Technical Highlights
- Intravascular Brachytherapy Source Dosimetry. The use of
beta-particle emitting brachytherapy sources for the prevention of restenosis
(re-closing) of coronary blood vessels after angioplasty continues to be
actively explored. NIST has taken an early and leading role in the calibration
of the sources used for this therapy, employing the NIST extrapolation chamber
equipped with a 1 mm diameter collecting electrode to measure dose rate at
a depth of 2 mm in water-equivalent plastic. These measurements are
confirmed using radiochromic dye film, which is also used to characterize
sources in the cylindrical geometry for transaxial uniformity. In addition,
irradiation of planar sheets of film at various depths in water-equivalent
plastic were used to construct data sets which can be used to predict the dose
rate at arbitrary locations around the sources using a modified form of the
AAPM
Task Group 43 Protocol. A publication describing this work has been
published in the journal Medical Physics. The equipment used for these studies
was augmented with the addition of an automated microscintillator detection
system and various well-ionization chambers. Use of all this equipment was
centralized in a newly refurbished laboratory. Collaborations were continued
between NIST and NeoCardia for dosimetry of a 32P wire, with
Washington Hospital Center for dosimetry of various sources, with Radiance
Medical Systems for dosimetry of radioactive balloon sources, and with Best
Industries for 188W/Re wire and 192Ir seed sources.
Collaborations were also begun with Cordis for 125I wire dosimetry.
(C.G. Soares and M.G. Mitch)
- Prostate Brachytherapy Source Characterization. There is currently
great demand for low-energy photon brachytherapy seeds used in the treatment of
prostate cancer. As a result, in 1999 the number of source manufacturers
submitting seeds to NIST for calibration has increased from two to nine.
Effective treatment planning requires accurate knowledge of source air-kerma
strength, which is used to calculate the dose rate to tissue that is delivered
by the seeds. The primary standard measurement of source air-kerma strength is
made using the Wide-Angle Free-Air Chamber (WAFAC) for seeds containing
103Pd or 125I. On-site characterization at seed
manufacturing plants for quality control, as well as at therapy clinics for
treatment planning, relies on well-ionization chamber measurements. Therefore,
calibration factors relating well-ionization chamber response to air-kerma
strength for each seed design must be known. The responses of several different
types of well-ionization chambers to the seeds were measured, and calibration
factors were calculated. The significant influence of seed composition on the
response of well-ionization chambers relative to measured air-kerma strength
for prostate brachytherapy seeds has been demonstrated. (P.J. Lamperti,
M.G. Mitch, B.E. Zimmerman, S.M. Seltzer, and
B.M. Coursey)
- LabVIEW Automation of Brachytherapy Dosimetry Measurements. Data
acquisition and instrument control for all measurement stations in the NIST
beta brachytherapy laboratory (including well-ionization chambers, extrapolation
chamber and plastic scintillator) have been automated using LabVIEW, a
graphical programming language. LabVIEW allows live data transfer between
computers over the internet. This can be used in conjunction with a video
teleconferencing link, providing a "virtual presence" at NIST for
scientific collaborators and calibration customers. This mode of operation was
demonstrated at the 1999 Council on Ionizing Radiation Measurements and
Standards’ annual meeting at NIST with the cooperation of a customer in Houston,
TX. Such real-time interaction between NIST scientists and their industrial
colleagues during a measurement will improve the overall efficiency of the
calibration process. (M.G. Mitch and C.G. Soares)
- Mammography Calibration Facility. In an effort to meet the needs of
calibration facilities that offer NIST traceable calibrations of air-kerma
measuring instruments used in the mammography energy range, NIST now offers
proficiency tests. The Food and Drug Administration’s Mammography Quality
Standards Act "Final Rule for Quality Mammography Standards" requires
that all clinical mammography units be calibrated with an instrument that is
traceable to a national standard. Any facility that provides the NIST traceable
calibration must participate in a biennial proficiency program directly with
NIST. A replacement Mo anode x-ray tube was purchased and has been characterized
for the NIST mammography proficiency testing and for routine calibrations. Upon
repair, the previous Mo anode will be characterized and integrated into the
calibration system. The data-acquisition software upgrade is in the final
testing stage and is being used to calibrate NIST reference chambers for
