuclear reaction is shown below. And the total thermal neutron cross section
for this reaction is about 0.231 barns.
"Al+n >Al+y
To take this into consideration, as little as possible aluminum should be
used. Here we do the Monte Carlo calculation for 8mm lithium-6 filter
without aluminum clad, and later compare this result with using aluminum
clad and see if it affects the neutron beam substantially.
Table 2.4 shows the neutron flux in air at the surface of the phantom, and
Figure 2.19 to 2.21 shows the neutron flux, boron dose and neutron dose along
the central axis respectively.
Table 2.4 neutron flux (cm' 2 s') in air at the surface of the 12cm aperture
(8mm Lithium-6 filter without aluminum clad)
-
0. 5eV
Flux
Error
2.09E+06
0.088
2.09E+061 O.088
0.5eV
-
lOkeV
lOkeV - 20MeV
Flux
Error
.52E+09
0.014
1.52E+091
0.014
54
Flux
9.58E07
Error
0.041
9.58E+07 O. 041
Total
Flux
Error
1.62E±09 0.014
1.62E+09 O. 014
06
E2
cl
Cr'
C\J
a)
u)
E
E
-
o0
E
0
E
0-
00
0:
01
-a~I
0
C
0
x
E
*£
,,
=
. _
-
0~
0)
I
0I
01
0o
[0
z7
01
CN
0
4-
I
4
:n
_
d
O
~
-t
~
-t,
c
cln
c
C-
cli
i'-,n8 ,-
(SgwD/I)
cli u:
xnl{
55
o
o
4
Lr
-£0
o
a1)
-1
0
Ca)
00
00
II
.S
O,
U)
*E
0.)
0.)
Cr'
Cf2
Cr'
-
0
CI
I "I
<
00
X
ii
OS
X.)
0
0Ctl 000
. X0a
0
Cr1
0)
00
0
Cr'
,E E
00~
00
>0
c--
0
0 0
0
00
cj
2
O
m
u
cn
01
0
0:"
C
C
i
0
01
CT
CIA
C\4
C
8
c4
c4$
ci
(U-LW/AD)
088
0
Ncp~
~
j
So
alpm asoa
56
s
Aqm
88
8-8
Un
o5
0
o
."
C.4
0
C)
t~
:> Z
o
LO
E
a2)
Cl)
E
0
4
e0
4
O
0
0
00+
0
cX
CD
ou
_
a
od
oo.
0
"-
ZCIlJ
s..
o
,a)
4-t
00 u
.-0
0
00
+U
Ic(C1888
r"ll
I-
(UIU/AXD)o~?~ Oso0 zJ
57
- I
2.5 Results of Analysis
Below we are going to analyze the results from three aspects, difference
between variant thickness of lithium-6 filter, difference between 8mm lithium
and 8mm lithium oxide, and difference between using and not using
aluminum clad for 8mm lithium-6 filter.
Also based on the results we got, we are going to develop therapeutic ratio
curves and percent change in therapeutic ratio curves. These two curves can
show clearly the difference between various situations and help us analyze the
results.
To get the therapeutic ratio (TR) curve, first calculate the total dose rate
for normal tissue and tumor tissue at different depths with specific BPA
content and RBEs respectively. Then find the maximum dose rate along the
central axis for normal tissue. Finally divide the total tumor tissue dose rate by
the maximum dose rate of normal tissue to obtain the therapeutic ratio. It is
desirable that a TR> 1 be maintained to the greatest possible depth in order to
treat deep seated tumor.
After obtaining the therapeutic ratio curve, we can use the formula below
to get the percent change in therapeutic ratio curve.
TR = Therapeutic Ratio
Percent change = [TR(with filter) - TR(without filter)]/TR(without filter) X 100 %
58
This curve shows the net effect of using the filter. Deep into the head
phantom model, the bigger the percent change is, the more dose that can be
delivered into this depth.
2.5.1 Selecting the Thickness for Lithium-6 filter
Table 2.5 neutron flux (cm 2 s l) in air at the surface of the 12cm aperture
(With and without lithium-6 filters of different thickness)
Energy
0.5eV -
0 - 0.5eV
Condition
Flux
No Filter
5.52E+07
6mm Lithium
Error
0keV
Ratio
Flux
Error
Ratio
0.025
100%
3.01E+09
0.009
100%
3.14E+06
0.077
5.7%
1.67E+09
0.014
55.5%
8mm Lithium
2.21E+06
0.104
4.0%
1.52E+09
0.014
50.5%
1cm Lithium
1.85E+06
0.088
3.4%
1.36E+09
0.015
45.2%
Energy
Condition
10keV -
20MeV
Flux
Error
No Filter
1.15E+08
6mm Lithium
9.60E+07
Total
Ratio
Flux
Error
Ratio
0.034
100%
3.18E+09
0.009
100%
0.041
83.5%
1.76E+09
0.013
55.3%
8mm Lithium
9.41E+07
0.041
81.8%
1.61E+09
0.014
50.6%
lcm Lithium
9.79E+07
0.040
85.1%
1.46E+09
0.014
45.9%
From table 2.5 we can see that, after using the lithium-6 filter, the thermal
neutron flux drops to around 4-6% of that before using the filter, the
epithermal neutron flux decreases to approximately 50%, the fast neutron flux
drops less than 20%. And the total neutron flux drops by about 50%. And it
can be seen that, both the thermal and epithermal neutron flux drops a lot
while the thickness of lithium-6 filter increases. Because the fast neutron
cross section for 6Liis very small, the fast neutron flux doesn't drop that much
according to the increase of the thickness.
59
Iz
4-)
0
o- I
:
L0
C~
~ ~~\
cjl
r
o o
I
!
E
._
VD
O
I
v
U--
oa
I
E
00
._
s:
0
.0
I-
Clt ~
,~tl
'2+.-
.-
.!
5:
02
!
I
CS
E
C0
t
+
C+
-
m
+
+
--
C
+
-oo t(sZw;/I)
+
C
-t
c
xnlA
60
+
C
CAC o
+
-
=
~~~~~~
C
ES
v
a
9
C
-I-
rC
C']
To
110
.
5
E
a)
-a
ti-
0
ocn
z0'A
cn
+u
04
v
!
0
CSCC
i
vo
+
I
.,
-.I
-+2
T
0
.
C"
0C)
+t
02
0°-
C
-+
-C
0
0n
+
CD2
02
-
0f°
0 I
F
CD
HCD
0
02
0
+I
C
02
t~~~~~~~~~~-
CC,,-_
(szwu/1) xnlj
61
C
0")
+t
C)]
02
c
C0
-I'
C>
0
*
CD
C+
0
C_-
cq
.0
._
-T-
0
0
5:
a)
cjn
au
V)
V)
. _
4aC)
5
X.
CL
3
Ea:
aC
9-
C
0
)
z
0
H
0
v
N
zE
Z
'I
~02
41.
w
6
2
I
}
wo
Cl
C
CC:>
CD
(0
C)
_::
_~~~~~~~~~~~~I-
(sZwo/1) xnlA
62
C)oo
Figure 2.22 to Figure 2.24 shows the comparison of neutron flux along the
central axis of the phantom for various lithium thicknesses. Figure 2.25 to
Figure 2.32 shows the comparison of boron dose rate, neutron dose rate,
photon dose rate and total dose rate along the central axis. The neutron dose
has been separated into thermal neutron dose, epithermal neutron dose and
fast neutron dose. We select BPA content as 18ppm in normal tissue and
65ppm in tumor tissue. The RBE for boron dose in regular tissue is 1.3 and in
tumor tissue is 3.8. The RBE for neutron dose is 3.2 and for photon dose is 1.
From all these figures we can see clearly that, after adding the lithium-6
filter, the shape of the beam curves roughly remains the same, but the peaks of
both the total neutron flux and total dose rate after using the lithium-6 filter
have shifted deeper along the depth. Also the magnitude of the curves
decreases proportional to the increase of the thickness of the lithium.
63
b
-4-)
4 ,
. --...
0
tiI
._-.
q-,
t-
0
.
._i
00
1'
0 O2
4
U)
.
Q
s
I
0
i
i
I
(0
|
8
0 m 0E' s0
*-1 0 .
tiI
tiI
;-
0
C
~ I
00
0s
,i-
0
--
i
XSSD
S
SOS
SSS
(UTLH/XD) oWŽI
Gso(
64
~It
C
0C-.
C\
+_~
E
0r"
0
(0.)
4-
S
00
rv:
J
*-
: ._
o_
-z
-t
E
0
0
O 0
-Ip
rJ)
0
m
E
0
00
'C
C0
0)
-
~Tj
x
t--S
$ $88
8
C(
8
C~
Coi
88
U/
(UI/IAD) alem aSOU ]qs
65
o
o
S
°
G
O
O
..
-
I{
V
g::')
O
m
m°
0
t
C)
0
z
O E
o~,
-
00
V)
ao
V)
(A
'-,
.
4-)
-)
a)
C) a)
0
m
C
V)
0
Cxl
-4)
CSCX
Cj
El
t0
0T-~
a.)
_0-q
-4
e4
n(
t
cl
[
ci LOS
mO
03
Cj
(UiuI/XD)
C
OIPH
S
asoG
66
3E
C
S
1°
$
LO
CD
0'
C~l
:
.
E
.
O0
s-
o
a"5
0
+1
D)
V)
A
z
A:
J0
X
00Cz
oml
*H
X
4
S
c-4UI/A9)
¢'w-4
(u.Luz/xo)
alum
-so
HE
osoaI RIfi
67
S
t
CN
I4
.
+4)
r.r
00
>o
C)
o
C
SC.
4-z
@
Cl
-
co
· ~-'
0
C)
Cs
+
i
I
Cr'
0
-o
i
i
08
C)
zu
ON
._
cQ
Q
)
6
ud
LUD
(uUI/xID) 04ea
CA
c6
SO(l Ag
68
a2)
L;
4-)
C\]
+.I
3
'r--
i
s
O
,
+1
-4
p
E
0
E
CO
o8
o
0
-p
C)
;-
soX
t
:
-I
-pH
t
LX
L
~
Lo
cl~
CA
C-3c-i
G
S
(
~~(n Ao) Cow]
CN
UJ
(U l/,!)) aIM asoa
g
I
69
L
0_
m
0.
Ci,
0
._
I
)
"O
0o
.,--l
ZO
:
+
rJ~
C-)
E
(C
o
I
.
C,
i
0
..N
s z
E
lai
Z
I _C:
O'--o
ln
0
OC
I
C:)
C,-]
o
C) )
C)
oc
C
C:
11(
C--~ CD
(ulw/x9)
alp8
70
C
soa
C)
co
6
o
Co
-,
)
eq
H0
O
4r-
L
0
J2
.
