Therapy Educational Course - Impact of the National Institute of Standards and
Technology (NIST) on Radiation Dosimetry in Medical Physics
M Mitch1, M McEwen2, R Tosh1
(1) National Institute of Standards and Technology, Gaithersburg, MD
(2) National Research Council, Ottawa, ON, CA
Tuesday
9:00:00 AM - 9:55:00 AM
TU-B-224-1
Room: 224
The Calibration Chain:
Role of BIPM, PSDLs and ADCLs
M. McEwen
National Research Council Canada
2011 COMP/AAPM Joint Annual Meeting
Calibration Chain
PSDL
(NIST or NRCC)
Calibration
laboratory
(SSDL/ADCL)
Clinical beam
Dw
ND,w
cal lab
Q
ND,w
clinic
Q
Dw
clinic
What is a PSDL?
• Primary Standards Dosimetry Laboratory
• The national laboratory designated by the
government for the purpose of developing,
maintaining and improving primary radiation
standards
• North America:
NIST (Gaithersburg) or NRCC (Ottawa)
• In some countries there is no PSDL
Primary Standard
• Instrument that allows the determination of a
dosimetric quantity according to its definition
• Preferably with a direct path to SI quantities
not involved with ionizing radiation
• SI base unit: meter, kilogram, second, ampere,
kelvin, mole, and candela
• SI derived units: J, Gy, etc.
Primary Standard - Example
Exposure
X
Q
m V
dQ
dm
Free air ionization chamber
or cavity ionization chamber
Measurement of current,
time
air
Length, density
Density = mass / length3
No ionizing radiation needed to
determine the volume
Green: SI base unit
orange: SI derived unit
Air Kerma
Does require
the use
of ionizing
radiation
K air
Q
m V
W 1 dQ Free air ionization chamber
e 1 g dm or cavity ionization chamber
Measurement of current,
time
air
Length, density
Density = mass / length3
W/e is not a fundamental constant but has
“special” status with agreed value – 33.97 J/C
The “Bureau International des
Poids et Mesures” (BIPM)
• BIPM = International laboratory created by the
metre convention; has an Ionizing Radiation
Division
• Role: “The task of the BIPM is to ensure
world-wide uniformity of measurements and
their traceability to the International System
of Units (SI).”
• www.bipm.org
•
Through the BIPM intercomparison program the NMI
can declare its calibration and measurement capabilities
(CMCs)
• Key comparisons and database (http://kcdb.bipm.org)
• Mutual recognition arrangement (MRA)
What is an SSDL?
• Secondary Standards
Dosimetry Laboratory
• SSDL = Laboratory designated
by a competent national
authorities to provide the
necessary link in traceability
of radiation dosimetry to
national/international
standards for users within
that country
SSDL network
 IAEA has direct traceability to BIPM but linked to national PSDLs through
regular comparison programs
 IAEA lab calibrates some SSDLs (others have direct traceability to BIPM)
 IAEA also operates TLD audit service for users
What is an ADCL?
• Accredited Dosimetry Calibration Laboratory (the
SSDL equivalent in the US but different…)
• Accredited by the AAPM
• Provides calibrations to users for instruments and
radioactive sources for dosimetry in radiotherapy and
diagnostic imaging
• ADCL network links the ADCLs to the NIST with
oversight by the Calibration Laboratory Accreditation
(CLA) sub-committee
11
The AAPM CLA subcommittee:
what does it do?
“The Subcommittee’s task is to accredit, supervise and
maintain the highest level of confidence in the quality
of the ADCL system, with sufficient capacity in the
system to prevent undue delays in satisfying the
membership’s calibration needs.”
Main forum for discussion of issues relating
to calibration of ion chambers and
brachytherapy sources
NIST has permanent membership of the CLA
12
The CLA:
Develops criteria
Accredits laboratories
Carries out assessment visits
Monitors performance
Makes recommendations
13
Summary
• Clinical dosimetry in NA is traceable to
national standards through a chain connecting
clinics to ADCLs and PSDLs (NIST and NRCC).
• National standards are declared equivalent via
key-comparisons organized through the BIPM
• In the, ADCLs are monitored by the CLA
subcommittee of the AAPM
Thank You
Standards in Radiation Dosimetry
for Medical Physics
Michael G. Mitch, Ph.D.