proficiency testing. (C.M. O’Brien)
- Electron Detector Calibrations. In collaboration with Physical
Sciences Inc. of Andover, MA, a series of charged-particle sensor calibrations
was conducted using the electron Van de Graaff accelerator. To facilitate this
work it was necessary to restore beam to the zero-degree beam line in the south
target room. The investigators had strict beam quality requirements, including
high resolution and low fluence rate, which we were able to meet in tuning the
beam. The calibrations were completed at several energies ranging from
500 keV to about 2 MeV, enabling the users to conduct a thorough test
of the detector’s performance. These sensors were designed to be used in
space-flight applications to measure light charged-particle spectra encountered
in Earth orbit. Some of these detectors have been placed aboard commercial and
government satellites, and there are plans to use them on the International
Space Station. Consisting of solid state and scintillator elements, these Light
Particle Monitors are able to identify and energy-analyze electrons, protons
and alpha particles simultaneously over a wide range in energy. Previously
separate sensors had to be used to monitor electrons and protons. Knowledge
gained from these calibrations will help researchers refine the present
generation of detectors and aid them in the design of future prototypes. The
researchers plan to return to NIST over the next several months to test a new
prototype detector currently under development. (F.B. Bateman and
M.R. McClelland)
- Photon and Charged-Particle Data Center. The Data Center compiles,
evaluates, and disseminates data on the interaction of ionizing radiation with
matter. The data on photons and charged particles, with energies above about
1 keV, include fundamental information on interaction cross sections as
well as transport data pertaining to the penetration of radiation through bulk
material. Databases are developed and maintained on attenuation coefficients
for x rays and gamma rays, including cross sections for Compton and
Rayleigh scattering, atomic photo-effect, and electron-positron pair production,
as well as on energy-transfer, energy-absorption and related coefficients
relevant to radiation dosimetry. Work on charged-particle cross sections and of
radiation transport data has entailed significant effort on the evaluation of
the stopping powers and ranges of electrons, positrons, protons, and alpha
particles, the elastic scattering of electrons and positrons, and the cross
section for the production of bremsstrahlung by electrons. Current work
includes the development of new standard reference data for the elastic
scattering of electrons and positrons from neutral atoms; comprehensive
evaluations of the doubly differential cross section for inelastic Compton
scattering of photons, including the effects of Doppler broadening due to the
motion of the orbital electrons; and comprehensive evaluations of the photon
mass energy-absorption coefficient, a quantity widely used in radiation
dosimetry. With the help of the NIST Physics Laboratory’s Office of Electronic
Commerce in Scientific and Engineering Data, a number of the Center’s databases
can be accessed on the www. (S.M. Seltzer, J.H. Hubbell,
M.J. Berger, and D.V. Rao)
- Alanine-EPR Film Dosimeter. A new polymer-based film dosimeter
containing alanine has been tested. The film, obtained from Gamma Service
(Germany), is produced in commercial quantities in a variety of thicknesses.
The first films tested were 100 µm in thickness and irradiated over a
range of 1 kGy to 100 kGy. The relative standard deviation of the
response over this range was on average 0.8 %. Comparative measurements of
radiochromic films and alanine films irradiated at MIRF produced similar dose values
within the relative uncertanities of the two systems. A more comprehensive
study of the alanine system is planned. A small batch of 10 µm alanine
films was recently received and will be tested for use with low-energy electron
beams. (M.F. Desrosiers, J. Puhl, F.B. Bateman and
M.R. McClelland)
- International Gamma-Ray High-Dose Comparison. The national metrology
institutes of the United States (NIST), France (BIPM), Great Britain (NPL), Germany (PTB), Italy (INMRI-ENEA), National Institute of Metrology
(China), and the International Atomic Energy Agency, have undertaken a
high-dose comparison. The BIPM coordinated the comparison, analyzed the results
and will prepare the publication of the results. The protocol was created
through consultation with the two issuing laboratories, NIST and NPL. The
irradiating laboratories processed the dosimeters and returned them to the
issuing laboratories for evaluation. NPL more closely correlated with the
testing laboratories than NIST; the average deviation from NIST ~1.5 %.