.-
0
'r'-I
.)
0
5:
0~~~~~
0
.-Cm
.t
::::S00
3
* -
0S
-
o
rJ~
a)~~~
J
+--l
9
00
0
.
tS
.,-I
30
0
0
.
0
m
CN
t
;) . _)
0
CS
+
r~~
~
-
Oq~~~~~~
°2 CI) 02
-4
_--
02
Go
(uTw/X9)
S?
0S
2
- --
-t
s
a1PN asoa
71
Cj
fIŽ
02
CD
N
0
s
-o
.,--C-
(
C
0)
0
W
In order to decide on the appropriate thickness of lithium-6 filter, we need to
look at the therapeutic ratio and percent change in therapeutic ratio. Figures
2.33 and Figure 2.35 show these two curves. Here we define AD as the depth
at which the therapeutic ratio equals to 1.
From the therapeutic ratio curves we find that, near the beginning of the
curve, the thinner the lithium-6 filter is, the larger the therapeutic ratio is. But
after a depth about 3cm, this trend reverses. Also the curve of 8mm lithium-6
filter is much closer to the 10mm lithium curve than the 6mm one. In the
percent change in therapeutic ratio curve, we can see this characteristic more
clearly. Before 4cm, the percent change of 6mm lithium-6 filter is the biggest,
and 10mm lithium-6 filter has the smallest percent change. After 3cm, this
trend reverses. All along the axis, the 6mm curve has the smallest absolute
value and 10mm curve has the biggest one. Small absolute value means little
difference from the condition without filter. The AD values for these three
curves are 10.04 + 0.06cm, 10.11 ± 0.06cm and 10.15± 0.06cm respectively.
Compared with the AD of 9.84 +±0.05cm without any lithium-6 filter, by
adding the lithium-6 filter, the therapeutic effect has been improved slightly.
72
-Without
6mm Lithium
Filter
---
8mm Lithium --
lcm Lithium
8.0
7.0
6.0
5.0
O
0
c4
r0-
U
a)
a)
34.0
3.0
2.0
1.0
0. 0
-
0
I
2
I .
4
6
8
I
10
12
Depth (cm)
Figure 2.33 Therapeutic ratio for different thicknesses of lithium-6
Boron component, normal tissue: 18ug/g, tumor tissue: 65ug/g
RBE for boron dose, normal tissue: 1.3, tumor tissue: 3.8
RBE for neutrons : 3.2, for photons: 1.0
73
14
+
-
Without Filter
-*-8mm Lithium
6mm Lithium
-*-lcm Lithium
2.0
1.8
1.6
Thickness
AD Range
Omm:
9.84cm
6mm:
10.04cm
8mm:
10.11cm
1cm:
10.15cm
1.4
.o0 1.2
0
_c:
.IU
1. 0
a)
En-
0.8
0.6
0.4
0.2
0.0
9
10
11
12
Depth (cm)
Figure 2.34 Therapeutic ratio for different thicknesses of lithium-6
Boron component, normal tissue: 18ug/g, tumor tissue: 65ug/g
RBE for boron dose, normal tissue: 1.3, tumor tissue: 3.8
RBE for neutrons: 3.2, for photons: 1.0
74
13
I ...
6mm Lithium-6
-
8mm
Lithium-6
---- 1cm Lithium-6
n
NA
Ju. U
25.0
20.0
15.0
10.0
5.0
la,
oo
-CZ
-50
0.
a)
C
-5.0
at
-10. 0
-15.0
-20.0
-25.0
__0 A
-ou.
'
0
2
4
6
Depth
8
(cm)
10
12
14
Figure 2.35 Percent change from zero filter thickness in therapeutic ratio vs. depth for
different thicknesses of lithium-6
Boron component, normal tissue: 18ug/g, tumor tissue: 65ug/g
RBE for boron dose, normal tissue: 1.3, tumor tissue: 3.8
RBE for neutrons: 3.2, for photons: 1.0
75
After analyzing the curves, we find that the 8mm is a good choice for the
lithium-6 filter. In the tails of therapeutic ratio curve and percent change in
therapeutic ratio curve, the values of 8mm lithium-6 filter are much bigger
than the 6mm one. This means the 8mm lithium-6 filter can deliver dose much
deeper into the tissue than the 6mm lithium. The 10mm lithium does a better
job but not that much. The advantage depth of the 10mm lithium-6 filter only
extends 0.04cm beyond the 8mm filter. The 10mm lithium-6 filter decreases
the dose rate more than the 8mm one, so that much more time is needed to
achieve the same dose during the irradiation. Considering both the dose
delivered depth and irradiation time, 8mm is a good compromise for lithium-6
filter.
76
2.5.2 Analysis of the Lithium Oxide Option
Considering the worst situation when all the lithium is converted to
lithium oxide, we hope this won't affect the neutron beam a lot. The analysis
below may support our expectation.
Table 2.6 neutron flux (cm-2 s') in air at the surface of the 12cm aperture
(With lithium and lithium oxide)
Condition
0.5eV -
0 - 0.5eV
Energy
Flux
Error
Ratio
Flux
0keV
Error
Ratio
No Filter
5.52E+07
0.025
100%
3.01E+09
0.009
100%
8mm Li20
2.26E+06
0.087
4.1%
1.41E+09
0.014
46.8%
8mm Li
2.21E+06
0.104
4.0%
1.52E+09
0.014
50.5%
lcm Li
1.85E+06
0. 088
3.4%
1.36E+09
0. 015
45.2%
Energy
Condition
10keV -
20MeV
Flux
Error
No Filter
1.15E+08
8mm Li20
9.22E+07
8mm Li
lcm Li
Total
Ratio
Flux
Error
Ratio
0.034
100%
3.18E+09
0.009
100%
0.040
80.2%
1.51E+09
0.014
47.5%
9.41E+07
0.041
81.8%
1.61E+09
0.014
50.6%
9.79E+07
0.040
85.1%
1.46E+09
0.014
45.9%
Table 2.6 shows the neutron flux on the surface of the 12cm aperture. The
areal density of 6Li atom for both 8mm lithium-6 filter and 8mm lithium oxide
filter is 4.03 X 1022atoms/cm2 . For the 1cm lithium-6 filter, the areal density
of 6Li atom is 5.03 X 1022 atoms/cm2. After the 8mm lithium-6 filter changes
into lithium oxide entirely, epithermal neutron flux decreases about 7% ± 1%.
The magnitude of the neutron flux is between that of the 8mm lithium-6 filter
and the 10
Omm lithium-6 filter.
77
i
-3
C'
.-
C)
M
A
v
X
)
.-
-
i
tC
-
i
.1
x
Q0
-z
+.1
A
es
-C +-.-C
0
I
CCO5.
N
i
C
I
I
i
,-A
Mo
O
0
0
00
-:
oS
t
O
t
9
uc
(sZwa/T) xnl7
78
O
c6
°°
c
°°
CT
r1
~L
CIA
'4
-C
3~
E
a)
c
3
A
C:
&
:>
iJ:
IV
0
©
C)
Z
i
a
0N
z
N+
r-
A
+
,I4-0
.
CO
t cc86
o
C
o
CS
C)
L )
C
C
C
(snwo/T) xnlgj
79
8
N CO
8
3
Cl
;
V:
0
..
2E
X)
4
o
10
Q0
-to
o
+.
+.
I
..t
IlA
0
x
.i
x0
z
00
e~
it
j
.
C
4-.H
3
U-
1C) 1
s
Lo
1
8
9
'-t
D
m
m
cN
(sZuD/I)
', e4
xnIl
80
.
C4
,.C-
-
--
1i
C:
,grCt2
E.
,.H
~z
4.)
0
E+_
0n
NA
i
_rJ
*C
]
f
M
--I'
i:
N~
N\
CI
C) 0
ai
CMI
CI
0
0
0
6.C- :t
C/ ,7
L5ad
N\
N~
0
81
N\
0
CD
c4
e4
CN
CO
0
,-4
cn
acli
Cl
4-
'-
0._
C0
-
O
0
._o.
8
Co
:
X
O
sz
C-)
0c
,0
.1
3
13
C)
0
Cl
0
0
O
S:0 O ;
+J
e4)
O
+
r.~
- _-q-4
*
SC Sr.4
,---
(uTu/xD)
Sq
X
asoG H
4alP?
82
SC']--
SO
- .
6
a)
00
5-
00
-)
a
Cl
C
i~ CD88
Lo
t
C,*
-~
C'
CS
(uluI/XD)
L
C
oal.PŽ asoa
83
S
HoWŽ
8
8
S
e
a)
rr
*Hc
c~
o0,l
3H
C)
, vvH
0
.
l LH
z0
e
0
Cl
*H
0,
0
3
-O
Z
0
-1
U'
-. 4
-- 4
LO
cl~ cl
~Ci
-4
--. i
U-
Ci
-4
Lr3
84
CIl
CIl
0-
Cr1
E
V)
~0
-
o
m
c.
O
0
.0
-a
AI-z/
i
e
.- 4
Cl
2
.-C
ZCt
,.iZ
-(1
4--4
-4-j
I
i
S
El-
Q.
Cl
Inkr5)L
0
(uTW/Ax) a1PJ asoa ]
85
V
-
---I
co
a
-4-
9
Cq
3H
_C)
;>
.,=i:
0
0
0
*,,
0
3
0
1
i
m
I
(~t
~ ~~~
_
0
C
o-z
©
*-
*
cl
-o3
0
M-
i
0,
I
t
I
4C)
7H
0
+ ° Do
c
8
Lo
C0'
C'i
* ,
LO
§
LL
,-4
(UUIn/x9)alet osofI
86
d6
0
oI
H
CIA
I
C\]
-4
H.
J
-I
CC
)
.zH
)
.**?.
i
oc
a
0
C
O
-
s-
It
.
3-
0
0
3.-'
0
a)c+
coR
*-q.a
i
3 I
c
i
I
,t
I
i
.q
4-~
C4--q
+-~
'
_t
CJ
l
cq
C
cq
Co
C'j
cl
C-j
Cl
C ,-
0 L- f Lo5i5
(utu/A9)
CS
a PleNasoG 7gtf
87
O
-
on
*a
.,---
IA
rjm
E
-
V:~
0m
.-H
) r o.