Leader, Radiation Interactions and Dosimetry Group
Ionizing Radiation Division
Physical Measurement Laboratory
National Institute of Standards and Technology (NIST)
Materials Measurement Laboratory (MML)
Physical Measurement Laboratory (PML)
Engineering Laboratory (EL)
Information Technology Laboratory (ITL)
Center for Nanoscale Science and Technology (CNST)
NIST Center for Neutron Research (NCNR)
Ionizing Radiation Division, PML
Radiation Interactions and
Dosimetry Group
Neutron Interactions and
Dosimetry Group
Radioactivity Group
Radiation Interactions and Dosimetry Group Staff
Group Leader (Supervisory Physicist)
Group Secretary
7 Physicists
1 Research Chemist
1 Physical Scientist
2 Electronics Technicians
Research Associates
Radiation Interactions and Dosimetry Group Strategic Element
Develop dosimetric standards for x rays, gamma rays, and electrons
based on the SI unit, the gray, for homeland security, medical,
radiation processing, and radiation protection applications.
How do we know what to do?
• Council on Ionizing Radiation Measurements and Standards (CIRMS)
• Consultative Committee for Ionizing Radiation (CCRI)
• National Academy of Sciences review panel
• Feedback from colleagues and calibration customers
• Membership in professional societies and committees
How do we know what to do?
AAPM committees and task groups with NIST members:
Calibration Laboratory Accreditation Subcommittee (CLA)
Therapy Physics Committee (TPC)
Brachytherapy Subcommittee (BTSC)
Low-Energy Brachytherapy Source Dosimetry Work Group (WGLEBS)
TG-136 (Induced Radioactivity Produced by Radiotherapy Accelerators)
TG-138 (Uncertainty in Brachytherapy Dosimetry)
TG-144 (Y-90 Microsphere Brachytherapy)
TG-145 (Quantitative PET/CT Imaging)
TG-200 (CT Dosimetry Phantoms)
Calibration and Measurement Capabilities (CMCs)
Quantity
Parameter
Reference
Standard
Key
Calibration
Comparison Service?
BIPM.RI(I)-
Air Kerma
x ray (10 to 50)
mammography
x ray (50 to 300)
Cs-137
Co-60
free-air chamber
free-air chamber
free-air chamber
graphite cavity chamber
graphite cavity chamber
K2
K7
K3
K5
K1
Yes
Yes
Yes
Yes
Yes
Absorbed Dose
to Water
Co-60
MV x rays
Sr-90/Y-90
water calorimeter
water calorimeter
extrapolation chamber
K4
K6
Yes
No
Yes
Air Kerma
Strength
Cs-137 brachy
graphite cavity chamber
Ir-192 brachy
graphite cavity chamber
HDR Ir-192 brachy
I-125 brachy
WAFAC
Pd-103 brachy
WAFAC
Cs-131 brachy
WAFAC
K8
Yes
Yes
No
Yes
Yes
Yes
Free-Air Ionization Chamber (< 300 keV)
20 keV to 100 keV
K air
Qair W
e
airV
C
J
x 33.97
kg
C
Primary Standard for Mammography X Rays (≤ 50 kV)
Electrometer
Mo, Rh anode
x-ray tubes
filters
V
Attix free-air
chamber
Cavity Ionization Chamber (> 300 keV)
K air
Qair W
e
airV
1
1 g
S/
S/
graphite
air
(
/ )air
( en / )graphite
en
Water Calorimetry (MV photons, electrons)
Dwater
c T
Vessel with thermistors
Vessel with an ion chamber
Extrapolation Ionization Chamber (electrons)
Electrometer
79.54 pA
Collecting
electrode
Insulating
gap
Air gap=0.40 mm
Source
Ionization
Current, pA
Water-equivalent plastic
High-voltage
electrode/window
Air gap, mm
Extrapolation Ionization Chamber (electrons)
D water
1
air
A
d
I ( x)
dx
x 0
W
e
slope of current vs. air gap
• Ophthalmic applicators
1. Planar (90Sr/Y)
2. Concave (106Ru/Rh)
• IVB seed and line sources (90Sr/Y, 32P)
( S / ) water