The comparison was dose dependent with the best correlation between all labs
occurring at the highest dose (30 kGy). (M.F. Desrosiers,
V. Nagy and J. Puhl).
- Techa River Study. One of the most massive 90Sr releases
in mankind’s history was produced by the first Soviet nuclear weapon plant Mayak
near the Techa River, Urals, Russia. In 1949-1956 about
7.6 x 107 m3 of liquid radioactive wastes
with a total activity of 1017 Bq
(2.7 × 106 Ci) was released into the Techa River,
with an 11.6 % contribution of 90Sr to the total activity. As a
result of Mayak’s radioactive discharges, the population of the Techa riverside
(about 28,000 people) was exposed to high levels of ionizing radiation. An
extensive EPR dose
reconstruction study (Institute of Metal Physics, Russia) with 29 teeth of the
Middle and Lower Techa riverside population was carried out. It revealed
ultrahigh doses (up to 15 Gy) absorbed in tooth enamel of the individuals
born in 1945 tp 1949. The introduction of corrections developed at NIST based
on the effective time of the onset of dose accumulation and dose attenuation
effects has eliminated serious contradictions of the original reconstructed
doses. The tooth most sensitive to 90Sr internal exposure is the
first molar (both tooth enamel and dentin). A tooth from this position can give
an additional enhancement factor of 4 to 6. Thus, selection by age group and
tooth position at sample collection can improve the detection and measurement
of former 90Sr releases by a factor of 200. (A.A. Romanyukha,
S.M. Seltzer, and M.F. Desrosiers)
- Asymmetries of the Weak Interaction and the Neutron Lifetime. The
cold neutron guide hall at the NIST Center for Neutron Research (NCNR) provides
a unique opportunity for the U.S. to compete for a leading role in research on
the physics of fundamental particles. High precision measurements at very low
neutron energies complement high-energy research at national and international
particle accelerator laboratories. Measurement techniques developed for this
research improve NIST’s ability to provide measurement services and calibrations.
The Neutron Interactions and Dosimetry Group pursues a research program of its
own as well as supporting a national user facility for industrial and university
researchers.
Two different neutron lifetime experiments, one a Harvard-led ultra cold
neutron (UCN) experiment, the other a NIST-led cold neutron beam experiment,
have taken data on our polychromatic neutron beam at the NCNR this year. In an
experiment using a cryogenic lithium calorimeter at our adjacent monochromatic
neutron beam, collaborative work with Indiana University is aimed at
determination of the efficiency of the neutron fluence monitor belonging to the
neutron lifetime experiment.
Complete three-dimensional magnetic trapping of ultracold neutrons was
demonstrated by the Harvard/NIST collaboration this year for the first time
anywhere. A report of this work has been accepted for publication in the
journal Nature. Trapping of neutral and charged particles is an
invaluable tool for the study of both composite and elementary particles. The
main advantages of trapping are long interaction times and isolation from
perturbing environments. In the present work, inelastic scattering in
superfluid 4He is used to load neutrons into the trap and also to
act as a scintillator for detection of neutron decay. This work verifies the
theoretical predictions of the loading process and the technique of magnetic
trapping of neutrons. Further refinement of this method should lead to improved
precision in the measurement of the beta-decay lifetime of the neutron.
In CY99 the large collaborations responsible for two previous experiments on
our polychromatic beam continued major drives to upgrade those experiments in
preparation for further running on our beam. Both projects, a search for
time-reversal symmetry violating correlations in neutron decay and a search for
parity violating spin rotation of neutrons in bulk media, are expecting to be
ready for beam time sometime in late CY2000. These projects have produced three
recent Ph.D. theses and have already made modest improvements on the best
preceding results. (M.S. Dewey, J. Nico, P. Huffman,
F. Wietfeldt, J. Adams, M. Arif, T. Gentile,
D. Gilliam, D. Jacobson, G. Jones, and A. Thompson)
- Polarized 3He for Neutron Spin Filters and MRI Applications. The primary
focus of the polarized 3He program is the development and application
of neutron polarizers for both condensed matter science and fundamental physics.
As a spin-off of this technology, the program also includes collaboration with
medical researchers in polarized gas magnetic resonance imaging (MRI). A unique
feature of the 3He effort is the concurrent development of the
spin-exchange and metastability-exchange optical pumping methods. These two
methods have different strengths and weaknesses, and pursuit of both allows for
a versatile approach to the needs of different applications.
Last year we demonstrated separation of coherent scattering from incoherent
scattering using 3He-based polarization analysis on the NIST Center
for Neutron Research (NCNR) NG7 small
angle scattering spectrometer (SANS). Following upon this work, we have
established a series of experiments to be done in collaboration with NCNR staff
to investigate separation of magnetic from nuclear scattering.