0o
oCD
L~
4-4
CG
cno
m~
*z
_ C -*_
Oo 'au
t
Oa)s
-oOm
- m;
o°
o
c~
O
O
cn (~
:
.00-c;
.4-1
3
*H
,HS
._
m'~
.,-,I
C~
3C
+
N1
,-4
S
(UIU/D)
CS
a6
s
solg
88
8
8
-4i
o
o~
0
n
From Figure 2.36 to Figure 2.46 we can see that, in all the neutron flux,
boron dose rate, phantom dose rate, neutron dose rate and total dose rate
curves, the lithium oxide doesn't change the shape of the curves. It only
reduces the magnitude at all the depths a little bit. This decrease will not
prolong the irradiation time very much. In these figures, we also can see the
other Lithium compounds have roughly the same performance.
It's much clearer to see in the therapeutic ratio curve and percent change
in therapeutic ratio curves shown in Figure 2.47 to Figure 2.49. The tail of the
percent change occur in therapeutic ratio curves when using lithium oxide
instead of lithium is a little higher than the curve when using lithium. And the
advantage depth is 10.13 ± 0.06cm which is also slightly larger than the
lithium curve with 10.11 + 0.06cm but within the error bar. This means even if
the lithium completely changes into lithium oxide, the filter still can help to
deliver the dose into deep distance.
The bad effect of lithium oxide is cutting down the total dose of the beam
along the axis of phantom. If during use, by accident, the lithium-6 filter is
exposed to the air and slowly turns into lithium oxide, and the operators don't
become aware of this, the result will only be a reduction of irradiation dose
rate. This will not be dangerous to the patients themselves. So it's still safe
after the lithium turns into lithium oxide entirely.
89
+With
-
Nothing
+
-u-With
8mm LiF
-- With 8mm Li2C03
With 8mm Li
-
With 8mm Li20
9. 0
c)
w ,,r~
o.u
7.0
6.0
0
.,o
4.0
.0
U0
3.0
2.0
1.0
0. 0
0
l
I
2
4
I
I
6
8
Depth (cm)
10
12
Figure 2.47 Therapeutic ratio for filter with different materials
Boron component, normal tissue: 18ug/g, tumor tissue: 65ug/g
RBE for boron dose, normal tissue: 1.3, tumor tissue: 3.8
RBE for neutrons: 3.2, for photons: 1.0
90
14
+
-- With 8mm Li20
With Nothing
With 8mm Li
With 8mm LiF
-- With 8mm Li2C03
2.0
1.8
1.6
1.4
0 1.2
1.0
0
$2
0l
C0
0.8
0.6
0.4
0.2
0.0
9
9.5
10
10.5
11
Depth
11.5
12
12.5
(cm)
Figure 2.48 Therapeutic ratio for filter with different materials
Boron component, normal tissue: 18ug/g, tumor tissue: 65ug/g
RBE for boron dose, normal tissue: 1.3, tumor tissue: 3.8
RBE for neutrons: 3.2, for photons: 1.0
91
13
- --- 8mm Lithium -
8mm LiF ----- 8mm Li2C03
- 8mm Lithium Oxide ------.
30.0
0
20.0
0
~.
0
-
0
:03
A.,%
x
1
.4. AIX
'
Li2
0
10. 0
Li
LiF
C)
.Id
c
0.0
a)
-10. 0
Material
Li:
AD Range
Li20:
10.11cm
10.13cm
Li2C03:
10. 19cm
-20.0
-30.0
l
0
2
4
6
8
Depth (cm)
10
12
14
Figure 2.499Percent change from zero filter thickness in therapeutic ratio is.depth for
filter with different materials
Boron component, normal tissue: 18ug/g,tumor tissue: 65ug/g
RBE for boron dose, normal tissue: 1.3, tumor tissue: 3.8
RBE for neutrons: 3.2, for photons: 1.0
92
2.5.3 The Aluminum Clad of the Lithium-6 Filter
In case the 0.254mm aluminum clad may attenuate the neutron beam and
add gamma contamination, the results with and without using the aluminum
clad are compared here to see whether it changes the beam character
significantly. The gamma rays generated by the aluminum clad are also
investigated.
Table 2.7 Neutron flux (cm-2s-1) in air at the surface of the 12cm aperture
(With and without 0.254mm aluminum clad for 8mm lithium-6 filter)
Energy
0
0. 5eV
-
0.5eV
Condition
Flux
Error
Ratio
No Filter
5.52E+07
0.025
100%
With Al Clad
2.21E+06
0.104
2.09E+06
0.088
No A
Clad
Energy
Condition
lOkeV -
Flux
Error
0lkeV
Error
Ratio
3.OlE+09
0.009
100%
4.0%
1.52E+09
0.014
50.5%
3.8%
1.52E+09
0.014
50.5%
20MeV
Flux
-
Total
Ratio
Flux
Error
Ratio
No Filter
1.15E+08
0.034
100%
3.18E+09
0.009
100%
With Al Clad
9.41E+07
0.041
81.8%
1.61E+09
0.014
50.6%
No Al Clad
9.58E+07
0.041
83.3%
1.62E+09
0.014
50.9%
As shown in table 2.7, it's clear that for the 8mm lithium-6 filter, whether
using or not using aluminum clad will not significantly affect the neutron flux.
Since the MCNP program doesn't include the gamma emission reaction of
aluminum, we will do a hand calculation here to see whether it influences the
beam gamma contamination.
93
The gamma emission nuclear reaction of aluminum is as,
UAl
+
n
28Al
> 13Al
+
n
+ y(1779.0keV)
Assuming the reactor works at 5MW power level, the thermal neutron
flux before the beam goes through the lithium-6 filter is about 2.2E+8cm -2 s-' .
The thickness of total aluminum covers is 0.02 or 0.05cm. The cross section
of thermal neutron is 0.230 barns. We can use the formula below to calculate
the photon production rate by the aluminum covers.
XY
X
X
I= JARdx = JAncVp(x)dx= JAncpoe-'dx
0
0
= Apo(l -e-naX)
0
Reaction rate density:
R=no(p(x)
Neutron flux:
qp(x)= yoe-n
Area of aluminum:
A= 935.9 cm2
Thickness of aluminum:
X = 0.05 cm
Atom density of aluminum: n = 6.0E+1022 cm
-3
The photon production after calculation is about 1.24E+8s-'. Assuming all
these generated photons emit isotropic away from the aluminum cover and go
through the passage without attenuation, the increased photon flux at the
.
12cm
of
aperture
collimator will
will be 1.1I
.E+5cm2s
the 1800keV gamma
gamma
For the
12cm aperture
E+5CM s 1800keV
94
2
rate, the fluence-to-dose factor is about 1.OE-11Gycm2 . So at the surface of
the 12cm aperture of the collimator, the photon dose result from the
irradiation of aluminum is about 6.6E-5Gy/min, which is much less than
3.9E-2Gy/min the photon dose rate calculated by MCNP with no lithium-6
filter inserted.
Figure 2.50 to 2.52 show the therapeutic ratio curve and percent change in
therapeutic ratio curve. These figures also show no big difference between
using and not using the aluminum clad. This proves that adding little
aluminum into the lithium-6 filter will not contaminate the beam line and will
not affect the therapeutic effect.
95
I *
Without Filter
S 8mm Lithium -a-8mm
Lithium Without Al Clad
9. 0
8.0
7.0
6.0
0
o
:,4 5.0
(D
C)
3.0
.0
0.
C:e
2. 0
1. 0
0. 0
0
2
4
6
Depth
8
10
12
14
(cm)
Figure 2.50 Therapeutic ratio for 8mm lithium filter with and without 0.01 inch Al covers
Boron component, normal tissue: 18ug/g, tumor tissue: 65ug/g
RBE for boron dose, normal tissue: 1.3, tumor tissue: 3.8
RBE for neutrons: 3.2, for photons: 1.0
96
-4Without
Filter
-8mm Lithium -u-8mm
Lithium Without Al Clad
2.0
1.8
1.6
1.4
0 1.2
©
4-)
0.-o
1. 0
C)
.
[_.'
0.8
0.6
0.4
0.2
0.0
9
9.5
10
10.5
11
Depth
11.5
12
12.5
13
(cm)
Figure 2.51 Therapeutic ratio for 8mm lithium filter with and without 0.01 inch Al covers
Boron component, normal tissue: 18ug/g, tumor tissue: 65ug/g
RBE for boron dose, normal tissue: 1.3, tumor tissue: 3.8
RBE for neutrons: 3.2, for photons: 1.0
97
-With
Without Al Clad
Al Clad
30. 0
25. 0
20. 0
15. 0
10. 0
5. 0
a)
beO
0
Q-)
0. 0
0
c0
a)
c,
-5. 0
-10. 0
-15. 0
-20. 0
-25. 0
I
-30. 0
0
2
4
6
Depth
8
10
12
14
(cm)
Figure 2.52 Percent change from zero filter thickness in therapeutic ratio vs. depth for
8mm lithium fitler with and without 0.01 inch Al covers
Boron component, normal tissue: 18ug/g, tumor tissue: 65ug/g
RBE for boron dose, normal tissue: 1.3, tumor tissue: 3.8
RBE for neutrons: 3.2, for photons: 1.0
98
Chapter 3: Engineering Design of the
Lithium-6 filter
3.1 Introduction
After finishing the physics design of the lithium-6 filter, we will go on to
the engineering design. I want to thank Yakov Ostrovsky again. After I made
the draft design, he helped me make several important changes and tutored me
using AutoCAD draw the engineering design. With his modification, the final
design is more practical and professional.
Lithium is a highly reactive and flammable metal. It reacts with the moist
air and turns into lithium oxide. So first of all, the mechanical design of the
filter needs to protect lithium very well and isolate it from the air.
The filter is an optional component of the FCB's collimator. In certain
clinical cases it may need to be installed, and for others, it should not be. In the
case of an emergency (e.g. catching fire) the lithium-6 filter has to be
uninstalled quickly and easily.
We are going to add this new filter component to an existing collimator
that has been used for a long time, any big modification of the existing parts
should be avoided. This is also a challenge of the design.
In order to meet all these requirements, the design is separated into fours
99
parts: core of lithium-6 filter, fixed frame, removable frame and moving rails.
The core of the lithium-6 filter includes lithium-6 metal core, aluminum
clamping rings, gasket, protective aluminum sheets and aluminum filter
housing that is used to protect the lithium.
The fixed frame includes a fixed steel frame and the right hand part
(looking from the patient side) of the RICORAD shielding that helps shield
the neutron particles.
The removable frame includes moveable steel handle and the left hand
part (looking from the patient side) of the RICORAD shielding. Along with
the fixed frame, it provides good shielding from the neutron beam.
Finally the moving rails allow the removable frame to travel along the
fixed part easily.
3.2 Lithium-6 filter Structure
In this section, the detailed mechanical design of the lithium-6 filter will
be illustrated.
3.2.1 The Core of the Lithium-6 filter
As shown in Figure 3.1, there are five components made of aluminum
1100 that make up of the core of the lithium-6 filter.