( S / ) air
Wide-Angle Free-Air Chamber (WAFAC)
160 mm Al Center
Electrode
Al Filter
Electrometer
W
Aperture
Rotating
Source
V/ 2
V = - 1674 V
Aluminized Mylar
Electrodes
Wide-Angle Free-Air Chamber (WAFAC)
43 mm Al Center
Electrode
Al Filter
Electrometer
W
Aperture
Rotating
Source
V/ 2
V = - 450 V
Aluminized Mylar
Electrodes
Original and Automated WAFACs
HPGe
Spectrometer
Al filter
wheel
Automated
WAFAC
seed
Original
WAFAC
seed
Air-Kerma Strength from WAFAC Measurements
SK
K air (Q)d 2
W
e
d2
K dr ( K ) M det ( K , Q)
air Veff
Ki
i
K j (Q)
j
125I
Value
Net current, M det ( K , Q)
W /e
Air density, ρair
Aperture distance, d
Effective chamber volume, Veff
Decay correction, K1
Recombination, K dr (K )
Attenuation in filter, K3(Q)
Air attenuation in WAFAC, K4(Q)
Source-aperture attenuation, K5(Q)
Inverse-square correction, K6
Humidity, K7(Q)
In-chamber photon scatter, K8(Q)
Source-holder scatter, K9
Electron loss, K10
Aperture penetration, K11(Q)
External photon scatter, K12(Q)
Combined standard uncertainty, uc
Expanded uncertainty, V
sI
0.06
33.97 J / C
1.196 mg / cm3
T1/2 = 59.43 d
< 1.004
1.0295
1.0042
1.0125
1.0089
0.9982
0.9966
0.9985
1.0
0.9999
1.0
Type A (%)
Type B (%)
s
0.11
-
0.06
0.15
0.03
0.24
0.01
0.02
0.05
0.61
0.08
0.24
0.01
0.07
0.07
0.05
0.05
0.02
0.17
(s2 + 0.7622)1/2
2uc
Monte Carlo Simulations
• Photon and electron source modeling
• Detector response calculations
• Ionization chamber correction factors
• Stopping power ratios
S/
S/
k wall
graphite
air
• Mass-energy absorption coefficient ratios
(
/ )air
( en / )graphite
en
Photon and Charged-Particle Data Center
1.0E+04
104
1.0E+03
103
Water
Water
1.0E+03
103
2
1.0E+02
10
2
1.0E+02
10
1
10
1.0E+01
2
(MeV cm / g)
2
(cm / g)
1
1.0E+01
10
0
Collisional
S/
/
1.0E+00
10
Total
-1
1.0E-01
10
0
1.0E+00
10
Total
-2
1.0E-02
10
Incoherent
Photoelectric
-1
10
1.0E-01
Radiative
Pair
-3
1.0E-03
10
-4
-2
1.0E-04
10
1.0E-03
10-3
1.0E-02
10-2
1.0E-01
10-1
1.0E+00
100
1.0E+01
101
1.0E+02
102
1.0E-02
10
1.0E-02
10-2
1.0E-01
10-1
1.0E+00
100
1.0E+01
101
Electron Energy (MeV)
Photon Energy (MeV)
pe
coh
incoh
pair
uA
trip
ph.n.
S
S col
S rad
1.0E+02
102
1.0E+03
103
Web-based Photon and Electron Databases
XCOM: Photon Cross Sections Database
http://www.nist.gov/pml/data/xcom/index.cfm
http://www.nist.gov/pml/data/photon_cs/index.cfm
http://physics.nist.gov/PhysRefData/Star/Text/ESTAR.html
“Accurate MCPT-aided brachytherapy dosimetry would not be
possible without the evolution of accurate photon cross-section
libraries made over the last 30 years (Hubbell 1999), an effort
led by [the late] John Hubbell of NIST.”
Chapter 14, Thermoluminescent Detector and Monte Carlo Techniques for
Reference-Quality Brachytherapy Dosimetry by J. F. Williamson and M. J.
Rivard, in Clinical Dosimetry Measurements in Radiotherapy, D.W. O.
Rogers, Joanna E. Cygler, Editors, Proceedings of the American Association
of Physicists in Medicine Summer School, Colorado College, Colorado
Springs, Colorado, June 21–25, 2009, p. 471.
CLA Criteria – Traceability to NIST
The only type of laboratory accredited by the AAPM is a secondary standard
laboratory with the capability of providing direct traceability to the National
Institute of Standards and Technology (NIST). Such a laboratory is referred to as
an Accredited Dosimetry Calibration Laboratory (ADCL).