100
I
c)
>1C
<I
cc
U-
V.
I
> ©
DOc2D
C
LI)
Cn LI2
'F
%
Loi
I
I
I
CDc
o _
Cc
c
CDi
Li
-J
--N
.6 - C - D Ic-~
D3
c
Cl
.
C
F
-
C
C1
C
0)1
0Th 'c
C
>
0
E EC)
aE
,3
u '
C
,._
C0)
CUI
0
C
0
aC
95
U
Co
Oo
o
.m
en
!
(U
._p
101
The central part is an aluminum ring named the filter housing that is made
of aluminum 1100. The inner diameter is 13.59" which is slightly larger than
the diameter of neutron beam at the same position. The outer diameter is
16.30" and the thickness is 0.354" which is about 9mm. The 8mm thick
lithium metal will be located inside this ring.
Two pieces of aluminum (1100) are used as covers to seal the lithium
metal in the filter housing from the outside air. They are 0.01" thick, and their
diameter is 16.30", the same as the outer diameter of filter housing. An
airtight graphite gasket is used between the cover plate and filter housing. The
detail design of seals like the number of clamping bolts and locations was
determined by the manufacturer.
One front and one back clamping ring are used to assemble all these five
pieces together using multiple bolts around the ring. The clamping ring has
the same radial dimension as the filter housing, and its thickness is 0.125".
The front and back clamping rings have to be rotated against each other to use
different threaded holes while assembling. The back clamping ring (looking
from the patient side) has three additional tabs that are used to connect the
core of the lithium-6 filter to the movable frame.
102
3.2.2 The Fixed Frame
The fixed frame consists of 6 components, a steel frame, three pieces of
RICORAD and two cover plates for a groove that will contain a sensor in the
future.
The detailed design of the steel frame is shown in Figure 3.2. It has
roughly the same outside shape as the other parts of the collimator that look
like an octagon. Its thickness is 0.75". Two big mounting holes are located in
the upper-right and upper-left corner of the steel frame. The entire fixed frame
is installed onto the mounting plate of the patient collimator using these two
mounting holes. There is a small groove in the left side of the steel frame that
is reserved for a sensor in the future. The sensor will be used to detect whether
the lithium-6 filter has been installed or not. A piece of steel cover plate will
be mounted on the groove to keep the sensor inside.
The inner shape of the steel frame is a little complicated. In order to
assemble all the pieces together, a lamella of the inner margin of steel frame
has been pared off. The thickness of the remaining part is 0.125". Several
holes have been drilled through this layer to attach the RICORAD frames and
mount the rails.
103
I II
1-
I. .
I
.i
.
i -....
l-
.
¥ "-
i !6
I
'e II--1
I
I
l[
E
0
ct
-0
au
-o
0b,_
. _
I r_
*U
!I
E
e
I qn
f._k
104
Three pieces of RICORAD have been used to shield the neutron beam. Since
the biggest RICORAD plate we can find is not large enough to cover the entire
area, the RICORAD must be made of three pieces instead of one. Figure 3.3
shows the detailed design of the RICORAD shielding. In order to fit into the steel
frame, the thickness of the margin of the RICORAD shielding is 0.625", exactly
the same as the depth of the concave space in the steel frame. The thickness of the
other part of the RICORAD shielding is still 0.750". Several threaded holes are
drilled through on the margin.
The RICORAD shielding has an inner diameter of 16.70", slightly larger
than the diameter of the filter housing which is 16.30". So a clearance will be
reserved between the RICORAD and filter housing to allow air flow around.
Also there is a small groove reserved for the sensor on the left side of the
RICORAD, a RICORAD cover plate can be mounted over the groove.
105
7q
OrEl
,
_Ir
I
H
0
2
6
p.z
{
.9~
-j ; .
Qo
m-,I
6.
I-
ID,
0a
I
0
0
-,
0
b0
._
0
IC
©
._
I
Lo
F
an
O)
O;
To
..
'V
'F
-0
.
Vo
._4
I
I
I
I
-a
uS
I
o0
s
0ot
I
0
u
-I
I-IC
v-j I
I
106
3.2.3 The Movable Frame
The movable frame consists of two components. One movable steel frame
and the right hand part of the RICORAD shielding.
Using the same approach as with the fixed frame, a lamella has been cut
off on the margin of the steel frame as shown in Figure 3.4. The right side of
the RICORAD and the movable parts of the rails will then be mounted into
this surface. A handle will be fixed on the steel frame so that the movable
frame can be opened and closed easily. The handle will be bought from
McMASTER-CARR, item number #1435A41.
The detailed design of the right side of RICORAD is shown in Figure 3.5.
Three square grooves are made around the inner radius to mate with the three
tabs of the back clamping ring of the core of the lithium-6 filter. The outer
edge has also been shaved off a little, so that it can be mounted into the steel
frame.
107
Li
'~ '
I
I
''I
I
I
l I
'T
I'''
'l
'''
+
Nd)
Us
+1
,
0)
IP.
a)
)
a)
E
m
0
a.)
._
-o
X
U
i
I
a)
4-
0
E
.2
v-
.r
Ca
_ _ .
7
1K
108
C4
N ., O
(N,-
~
O~
~
I
I
N
'
'I
t
,
C
~~~
C
1
03OI
0 e Ol O~
o
0
I(,
I
.
,a
-r
o
0
C,
=
o
F
In
(N
U
~E
0C
1o
rs
*
Ik
1
0D
*t
C3
H-
X
)E
N
a
L
US
(N
_
J
_.
_N
(N
(Na
ill- -1
IIl l
•
_ _
--- --- -- ----I ll ----------- ll----- -4l-=-11l 1.
.L1
~~
_
_
(
~
>.
N
_
a
?
U1)
109r
109
Ji
~1
_
~
_
II L't
~~~~~_
@
H0~n
_~~~~~~~~~~~~~~~~~~~~~~~~l
O~~~~~~~~~~~~~~~~~~(
3.2.4 Roller Bearing Tracks
A pair of roller bearing tracks has been used to connect the fixed frame
and the movable frame together, and allow the movable frame to slide
smoothly into and out of the fixed frame.
The roller bearing tracks were bought from McMASTER-CARR. The
item number is #105 7A5 1. The fixed part of the rails will be mounted on the
fixed steel frame. The movable part will be connected to the movable steel
frame. Figure 3.6 (from catalog of McMASTER-CARR)
shows the
mechanical drawing of the rails which is quoted from McMaster's catalog.
These rails have a hold-open detent that holds the rails firmly in place when
it's opened.
The entire weight of the movable frame and the core of the lithium-6 filter
is no more than 50 lbs. The load rating of the tracks is about 88 lbs/pair. This
load rating is base on 50 cycles per week. So the practical load is far from the
design limitation.
110
.
i
I
s
I
7
II
I.[1
;
!. 2
w
UI
10
I
I
i
be
A
Ii1
4
_f1
is
I
'
i
Il
cj
U
,jI
03 X
Ig a
Ii'
ji
i
I
.
at
D
r
_m
a'
I.
X
a)
u
_
8^
.., I
i
vo
-0
-2
a)
O
0
.a_jV)~~t
v
-0
C)
f
u
r
~~~en
.w
8
t
111
Chapter 4: System Installation
Instructions
4.1 Introduction
In this chapter, the details are given for assembling all the components
together and installation of the lithium-6 filter is illustrated.
In order to install the lithium-6 filter conveniently and quickly, we design
and separate the lithium-6 filter system into three parts, one fixed frame, one
movable frame and the core of the lithium-6 filter. The fixed frame will be
mounted at the base of the collimator. After this fixed portion has been
installed, it need not be removed again because it provides a slot for placing
and removing the other parts of the filter. The movable frame will be
assembled separately and connected to the fixed frame through a pair of roller
bearing tracks. The movable part of the filter can then slide in and out like a
vertical drawer. When the movable frame slides out, the core of the lithium-6
filter can be attached or removed from the frame with three screws. Figure 4.1
shows the three dimensional drawing of the filter after it has been assembled.
When the lithium-6 filter is not used, the filter with its aluminum covers will
be put in a special storage container.
112
a)
t4
a)
a)
-
0)
---
Zi~~~~~~~
RJ
C,7)
d~~~~C7
:5
-I
0
0
0
-9
FY
(
(9)
(7)
FV
i
0
(9I-9(9
U
-
Q
-A
o
0
-
Qd
C
Ou
U Y
o
CD
-P
7
0) c
Cd'
->
OJ>
-0L i
(
o
PqU
Co
-E
-u - (y
i
'
s-
0
L(
C1
o (1Q
L ~_
0 0
L9
o0
0+ <0
00
-
E
•
co
113
-
0
q-
CY
U
0
o~
0-
F
-5
©
O
-p
_
-(5 (Ij
4.2 Assembly of the Fixed Frame
The Fixed Frame constitutes six components, one piece of steel frame,
three pieces of RICORAD shielding frames and two pieces of cover plates for
the sensor groove.
The fixed frame were assembled in the workshop. First put the steel frame
on the assembling table, the surface with the groove up. Mount the three
pieces of the RICORAD shielding ring onto the steel frame. The top shielding
ring and bottom shielding ring each needs three steel 10-32UNF flat head
screws. The left shielding ring needs eight screws.
If a sensor has been prepared to detect the installation of the core of the
lithium-6 filter, put the sensor into its groove now. After placing well the
sensor, fix the two sensor cover plates onto the steel frame and left RICORAD
shielding respectively with eight 6-32UNC flat head screws.
Finally a pair of roller bearing tracks has to be mounted to the steel frame.
The fixed part of the rail will be fitted into its slot and fastened with four
10-32UNF flat head screws each.
Figure 4.2 shows the fixed frame after all components have been installed.
114
U)
0
£3
09
ci~~~~~~~~~~~~~~~~0
09
O
09
0
C-)
as
-ci
L
-e
v)
S
01)
CA
Ci)
0t
a)
m
x
Ed
iC.)
-C
'I
u-
-c
33
-
-if
(l
C)
-c7
+1
(3
2
c
GI
+>
>9
J1
C
75ci
Lo
C
Li
as
0~
F-
-
j
7n
60
ci
-H
4-
115
-U
~~~~~~~~~~~~~To
~~~~~~~~~~09
0~~~~~~
4.3 Assembly of the Movable Frame
The movable frame consists of two components, one movable steel frame
and one movable RICORAD shielding.
The movable frame will be assembled in the workshop. First put the
movable steel frame on the assembling table with the concave surface
upwards. Fit the right RICORAD shielding in the slot of the steel frame.
Seven 10-32UNF flat head screws will be used to fasten these two pieces
together.