A3.6.1.6 Calibration traceability to NIST dosimetry standards shall be maintained
by participation in NIST measurement quality assurance tests and in ADCL
intercomparisons at intervals prescribed by the Subcommittee.
CLA Criteria – Equipment Calibrations
A3.6.1.2 Each reference class ionization chamber, which serves as the laboratory’s
standard for accredited beam qualities, shall be calibrated by NIST. The laboratory
must have standard ionization chambers calibrated at beam qualities sufficient to
cover the laboratory’s accredited beam qualities.
A1.5.1.11.2 At least one of the laboratory standard voltmeters and one of the
capacitors used for charge measurement, shall be calibrated at least biennially at
another facility. Alternatively, an electrometer with a precision and stability of 0.1%
or better may be calibrated biennially by NIST. These calibrations shall be
documented as traceable to NIST.
CLA Criteria – Equipment Calibrations
5.5.13 The laboratory shall have at least two high-quality barometers (resolution
of 0.5 mm Hg or better) and two high-quality thermometers (resolution of 0.1 C
or better). At least one barometer and one thermometer shall have a
calibration documented as traceable to NIST.
5.5.15 The laboratory shall have a device to measure relative humidity (RH). The
device shall have a calibration traceable to NIST with an uncertainty of +/- 7%
RH or better.
CLA Criteria – Brachytherapy Source and Chamber Calibrations
A2.4.1 Standard Source Traceability: The ADCL shall obtain all model-specific
calibrations of standard sources used as reference standards for calibrations
directly from NIST.
A7.5.2 For chamber calibrations the laboratory shall have at least one sealed source
of each radionuclide, manufacturer, model and encapsulation for which calibration
will be offered…This source shall have direct traceability to NIST.
Measurement Traceability for Brachytherapy Sources – New Source
SK
ADCL
5 sources
Manufacturer
Clinic
Measurement Traceability for Brachytherapy Sources – New Source
3 sources
ADCL1 ADCL2 ADCL3
SK
secondary standard
(SK / I)0
2 sources
Manufacturer
SK / I ?
ADCL calibration date
Clinic
Measurement Traceability for Brachytherapy Sources - Clinics
ADCL
Manufacturer
well-ionization
chambers
(SK / I)ADCL
Clinic
Measurement Traceability for Brachytherapy Sources - Clinics
ADCL
Manufacturer
verification for
treatment planning
(SK / I)ADCL
Clinic
sources (SKM)
SKClinic
Measurement Traceability for Brachytherapy Sources – Annual QA
3 sources
SK
3 sources
Manufacturer
ADCL1 ADCL2 ADCL3
3 sources
(SK / I)t
2.00 %
vs.
(SK / I)0
Clinic
Measurement Traceability for Brachytherapy Sources
sources
ADCL
well-ionization
chambers
SK
secondary standard
sources
Manufacturer
verification for
treatment planning
Clinic
sources
SKClinic
Clinical Brachytherapy Source Measurements
Well-ionization chambers, calibrated by an ADCL
S KClinic
I Clinic
SK
I
ADCL
AAPM Clinical Protocols
TG-43U1 (Brachytherapy Dosimetry)
“For experimental measurement of absolute dose rates to water, at least one source should have direct
traceability of SK to the 1999 NIST WAFAC calibration standard.”
“For calibrating radioactive sources, the AAPM has previously recommended that users not rely on the
manufacturer’s calibrations, but instead confirm the accuracy of source strength certificates themselves by
making independent measurements of source-strength that are secondarily traceable to the primary
standard maintained at NIST.”
TG-51 (High-Energy Photon and Electron Dosimetry)
“The primary purpose of this dosimetry protocol is to ensure uniformity of reference dosimetry in external
beam radiation therapy with high-energy photons and electrons. To achieve this goal requires a common
starting point and this is accomplished by starting with an ion chamber calibration factor which is directly
traceable to national standards of absorbed dose to water maintained by Primary Standards Laboratories
(National Institute of Standards and Technology, NIST, in the US, the National Research Council of
Canada, NRCC, in Canada). Direct traceability is also achieved via calibration factors obtained from an
Accredited Dosimetry Calibration Laboratory (ADCL).”
TG-61 (40-300 kV X-Ray Beam Dosimetry)
“Calibration factors NK shall be traceable to national standards, i.e., from an ADCL, NIST or NRCC,
preferably for a number of x-ray beam qualities.”