Turn around the steel frame with the other side upwards. Connect the
handle to the steel frame with two other screws.
Figure 4.3 shows the movable frame after it has been assembled. It will be
connected to the fixed frame when installing them into the collimator.
116
C,
ci
IL
C)
C1)
Ln
cn
ct
C1)
/
I
I
"'
/
C-
4-
41)
_Q
C)
-)
d
>
a)
0
0
'-lK
~0
q.(J3
117
4.4 Installation of the Lithium-6 filter Frame
Prior to installation, the patient collimator base must first be removed
from the mounting plate inside the medical room. Figure 4.4 shows the
components of the collimator that need to be removed in order to install the
filter frame. As the beam facing side of the patient collimator may be
activated from years of use, Reactor Radiation Protection Office (RRPO)
would be consulted prior to removal and will be present during removal.
Fit the fixed frame to the mounting plate using the two mounting holes
on the top corner of the frame. The surface with the roller bearing tracks
should be toward the patient side. Lift up the movable frame with the handle
towards the patient side. Connect it with the fixed frame, which has already
been installed on the mounting pins, through the movable parts of the rails
with four 10-32UNF flat head screws to each rail. Figure 4.5 shows the
Lithium-6 filter Frame after all the parts have been put together.
After the lithium-6 filter frame is affixed to the mounting plate, the
patient collimator will be reinstalled. Once the frame of the lithium-6 filter is
installed, it need not be removed again. The fixed frame will be like a slot for
the movable part. The movable frame will provide a holder for the core of the
lithium-6 filter and permit it to slide in and out like a vertical drawer.
118
VY\,-,
K
K>
K
I
I
i
I
I
7%OCoI/[m otor Base
I 00
I 9000
I 00
00
I 9000000
I 0000000000
00000000000
I 000000000000
I 000000000000000,
I
I
I
900000000000000000s
0000000000
000000 0
0000000000000000000
000000000000000000
000000000000000000 X
0000000000000000000 N 000000
00000000000000 NI11 I I 11
00000000
17
'I
I'--,
\\
I
00
00000,.
X
0 00 C
0 0 0
0 0 0 0 00
f00000
0 0
0 0 00
0 0 0
0 0 0 00
0 0
0000000000
I
I
I
I
I
I
I
K
K
>2
>2
K
:Fooooo0oooooooooooo
0000000000000000000
0000000000000000000
90000000000000000000
00000000000000000000O
00
0
90000
0/0
00
0 O)0O
Of
9
00000000000000000
0000000000000000
0000000 00000000
,p-
'
00000000000
0000000000
I DOOOOOOOOO
90000000
I )000000
0
I 00 0
O
I 9OO 0Y
I ,PI
I
I
I
I
I
I
I
I
Figure 4.4 Remove the components of the collimator downstream of the collimator base
119
'0
a)
-.
/,
E
ca)
U)
a)
E
a)
cc
U)
D
(U
a)
U.
C
a)
E
0cu
0
-
E
0r-
CD
:E
.)
-C
2
U)
MC
U)
I-
\\ c
120
aD
CL
O
r.n
... ,,..3,
-
._
Ed
C)
._C
0
121
4.5 Installation of the Core of the Lithium-6 filter
To install the lithium-6 filter, slide out the moveable part of the frame till
the hold-open stops of the rails are on. The core of the lithium-6 filter is
removed from its protective container, and attached to the frame with three
10-32UNF flat head screws. After assuring that the core of the lithium-6 filter
has been fastened well, push the frame containing the filter a little firmly
along the tracks until it stops. This will place the lithium-6 filter in the correct
position in the beam line. Figure 4.6 shows a diagram with the filter opened
and the core of the lithium-6 filter installed.
4.6 Removal of the Core of the Lithium-6 filter
Figure 4.7 shows a diagram with the filter closed after the core of the
lithium-6 filter is installed. To remove the lithium-6 filter when it is not
needed, slide out the moveable part of the frame till the hold-open detents of
the rails are on. Survey the filter with an ion chamber or GM tube to ensure
that dose rates are reasonably low. Unfasten the three screws which are used
to connect the core of the lithium-6 filter with the movable frame. One person
should hold the lithium-6 filter carefully while another person removes the
screws. The lithium-6 filter is then returned to its storage container. Finally,
the moveable part of the frame is closed to prevent the leakage of radiation
when the beam is on.
122
-0
Q
v0
-
H
.t:
V
E
a)
4
.lQJ
e-~
v1
/
/
/
123
Chapter 5: Safety and Storage of the
Lithium-6 filter
5.1 Introduction
Lithium is a very reactive metal. It's easy to turn into lithium oxide when
it interacts with moisture in the air or begins burning when the temperature is
more than 179°C. Special safety precautions must be considered when
handling of the lithium-6 filter.
When Lithium 6 is irradiated by neutron beam, tritium gas which has a
long half life (T1I 2=12.3 year) isotope will be generated. The tritium gas will
keep accumulating when the beam is on since it will be sealed in the lithium-6
filter housing and not be released unless the aluminum cover is opened. The
tritium would be released if the gas pressure from the 3H 2 and helium gas is
too high. If the tritium gas escapes, it will be dangerous because of its beta
radiation.
In order to prevent the lithium-6 filter form both physical and chemical
damage, a special storage system will be designed to protect the filter while it
is not installed.
124
5.2 Properties of Lithium
5.2.1 Nuclear Properties of Lithium 6
The atomic number of lithium is three. The standard atomic weight of
lithium is 6.94. For lithium 6, the atomic mass is 6.02. There is another
isotope which is lithium 7 with atomic mass 7.02.
Slow neutrons have a high cross-section for interaction with lithium 6.
Assuming the nuclear reaction proceeds only to the ground state of the
product and it can be written simply as
6Li
+ n
>
H
+
+
4.78MeV
For thermal neutron, the incoming neutron energy is negligible. The Q-Value
will be shared by the generated alpha particle and tritium. The thermal
neutron cross section for this reaction is 940 barns. There is also a resonance
when the neutron energy is around 250keV.
Lithium 6 occurs with a natural isotopic abundance of 7.40%. We use
6Li
enriched lithium metal with 95.0% lithium 6 to build the lithium-6 filter in
this project.
125
5.2.2 Physical Properties of Lithium
Lithium was discovered by Arfvedson in 1817. Lithium is the lightest of
all metals, with a density of 0.53g/cm3 only about half that of water. The
crystal structure of lithium is body-centered cubic. Atomic radius is about
1.52A. The table below show's some physical properties of lithium
Table 5.1 Physical properties of lithium
3)
Name
Lithium
Symbol
Li
Density at 293K
0.534 g/cm3
State
Solid
Characteristics
Soft, lighter solid
Color
silvery
Melting Point
453.74 K
Boiling Point
1620 K
Body-centered
Specific Heat
3582 J/kg.K
cubic
at 300K
Structure
Structure
cbca
0K3582
J/kg.K
Ignition Point In
Air
452.5 K
5.2.3 Chemical Properties of Lithium
Lithium is silvery in appearance, much like Na and K, other members of
the alkali metal series. It reacts with water, but not as vigorously as sodium.
The chemical equation can be written as
2Li + 2H2 0 = 2LiOH + H2
126
Lithium will be oxidized quickly when exposed in the air. At 179 °C in air,
it begins to bum and imparts a beautiful crimson color to a flame, but when
the metal burns strongly, the flame is a dazzling white. The chemical equation
is
4Li + 02 = 2Li2 0
5.3 Safety Consideration
5.3.1 Tritium Production
After irradiation by a neutron beam, the
6 Li
contained inside the
aluminum cover will generate tritium. Tritium is a long half life isotope. Its
beta decay half life is about 12.3 years. The amount of tritium produced has
been estimated as described below.
First we need to know the neutron flux in front of the surface of lithium-6
filter. We can measure the neutron flux at the surface of the 12cm aperture of
the collimator without the filter. The neutron flux incident on the lithium-6
filter is estimated to be about four times larger than that in the 12cm aperture.
Assuming the reactor works at 5MW power level, table 5.2 shows the neutron
flux on both of the surfaces.
127
Table 5.2 Neutron flux used for tritium production calculation
0 - 0. 5 eV
0.5eV-
10 keV
10 keV-20MeV
Total
Neutron flux (101 cm2s1) in air at the surface of 12cm aperture
0.0055
0.3000
0.0115
0.3175
Neutron flux (101 cm2s1) incident upon the lithium-6 filter (9)
0.022
1.20
0.046
1.27
For the convenience of calculation, we divide the neutron beam into three
groups, the thermal neutron range from 0 to 0.5eV, the epithermal neutron
from 0.5eV to lOkeV and the fast neutron from 10keV to 20MeV. The cross
section of three groups can be estimated as below with the assistance of data
available in the "RadiationDetection and Measurement").
Group 1 ( - 0.5eV):
The cross section at the most probable
energy (0.025 eV) in the Maxwellian
distribution is taken as 940 barns.
Group 2 (0.5eV-lOkeV):
Assuming the neutron flux drops inversely
proportional to the energy in a slowing
down spectrum. The relation between cross
section and the energy of incident neutron
128
particle is assumed to be as follows.
o(0.5eV) = 250 barns= C 2/0.51/2
C2=250*0.5 u2=17 7
I OkevC
Ikev
fq,(E)o(E)dE
>_
>
epi
x
f _
0.5-E
v
0.5ev
10kev
I0kev
= 177x -
f
E
dE
- C-2 X" 0.5ev
10ke
J,(E)dE
05ev
3
IOkev
dE
C'dE
f- dE
0.5evE
0000
0.5e
E
=50 barns
lnInIol05I oooo
Group 3 ( OkeV- 20MeV):
A constant cross section of 1 barn is
assumed, neglecting
the resonance
0.25MeV
Table 5.3 shows the cross section for all three groups.
Table 5.3 Cross Section a [6Li(n,a) 3H] (barns)
0 - O. 5eV
940
0.5eV-
50
129
10keV
1 OkeV-20MeV
1
at
The tritium production rate can be calculated use the formula below for
each group.
X
X
X
I = fARdx = fAn (cr(x)dx = fAnooe-"dx
0
0
= A o(1 -e
-n x
)
0
Reaction rate density:
R=nocp(x)
Neutron flux:
qp(x) = (poe
Area of lithium:
A = 935.9 cm2
Thickness of lithium:
X =0.8 cm
Atom density of lithium:
n
5.3E+10 2 2 cm -3
Table 5.4 gives the results of the tritium production rate after calculation.
Table 5.4 Tritium Production Rate (1012 s- )
0 - 0. 5eV
0.5eV - 10keV
10keV - 20MeV
Total
0.21
9.88
0.02
10. 11
130
Consider two situations. First suppose that the beam is continuously
operated. The number of tritium atoms at time T can be calculated as,
e -AT )
N(T) = Jle-A(T-t)dt = -(1
And the activity of tritium at time T is,
A(T) = AN(T) = I(1
e
T
)
Half life of tritium: T=12.3 y = 107748 h
Decay constant of tritium: X = 0.056 y-' = 6.43E-6 h-'
Table 5.5 shows the results.
Table 5.5 Activity of tritium when the beam is continuously operated
Time (year)
Activity
(1012
Bq / Ci)
Number (1021 atoms)
10
4.35 / 117.6
2.44
120
10.10/ 273.0
5.65
infinite
10.11 /273.2
5.66
More practically, the beam may only be turned on for one hour per day,
365 days per year. Then the number of tritium atoms produced during the time
of the beam has been turned on for an hour will be,
Nadd =
I~
(1-e11
Ie'-
ton
0A
131
-'t
)
The number of tritium atoms after n days is,
N(n) = -(1
-
e- At' )[1 + e - A24 + e -
2 24
. e (n1)24] = -(1 - e
/I
'
)
1
24
-
-e
The activity of tritium after n days is,
- e-A n' 24
A(n) = AN(n) = I(1-
e-i '° ) __24
Half life of tritium: T=12.3 y = 107748 h
Decay constant of tritium: X= 0.056 y-' = 6.43E-6 h-'
Table 5.6 gives the result after calculation.
Table 5.6 Activity of tritium when the beam is turned on for one hour per day, 365 days per
year
Time (year)
Activity (01 l °Bq / Ci)
Number (1019 atoms)
1
2.31 / 0.6
1.29
10
18.14/4.9
10.15
120
42.07 / 11.4
23.54
infinite
42.12 / 11.4
23.57
From the results we can see that, the total tritium production rate is about
1.01E+1 3s- 1.If we assume the beam will be turned on for an hour per day, 365
days per year the tritium activity will rise to 1.81E+11 Bq (4.9 Ci) after ten
years which is expected to be manageable.
132
5.3.2 Nuclear Heating
Both the alpha generated reaction of 6Li and beta decay of tritium produce
heat. Since the lithium metal will be dangerous when the temperature reaches
over 179 °C, we need to consider the temperature rise caused by the nuclear
heating carefully.
All the calculations below are based on the assumption that the beam is
continuously turned on for an hour. All the heat generated from the irradiation
during the hour will be totally absorbed by the lithium. No heat can transfer to
the environment outside the lithium-6 filter within the hour. This is obviously
extremely conservative.
First let's have a glimpse of the nuclear reactions which produce heat.
Alpha reaction:
3Li +6 n
> H ++ ~~~4
+ 4.78MeV
Beta decay:
H
> 3He + l-
133
+ 18.6keV
Suppose the irradiation time Tonis one hour. We can calculate the power
and energy generated as below,
Power generated from alpha reaction:
Pl =IxQ
1
=1O.llx l01' 2 x 4.78x 106 x 1.60x10- 9
=7.73J/s
Energy generated from alpha reaction:
El =I xTo xQt =10.llxlO0
t2
x x3600 x4.78x10
6
x 1.60 x 10 - ' 9 =2.78x10
4
J
Power generated from beta decay:
P 2 =A(inf) x Q 2 = 42.12x10'°
x18.6x10
3
x 1.60x 10-'9 =1.25xlO-3J/s
Energy generated from beta decay:
E2 = A(inf)xTn xQ 2 =42.12x10 0° xlx3600x18.6x1O
3
x1.60xO-'9 =4.51J
Then the temperature rise caused by the nuclear heating assuming no heat
loses will be,
Temperature increase: AT=(E,+E 2 )/(M*C) = 19.4 K
Specific heat capacity of lithium at 300K: C = 3582 J kg' K'Mass of lithium: M = 0.4 kg
Volume of lithium: V = 748.7 cm 3
Density of lithium: p = 0.534 g cm -3
From the result above we can see that, assuming no heat transfer from the
6 Li
filter, a one-hour irradiation will result in only a 19.4K temperature rise.
Assuming any reasonable level of heat transfer from the filter to the
134
environment, the temperature rise of the filter will for practical purposes be
negligible.
5.3.3 Pressure from Released Gases
Since the aluminum covers of the lithium-6 filter will be seldom removed,
the generated tritium and helium will keep accumulating inside the aluminum
box as gas. The thickness of the aluminum cover is only 0.01". The pressure
from the released gases needs to be calculated carefully to determine if there
is a danger that the gas pressure will cause the gas seal to fail and allow
release of tritium gas.
Suppose all the tritium and helium gas generated from the irradiation
will be sealed inside the clearance between the lithium-6 filter and its
aluminum cover, the beam will be turned for one hour per day, 365 days per
year. In each alpha reaction, one tritium and one helium atom will be
generated. In beta decay, one tritium will change into one helium atom. So at a
given time T, the relation of the number of helium atoms and number of
tritium atoms is,
N(helium, T) = number of helium atoms at time T
N(tritium, T) = number of tritium atoms at time T
N(helium, T)=2*I*Ttota-N(tritium, T)
The gas pressure generated inside the sealed lithium-6 filter is,
135
Pin=nRT/V
Mol number of total gas: n=[N(tritium)/2+N(helium)]/NA
Volume reserved for the gas in the sealed lithium-6 filter:
V(Space)
-= 0.0936
L
NA= 6.02E+23
R = 8.314 kPa L K-Imol1T = 300K
Assuming there is no distortion occurring in the aluminum covers due to the
gas pressure, we can calculate the net force on the aluminum cover as,
F=Pin*A
Area of aluminum cover: A = 935.9 cm2 = 0.09359 m2
The tensile stress on the aluminum cover is,
Tts=FIAts
Thickness of aluminum cover: Dal= 0.0254 cm
Side area of aluminum cover: Ats = 2.75 cm 2 = 275 mm 2
Table 5.7 shows the results of pressure at different time.
Table 5.7 Pressure from the released gases
Time
N
N
(year)
(tritium)
(helium)
1
8.48E+18
8.97E+18
10
6.68E+19
120
1.55E+20
n
TS
P
F
(kPa / psi)
(N)
3.34E-5
0.9/0.1
83
0.3/43.5
1.08E+20
3.57E-4
9.5/1.4
887
3.2/464.1
1.94E+21
5.10E-3
135.4/19.6
12670
46.1/6686.3
(mool)
136
-2
(N mm / psi)
From the results above we can see, assuming that the beam is turned on for
an hour per day, 365 days per year and assuming that all the generated tritium
and alpha particles diffuse out of the lithium into the clearance between the
lithium and aluminum clad as gas; after ten years the net gas pressure will be
9.48KPa (1.38psi), and the tensile stress in the aluminum cover plate will be
3.2 N mm 2 (464psi). The stress likely to be induced from the released gas is
therefore far below the yield strength of the alloy aluminum (1100) which is
about 35 Nmm
-2
or 5.0E+3psi.
5.3.4 Irradiation Levels on the Side of the Collimator
For the safety consideration, we also need to know how much the
irradiation dose is on the side of the collimator. It must be low enough to
protect the patients and operators. As shown in Figure 2.2, we select three
locations to test the irradiation dose, one is behind the lithium-6 filter at the
edge of the RICORAD shielding, one is in front of the lithium-6 filter at the
edge of the collimator base, and the other one at the edge of the lithium-6 filter.
We used the Monte Carlo method to run the simulation and calculate the result.
Table 5.8 gives the dose rate of neutron and photon at the side of the
collimator.
137
Table 5.8 Dose rates at the side of the collimator
Neutron Dose Rate (Gy/min)
Energy
Edge of
RICORAD shielding
Condition
Dose Rate
Error
Without Filter
7.78E-05
0.1690
With Filter
1.01E-04
0.2336
Energy
Edge of
RICORAD shielding
Dose Rate
Error
Without Filter
1.87E-04
0.2638
With Filter
2.90E-04
0.3592
Energy
2.81E-05
Error
0.3300
Dose Rate
Error
2.64E-05
0.3385
6.87E-06
0.0973
Lithium-6 filter
Dose Rate
7.98E-05
Error
0.7508
Collimator Base
Dose Rate
Error
4.75E-05
0.4643
2.45E-04
0.7249
10 keV - 20 MeV
RICORAD shielding
Condition
Dose Rate
Error
Without Filter
3.17E-04
0.4696
With Filter
2.02E-04
0.3568
Energy
Edge of
Dose Rate
Collimator Base
0.5 eV - 10 keV
Condition
Edge of
0 - 0.5 eV
Lithium-6 filter
RICORAD shielding
Condition
Dose Rate
Error
Without Filter
5.82E-04
0.2726
With Filter
5.92E-04
0.2181
Lithium-6 filter
Dose Rate
7.47E-05
Error
0.4036
Total
Lithium-6 filter
Dose Rate
1.83E-04
Error
0.3708
Collimator Base
Dose Rate
Error
1.93E-04
0.4772
1.OOE-04 0.5517
Collimator Base
Dose Rate
Error
2.67E-04
0.3567
3.52E-04
0.5283
Photon Dose Rate (Gy/min)
Energy
Edge of
RICORAD shielding
Condition
Dose Rate
Error
Without Filter
2.94E-02
0.5359
With Filter
4.32E-02
0.5821
0 - 100 MeV
Lithium-6 filter
Dose Rate
2.35E-03
Error
0.3316
Collimator Base
Dose Rate
Error
2.45E-03
0.4492
7.17E-04
0.7568
We can tell from the table that, after using the lithium-6 filter, both the
138
neutron dose and photon dose at the edge of the RICORAD shielding are
higher than the dose at the edge of lithium-6 filter or the collimator base. This
is because inserting the lithium-6 filter will cause a large amount of back
scattering and then increase the dose behind the lithium-6 filter. But the
increase is not so big, roughly lower than 2 times of the dose without using the
lithium-6 filter which is still acceptable.
With a large portion of RICORAD shielding component around the
lithium-6 filter, the neutron dose has been effectively reduced at the edge of
the lithium-6 filter compared with the neutron dose at the edge of the
collimator base which is built up by steel only. On the other hand, with less
steel shielding, the photon dose at the edge of the lithium-6 filter is larger than
that at the edge of the collimator base but still acceptable compared with the
photon dose before using the lithium-6 filter.
It's also obvious that, after using the lithium-6 filter, the epithermal
neutron dose has increased corresponding to the rise of the portion of
epithermal neutron in the beam after passing through the lithium-6 filter. We
can conclude from the table that it will still be safe to operate while the
lithium-6 filter is installed.