Current Research Areas in Medical Dosimetry at NIST
• Small-field therapy dosimetry using alanine/EPR
• Water calorimetry studies at room temperature and 4 oC (60Co and MV x rays)
• Air kerma standard for electronic brachytherapy
• CT dosimetry (ion chamber and phantom tests)
• New reference standard beams for digital mammography
• 3D dosimetry through ultrasonic tomography
Electronic Brachytherapy Calibration Facility
1.5 kV
x-ray source
leaded glass
shield
Lamperti
free-air
chamber
HPGe spectrometer
50 cm
I
Shield, free-air chamber,
and spectrometer rotate
around source
“Although radiation calorimetry for the measurement of
absorbed dose has a long history going back to the 1920s, the
modern era of primary standard devices can be traced back to
the graphite calorimeter designed by Domen and Lamperti
(1974) at the U.S. National Bureau of Standards (now NIST).
More than three decades later, this design, in modified forms,
remains the primary standard for 60Co and MV photon beams
at several national laboratories…”
Chapter 15, Primary Standards of Air Kerma for 60Co and X-Rays and
Absorbed Dose in Photon and Electron Beams by M. McEwen, in Clinical
Dosimetry Measurements in Radiotherapy, D.W. O. Rogers, Joanna E.
Cygler, Editors, Proceedings of the American Association of Physicists in
Medicine Summer School, Colorado College, Colorado Springs, Colorado,
June 21–25, 2009, p. 523.
Radiation Interactions and Dosimetry Group Staff
Dr. Fred Bateman, Physicist
Dr. Paul Bergstrom, Physicist
Diana Copeland, Secretary
Dr. Marc Desrosiers, Research Chemist
Dave Eardley, Electronics Technician
Dr. Larry Hudson, Physicist
Mel McClelland, Electronics Technician
Dr. Ronnie Minniti, Physicist
Michelle O’Brien, Physicist
Jim Puhl, Physical Scientist
Dr. Ron Tosh, Physicist
Jason Walia, Physicist
Water Calorimetry at NIST: Research and
Applications
Ronald E. Tosh, Ph.D.
National Institute of Standards and Technology
Physical Measurement Laboratory
Ionizing Radiation Division
2011 Joint AAPM/COMP Meeting
Aug 2, 2010
Water Calorimetry at NIST
Source
Timing Control
On/Off
LIA/PSD
60Co
source, ~ 10 k Ci
(~ 1 Gy/min at 1 m)
Insulation:
foam
wood
aluminum
Height adjustment
•Water phantom enclosed in a tank of (30 x
30 x 30) cm3 made with PMMA
•Entrance window thinned to 3.5 mm for an
area of 12 cm x 12 cm (radiation field 10 cm
x 10 cm)
Sealed glass core blown from Pyrex tubing
•ID < 35 mm
•wall thickness < 0.3 mm at the center
•fitted with two threaded Teflon (PTFE) plugs
Experimental uncertainties
Heat defect
?
~ 1 Gy/min
Heat transport
…
V(t)
~2 V
~ 0.239 mK
t
~ 60 sec
Heat Defect
Causes:
• Chemical reactions involving products of incident radiation
and various dissolved species within the water
Effect on signals:
• Transient – can be huge (~100%).
• Steady state – depends on dissolved species (0 to few %).
Remedy:
• H2 – saturated, high-purity water in a sealed glass vessel.
N.V. Klassen and Carl K. Ross, J. Res. Natl. Inst.
Stand. Techol. 107, 171-178 (2002).
Heat Transport
Causes:
• Dose non-uniformities
• Changes in heat capacity at material
interfaces
Principal mechanisms:
• Conduction (linear)
• Convection (nonlinear)
Effects on signals:
• Distortion throughout entire waveform.
• Errors amplified by extrapolation
procedures for extracting T.
Remedies:
• Restrict experimental runs to a few
exposures at a time  ~0.2% for
conduction.
• Operate calorimeter at 3.98 oC  0%
error for convection.
Heat Transport cont’d
Questions regarding existing remedies:
• Would the system exhibit a stable, steady-state behavior?
Interruptions to reestablish thermal equilibrium might not be necessary.
• If so, to what extent would convection contribute?
Possibility to operate at room temperature instead of 3.98 oC.