We can conclude that the dose rates with the lithium-6 filter are not
expected to be a problem.
139
5.4 Storage System
In addition to the aluminum covers, a special storage system has been
designed to protect the lithium-6 filter from both physical and chemical
damage. The storage system consists of three components, one zip-press bag,
one foam box and one steel case. Figure 5.1 gives a rough picture of the
storage system.
The lithium-6 filter will be stored in a special or possible two plastic
zip-press bags containing a valve. A vacuum can be drawn on the bag through
the valve to remove trapped air and then nitrogen gas can be introduced into
the bag through the same valve to protect the lithium-6 filter from humidity or
oxygen in the air.
The vacuum bag containing the lithium-6 filter with its aluminum clad
will be put into a small box with foam padding, which protects it from shock
or other damage.
The foam box will be stored in a strong steel case that is a corrosion
resistant watertight enclosure. This metal box also helps to protect the
surroundings if the lithium-6 filter were to burn despite the many precautions.
The lithium-6 filter and its storage container will be labeled to indicate the
hazardous contents. A class D (metal) fire extinguisher will also be available
in the reactor building near the medical room and where the filter is stored.
140
S
C)
b-0
C.
~ ~
z
0
2
tz_
E
-C
st'.
(1)
-C
4.
|
;>
rV)
T--oI
N
II
V
a)
U
C)
141
CI~~~~~f
wo
Chapter 6: Beam Performance with
Lithium-6 Filter
6.1 Introduction
In order to verify that the designed lithium-6 filter truly improves the
epithermal neutron beams from the FCB, a mixed field dosimetry method was
used to measure the photon, thermal neutron and fast neutron dose. The
thermal neutron flux is measured with gold foils using the cadmium
difference technique. The photon and fast neutron doses are determined with
two different ionization chambers, one neutron insensitive and one sensitive
to neutron and photon, using the dual chamber technique. The thermal
neutron and boron doses are determined by the kerma factor method based on
the measured thermal flux9 )' 10)
During the experiment, the 12cm beam with 3cm air gap collimator was
used. The medium ellipsoidal water filled phantom was used to simulate
human head. The Reactor power was between 3.5 - 4.0 MW. The FCB
converter power is 83kW at 5MW reactor power.
I would like to thank Dr. Kent J. Riley and Dr. Peter J. Binns again. They
helped me do the dosimetry measurement and data calculation. I am
presenting their results here and do a little further analysis.
142
6.2 Methods
6.2.1 Thermal Neutron Flux
Two sets of gold foils are weighted between 10 to 40 mg. One set is bare
gold foils. The other set is covered with cadmium. Each set of foils are
irradiated separately. The bare foils are taped on a thin plastic rod every one
centimeter while the cadmium covered foils are positioned every 2 cm. The
further distance between the cadmium covered foils can reduce the flux
depression at the foil from each other's cadmium cover. Then the rod is
inserted into the centre of the head phantom for irradiation9
) 10)
The reaction happen in the gold after irradiated by neutron is
Au- 197(n, Y )Au- 198*. The new generated Au-198 emits 411 keV photons in
95.5% abundance. Its half life is 2.696 days. The activity of the gold foil is
measured by HPGe detector.
The thermal neutron flux can be estimated by its saturated activity of the
gold foil per unit mass with the equation below,
0
MW
Asa,
A
m
here (p is the neutron flux averaged over the gold foil surface. MW is the
molecular weight. Av is Avogadros number.
is the microscopic activation
across section averaged over the spectrum. We use the 2200 m/s cross section
143
which is 98.8 barns. m is the mass of the gold foil. Asat is the saturated activity
of the gold foil, which can be calculated with the equation below,
A
s(1
A2C
e-At"
)(e- t" e-At2)
-
where X is the decay constant of Au-198. C is the net counts between
measurement time t and t 2, where the start of the irradiation is time zero. to is
the irradiation time. c is the overall counting efficiency.
Because bare foils are activated by both thermal and epithermal neutron
fluxes, while cadmium covered gold foil can only be activated by epithermal
neutrons. We can calculate the thermal neutron flux with the following
equation,
¢2200
As,) bare--cd(atCd]
-F (Aa
2200 _
m''~
-- MW R1
(/st
Ao2200
m
) Cd]
m
Here we us the 2200 m/s absorption cross section as the average thermal
neutron cross section. Fd is a correction factor used to account for the
absorption of some neutrons with energy above the cadmium cutoff by the
cadmium covers.
6.2.2 Photon and Total Neutron Dose Rates
The photon and fast neutron doses are determined by the dual ionization
chamber technique. One is brain equivalent ionization chamber filled with
144
tissue equivalent gas. This kind of chamber measures both photon and neutron
doses. The other is graphite ionization chamber flushed with CO2 . The
graphite ionization chamber can only detect the photon dose and is quite
insensitive to the neutron dose. The responses of the tissue equivalent and
graphite ionization chambers in the mixed neutron and photon field9 )' 10)can
be express as the equation below,
QTE= hD, +kD,
=
OCG
UDY+ kUD
,1
are the corrected currents of the tissue equivalent and graphite
where QTEQCG
ionization
157,/ n are
chambers.
the photon and neutron dose rates. ht , kt are
the fractional response of tissue equivalent ionization chamber to photons and
neutrons of all energies. hu , ku are the fractional response of graphite
ionization chamber.
Usually ht and hu are set to unity. Using the data from the calibration of the
ionization chambers, the equations can be changed to,
k, x
D = ECF
7
Cal~
CalT
E x QTE
C x QCG- k X
GAtE
CG
GA
GAE
GAg
~~-k
CalTE X Q-Ca
.r
D n = ECF
GATE
C
x QC
GACG
where ECF is the electrometer calibration factor. CalcG and CalTE are the
145
tissue kerma calibration coefficient for graphite and tissue equivalent
ionization chambers respectively. GACG and GATE are the gas/air current
ratios determined experimentally during the calibration of the graphite and
tissue equivalent ionization chambers.
6.2.3 Thermal Neutron and Fast Neutron Dose Rates
There are two major interactions of thermal neutrons in the brain. One is
N-14(n,p)C-14, the other one is H-l(n, ¥ )H-2. For the second one, the dose
from the prompt gamma has been included with the photon dose above. The
recoiling deuterium does not have enough energy to ionize and can be
neglected for adding to the kerma9 )' 10).The dose from the first interaction is
determined using the kerma factor method as the equation below,
DN-14
where
Fn
bN-1 4
Fn¢
is the thermal neutron dose from the N-14(n,p)C-14 interaction.
is the kerma factor.
is the thermal neutron flux we measured above. The
B-10 dose can also be easily calculated with the kerma factor method. The
N-14 kerma coefficient is 1.72E-11 cGy cm2 . The B-10 kerma factor is
8.66E-12 cGy cm 2 g 1 g-1.
The fast neutron dose will then be obtained by subtracting the thermal
neutron dose from the total neutron dose.
146
c0i
0
o
5:
x
~r
E
C
i
CO
00)
C
E
.-
o
Cl
3
cil
CA
-0x
V
Cs
S
V
E
a-
I-cu
._
0
C1
0
0+
00
C)
0±
00
0+
0
0
C
C
C
CC
0+
00
CD
0+
00
C)
s
C)
(sZwD/ I)
0+
0
0
C
CD
=
xnl,
147
C,
O
0+
00
CD
6
0
-e
14)
oi
C
CN
0_C
4e
0
0C-)
0I
C
co
O
Q~~~~4
O
r~~~~~~~l
E
0
CC
0
00
-C3
C=
C
o
00
£o
-e
C-
C'-
0~
N
C_)
X
0~
C
°
es~
,0
°-
0
o
-C,
CD
C/]
+<
o
C)
0"
C)
C
C,
0
C-C-:
C0
(SZoD/I) xnlA
148
CC01
C--0C- CCD0
*
C)
*
E
>~~
~~~~~~~~~~~~u
J...
6.3 Result
6.3.1 Without Filter
Figure 6.1 shows the 2200m/s neutron flux along the central axis of the
head phantom. Figure 6.2 gives the comparison between measured 2200 m/s
neutron flux and simulated thermal neutron flux with energy between 0 and
0.5eV. In Figure 6.2, both fluxes are normalized by setting the maximum
fluxes of the curves to unit one. During the measurement, in order to avoid
disturbance to the flux between the cadmium covers of the foils, we can not
have as many detection spots as in the Monte Carlo simulation. So the curve
of measurement is not as smooth as the curve of simulation. The difference of
two curves at the shallow depth is because of the difference between 2200 m/s
flux and thermal flux.
Figure 6.3 and 6.4 show the different dose rates. We can see that the
shape of the curves is almost the same as shown in the computer simulation.
Figure 6.5 gives the comparison of normalized total tumor dose rate between
measurement and simulation. The curve of measurement reaches the apex at
the depth about 2.4cm which is a littler lower than the curve of calculation
with its apex at about 2.8cm. There are potentially two reasons for this
difference. One is the FCB model we used in calculation may not perfectly
agree with the real facility. The other reason is that some nuclear reaction such
as gamma emission reaction of aluminum is not included in MCNP code.
Since these differences are consistent in the calculation, they do not affect the
design of lithium-6 filter significantly.
149
C"
I
C/)
0
0
O
(+)
4-4
0I.1.
C/)
c
0
0
,-
4-,
z
..
~.).*-
C/C
E
C)
o..
v
C)
=o
CT/
4-)
0
0
C
2
C")
Lri
CN
4-:
i
c
E
It
I)
CoT")
IC
1
-; I
i.
LO
C:
-.
o
C;
(UTWl/Aq)
81BM asoo
150
]gm
O)
O
o,-)
ca)
.)
.)
'm.
< .
-.
"1l
,)
ood
c
C
E
00
C)
;::
0
f.0
X0
U)
©
C/)
U)
.--
~ ',0
.-
oc~~.
U
0
to
U
CD
s
00
0
C)
c0
.-
*-
;0
s-
O
0
'
i
0
p
o
a) Es-_ 0tom-a
.- _ )
C
I
O0
U)
:::I
Cl=
U)
%
E--'
En
0-
i-
.
(uIm/AX)
U.)
11
oIpg
T
'11
C)
M
asoa
C"]
CIA
UN
151
-4
CD
O<
°
0
0
0
0
0
0
E
0
©
U
0
Q)
3
0
0
0
S
0
0
S
0
S
0
0
0
0
0
0
c/C
0
0
0
-o
0
0
CDi
N
0O)
0
0
E0)
0
O
CA
0rfi
Cl
Q)
0~
-O
cD
S
0
0
0
a
H
In
c1)
O6
o,.