10
y = sqrt(m1^ 2+m2 ^ 2 /m0^4)
9 10
-5
1
8 10
7 10-5
6 10
-5
0.003536 5
2.5438e-07
m2
0.000190 95
Chi sq
0.000108 9
NA
R
0.99999
NA
0.1
0.01
5 10-5
0.001
4 10
-5
0
2000
4000
6000
8000
1 10
4
0.001 0.01
~ time (sec)
Error
0.005988 6
-5
FFT amplitude
~ Temperature (K)
Val ue
m1
0.1
f (Hz)
1
Heat Transport cont’d
Variation of “apparent” dose rate with shutter freq/period demonstrates agreement with
heat equation (i.e. conduction only) except at higher shutter periods.
Measured dose rate (Gy/min)
 steady-state operation at room temperature appears feasible at shorter exposure periods.
1.20
0.98
0.96
0.94
0.92
0.90
0.88
Oct-09
1.15
1.10
May-Jul-2009
1.05
finite element
simulation
1.00
30
0.95
130
0.90
230
Vessel removed
0.85
0.80
10
100
1000
Shutter on time (s)
10000
330
Heat Transport cont’d
2
t

tu
2
u


D
g ( x ) f (t ) uu
cv
p
g zˆ
o
zˆ
even harmonics?
Application Note: chamber calibration
Bilateral comparison with BIPM:
• BIPM: primary standard graphite
calorimeter
• NIST: A12 reference ionization chamber
– “pre-calibrated” in 60Co
– Directly calibrated in 6- and 18-MV xrays using room-temperature, sealed
water calorimeter
Calibration runs in Clinac beams
- “run” = 20 to 30 cycles of 1 minute on/off
- A12 and calorimeter runs interleaved
• 6 MV – 14 runs
• 18 MV – 8 runs
• Convection test: vary dose-rate and look for
nonlinearity in calorimeter response.
App Note cont’d: chamber calibration
2.00
calorimeter
Comparison of calorimeter and scaled A12
(TG-51) measurements:
• Agreement within uncertainties at both
beam qualities.
• Results shown are provisional, as we are
determining revised uncertainties on heat
defect and beam perturbation estimates
(Monte Carlo).
chamber
Gy
1.90
18 MV
1.909 +/1.912
+/- 0.0022
0.0022
1.907 +/- 0.0014
6 MV
1.617+/-0.0021
1.615
+/- 0.0021
1.617 +/- 0.0025
1.80
1.70
1.60
0
5
10
15
20
25
Calorimeter runs/chamber
runs/chamber runs
Calcorimeter
runs
Test of calorimeter linearity over ~5x change
in dose rate for an 18 MV beam.
• Clinac allows 5 discrete dose-rate levels
(calibration runs were done at middle value).
• Slope of fit: (7.999e-3 ± 0.02e-3) Gy/MU
Expected: 8 mGy/MU
• Suggests that convection is negligible (even at
these elevated dose rates).
MU
“Everything becomes a project”…
•
•
•
•
•
60Co
high energy x-rays
electron beams
proton beams
HDR brachytherapy
•
•
•
Each one of these…
requires
…redetermining each one of these
or





Heat defect
glass
Excess heat
vessel
Heat transport
Perturbation
Field non-uniformity
or
…
What if we could measure this?
After all, water is a sort of 3D imaging medium for dose
(if we could just figure out how to extract the information).
Ultrasonic Thermometry – 1D
Co-60
Co-60 Shutter
Frequency
Counter
PPLL
Malyarenko E, Heyman J, Chen-Mayer H and Tosh R, “High Resolution Ultrasonic
Thermometer for Radiation Dosimetry,” J. Acoust. Soc. Am., 124 3481-90 (2008)
UST 1D cont’d
Co-60
First use of ultrasonic thermometry to detect
absorbed dose to water…
UST 1D – cont’d
Co-60
No convection barriers  convection…
simple behavior suggests that its effects
may be readily deconvolved.
UST 2D
Eugene V. Malyarenko, Joseph S. Heyman,
H. Heather Chen-Mayer and Ronald E. Tosh,
“Time-resolved Radiation Beam Profiles in Water
Obtained by Ultrasonic Tomography,”
Metrologia 47 3 (2010)
UST 2D cont’d
UST 2D cont’d
Spring 2008
Spring 2011
Summer 2011
????????
Water Calorimetry at NIST
1994-present
Summer 2011
Spring 2008
Spring 2011
????????