C
-0
Cl
"t
00
6
6
(UlW/AO) alum asoc
152
6
C
v
C")
0
00
o
oo
. 3x0
0
-S
-4-1
)
E2
a
O
cn
0
I
0
*
0
0+
0
cs~
('~
cle
0T
0
0+
0
0+
0
m
C)
m
C)
C'
C
C
-4o
C-
cl
-4
0T
0
a)
CD
(sZw3/I)
C
0+
0
co
CDO
cc
0+
0
0
o
xnlA
153
0
0~~~~~
o
CD
cS
S
._
o
cJI
a
,]
-
oo
r
. -
fl.
a)
..
,,..
a ~
oo0. E
E
n
o.
a)
)
'
CIm )
C
(I..
o
C_
a.)
.
o "
C)
0c
-x
.~
.I
~
_
I
X.0
I
.
'
C,
6
(0
00
o
(ULW/AUJ)I I -1
- - -
04Ud
- - - -
OuUU
0
1.11 -
154
Ud
C
C; 6
m
C)
Cl)~
0
E
0
0
C)
Do
C)
~ ~
o
00o~~~~~~o
03
o0
Cl)
C2
C)
cr.
E
00
E
0i )
C)
CZ
E
C/
..
C:
00
O
"Ci) "
(a)
-,cCi
E moo
O*
0
0
,...-m.
H-
C
ooO
.--~
o
3
']
Cl
.
o
C1)
C)
0
Cl)
::
U)
F-
C
O
tct
C-
D ,4
C.-i
o
(ufw/AD)
OIP
O
o
osoU
155
6.3.2 With 8mm Lithium Metal Filter
Figure 6.6 presents the 2200 m/s neutron flux at different depth. Figure
6.7 and 6.8 show the different dose rates along the central axis of head
phantom. Figure 6.9 and 6.10 give the comparison between measurement and
simulation. Figure 6.9 is the normalized thermal neutron flux. Figure 6.10 is
the normalized total tumor dose rate. The results of measurement are
consistent with the results of computer simulation except the curves of
simulation reach their apex a little deeper than the curves of measurement.
The intensity of incident neutron beam is not precisely constant during
the measurement with and without the lithium-6 filter which is true in the
computer simulation. But it had been maintained in approximately the same
level by the control the reactor power. The same as shown in the simulation,
after using the 8mm lithium-6 filter, both the neutron flux and neutron dose
rate drop about 50% at the depth where they reach maxima. This also shows
the consistence between measurement and simulation.
156
a))
!
La
00
0
._
E
E
00
-
0©:
.)
v c.,.
._
cn
0
a)
0
rE~
0a)0
o
a)
0~
a)
E
a)
cD a)
eN-
C0
Q0U)
0
cn
a)X
U)
o
0t)0'
0
0
0D
O
0
U)
a)
C
0
Oa)
sr
t-
C)
o
o,
-4
00
-4
CD
o
C)
0
6
o
.t
C;
6;
(SZUwD/) xnl
157
C:,
q
6
C)
C
C
Q
=
EI
E
01
0~
C
ES
00oo
:
C
O
0
cS
O
E
-C0
CD
C)
©
i
_I
O
Om
o
,
-
i
I
0
°
0=
~0
C')
t
0
0
SC/
0
O
_H
._
.
oT
Oo
o
(UWf/3)
6
-6
1 asoa
158
6
L
=
6.3.3 Data Analysis
Figures 6.11 and Figure 6.12 show the comparison of the therapeutic ratio
between using and not using the 8mm lithium-6 filter. The curve presents that,
up to a depth of about 3cm, the therapeutic ratio is higher when we do not use
the lithium-6 filter. But after that, the trends reverse. In Figure 6.12, we zoom
in the tails of the curves. It shows that the advantage depth is 9.3 ±0.1cm
when we do not use the lithium-6 filter. After using the filter, the AD extends
0.6cm deeper which is 9.9±0.1cm.
Recalling the results of computer
simulation, the advantage depths are 9.84cm and 10.11cm for with and
without lithium-6 filter respectively. Both are a littler larger than the
measurement.
The results of measurement show that after using the 8mm lithium-6 filter,
the apices of both neutron flux and total dose rate have been pushed deeper
than the apices without the filter. The advantage depth also has been improved.
The AD depth is 0.6cm deeper with lithium-6 filter than without the filter.
Meanwhile, the total dose rate drops significantly after we installed the
lithium-6 filter. It is only about 50% of the total dose rate without the
lithium-6 filter which means we need longer time to reach the same clinical
dose with the 8mm lithium-6 filter.
The results of measurement are consistent with the results of Monte Carlo
simulation. The therapeutic effect has been improved after adding the 8mm
lithium-6 filter.
159
CN
-
E
!-
C)
f
i
CO
0
E
O0
0+
I
I
%
E . *-
00~~~~~~~
o)
I
00
I
~~~
I
I
i
i
~~
-~
"00
i
I
i
i
I
i
i
I
731
+
-
C)
I
i
i
i
Ii
i
o
0o
O
0
O
O
O
cc
oTIU
OTlnadujaq£
OIPDI11Dd
160
160
0
3
"-'
C)
*-
O
2) ckl *
._o , . .
i
i
i
v
0~~
C)
4-
I
i
-c
o,,.C)
0
LL
1-
-
-~~~~~~~~~~~
E
.,;
s
E
0
C)
0
O0
_~ 5
=z3
0
c~bl
*-
C ) 0> __
5: ' -c :
0.
oI~~~. 0 ._- c=~
_
103
00
O
+
~0
CS1
+1
0
i00
ii;
c~~i
C-i
o ll
-4
DiTnad~aq1
161
6
6;
m
co~
3 ,
3 _o
Chapter 7: Conclusion
7.1 Summary
Based on the Monte Carlo calculations, an 8mm thick lithium-6 filter
was selected to be the optimum filter for the FCB. It was shown that the
system is well shielded within the steel and RICORAD frames. The aluminum
covers of lithium-6 filter do not contaminate the neutron beam significantly.
Within the carefully designed covers, the lithium metal is isolated from the air
to keep the system safe. The mechanical design of the filter system makes it
easy to install and uninstall the filter during operation. The results of
dosimetry measurement proved that successfully using the new designed
lithium-6 filter, the therapeutic effect of BNCT in FCB can be noticeably
improved. The advantage depth has been increased by 0.6cm from 9.3cm to
9.9cm after applying the 8mm lithium-6 filter.
7.2 Suggestions for Future Work
At present, by optimizing the system, only one 8mm thickness lithium-6
filter had been made. According to different clinical situation, we can install
or uninstall the lithium metal filter respectively. To add more flexibility, and
optimize the therapeutic effect for each individual clinical case, we may
162
consider design a series of different thickness lithium-6 filters to satisfy
different situation.
The system is manipulated completely manually now. The operator has
to install and uninstall the filter all by hand. One automated sub-system can be
added. This had already been considered in the existing system. A special
sensor device can be designed and assembled within the filter. It gives the
signal that if there is one filter is installed, and which filter is in the collimator.
The operator can detect this signal directly on the control panel without
mistake. The operator may even be able to slip out the filter automatic out of
the medical room during an emergency.
163
References
1. Glenn F. Knoll, "Radiation Detection and Measurement", Third Edition.
2. O.K. Harling, K.J. Riley, T.H. Newton, B.A. Wilson, J.A. Bernard, L-W. Hu, E.J.
Fonteneat, P.T. Menadier, S.J. Ali, B. Sutharshan, G.E. Kohse, Y. Ostrovsky, P.W. Stahle,
P.J. Binns, W.S. Kigern III, P.M. Busse "The Fission Converter-Based Epithermal Neutron
Irradiation Facility at the Massachusetts Institute of Technology Reactor", Nulcear Science
and Engineering, Volume 140, Pages 223-240, March 2002.
3. Syed Jameel Ali, "The Design, Optimization, and Construction of a Patient
Collimator for the Fission Converter Beam", Master thesis, Massachusetts Institute of
Technology, February 2001.
4. Michelle N. Ledesma, "Medical Room Design for a Fission Converter-Based
Boron Neutron Capture Therapy Facility", Master thesis, Massachusetts Institute of
Technology, August 1998.
5. J.Benczik, T. Seppala, M. Snellman, H. Joensuu, G. M. Morris and J. M. Hopewell,
"Evaluation of the Relative Biological Effectiveness of a Clinical Epithermal Neutron
Beam Using Dog Brain", Radiation Research 159, 199-209 (2003)
6. Jeffery A. Coderre, Phd, Michael S. Makar, B.A., Peggy L. Micca, B.S., Marta M.
Nawrocky, B.A. Hungyuan B. Liu, Phd., Darrel D. Joel, Phd., DVM, Daniel N. Slatkin,
M.D. and Howard I. Amole, Phd., "Derivations of Relative Biological Effectiveness for the
High-LET Radiations Produced During Boron Neutron Capture Irradiations of the 9L Rat
Gliosarcoma in Vitro and in Vivo", I. J. Radiation Oncology Biology Physics, Vol 27, pp
1121-1129.
7. Kent J. Riley, Peter J. Binns, Dennis D. Greenberg, Otto K. Harling, "A physical
164
Dosimetry Intercomparison for BNCT", Medical Physics, Vol. 29, No. 5, May 2002.
8. Cecil Jensen, Jay D. Helsel, "Engineering Drawing and Design", Fourth Edition
9. Ronald D. Rogus, Otto K. Harling, Jacquelyn C. Yanck, "Mixed filed dosimetry of
epithermal neutron beams for boron neutron capture therapy at the MITR-II research
reactor", Medical Physics, Vol. 21, No. 10, October 1994.
10. K. J. Riley, P. J. Binns, O. K. Harling, "Performance characteristics of the MIT
fission converter based epithermal neutron beam", Physics in Medicine and Biology, 48
(2003) 943-958
11. Documentation for MCNP4C2 Monte Carlo N-Particle Transport Code System,
Los Alamos National Laboratory, Los Alamos, New Mexico
12. O.K. Harling, K.A. Roberts, D.J. Moulin, and R.D. Rogus, "Head phantoms for
neutron capture therapy", Med. Phys. 22(5), May 1995
13. http://www.scescape.net/-woods/elements/lithium.html
14. "Standard Test Method for Determining Thermal Neutron Reaction and Fluence
Rates by Radioactivation Techniques", Annual Book of ASTM Standards, Vol 14.02,
Published August 1998.
15. Waldemar Seton, "Technical Committee on Combustible Metals and Metal
Dusts".
16. Kemt J. Riley, Otto K. Harling, "An improved prompt gamma neutron activation
analysis facility using a focused diffracted neutron beam", Nuclear Instruments and
Method in Physics Research B 143 (1998) 414-421.
165