Recently we have obtained a significant improvement in the achievable optical
pumping cell pressure in the diaphragm compressor apparatus which is used for
the metastability-exchange polarization method. This improvement should allow
us to reliably fill cells with over 40 % polarization, and we hope to push
up towards 50 % with more efficient optical pumping. (T. Gentile,
A. Thompson, G. Jones, and D. Rich)
- Neutron Interferometry and Optics Facility (NIOF). During the past
year at the NIOF a number of fundamental and applied physics experiments have
been carried out and new collaborations with Indiana University, the University
of North Carolina, and the Nuclear Physics Institute (Czech Republic) have been
established. Three-dimensional neutron imaging of industrial specimens has been
developed under ATP
sponsorship. Five refereed articles have been published (or are in press).
Invited presentations were also made at the SPIE and
AOS (Australian Optical Society) organized conferences.
Major progress has been made in the design and construction of a highly
sensitive experimental assembly for the precision measurement of the
neutron-electron interaction length bne and in the actual
conduct of the experiment. This very fundamental quantity bne
is critical to the understanding of the charge structure of the neutron and of
the deuteron charge structure in atomic physics. After four decades of
sustained effort discrepancies between different experimental values and
between experiment and theory persist. Using a novel dynamical diffraction
effect for a perfect single crystal inside a neutron interferometer, we are
attempting to measure bne directly and accurately.
Researchers from the University of Innsbruck, Austria, have successfully
completed experiments involving dynamical diffraction of neutrons from
macroscopic objects. These experiments are also the primary component of the
Ph.D. dissertation of a student (Annette Lacroix) of Prof. A. Zeilinger
from the University of Vienna.
After successful neutron interferometric measurement of the mass density of a
thin polymer film (~0.4 µm) in 1998, the measurement was extended to
measure film thickness between (0.01 µm and 0.05 µm). For
the first time an effectively "zero" phase shift substrate was used to enhance
the sensitivity of the experiment. This interferometeric technique may provide
an alternative, highly accurate and self-calibrating method for the routine
measurement of polymer thin film mass density. The experiment was carried out
in collaboration with the NIST Polymers Division. (M. Arif,
D. Jacobson, A. Ioffe, P. Huffman, T. Gentile, and
A. Thompson)
- Neutron Dosimetry. An unusual amount of measurement service work was
provided this year at the request of U.S. Navy contractors from Bettis and
Knolls laboratories. Special irradiation facilities were constructed for
testing neutron fluence monitors in the presence of high level gamma-ray
fields.
Developments in metrology services and consultation services on behalf of the
Nuclear Regulatory Commission Office of Regulatory Research included
development and successful balloting of a new ASTM standard on reactor
dosimetry and major supporting efforts for the Tenth International Symposium on
Reactor Dosimetry in Osaka, Japan.
Calibration services for isotopic neutron sources and for neutron radiation
protection survey instruments were both upgraded and continued to have a steady
customer base. The Manganese Bath facility for comparing customers’ sources with
the national standard NBS-I was generally refurbished and modernized with
improvements including new highly reproducible source positioning fixtures and
a new shield window. Continued progress was also made in the automation of the
survey instrument calibration range. (J. Adams, A. Thompson,
J. Nico, and D. Gilliam)
- Neutron Cross Section Standards. The NIST Neutron Cross Section
Standards Project has played an important role in the improvement of the
neutron cross section standards through both evaluation and experimental work.
We are leading an effort that will result in a new international evaluation of
the neutron cross section standards. This has involved motivating and
coordinating new standards measurements, examining the standards database, and
pursuing the extension of the standards over a larger energy range. This work
is taking place through participation in the U.S. Cross Section Evaluation
Working Group and two international committees, the International Nuclear Data
Committee and the Nuclear Energy Agency Nuclear Science Committee. An objective
is to complete the evaluation in time for the major international cross section
evaluation projects to use the improved standards in forming new versions of
their libraries. (A. Carlson and D. Gilliam)
- Dissemination of National Standards of Radioactivity. The
Radioactivity Group disseminated the National Standards of Radioactivity mainly
through the following three activities: (1) Over 550 radioactivity
Standard Reference Materials (SRMs) were sold in 1999. (2) Over 200
comparative measurements and Reports of Traceability were provided to federal
regulatory agencies, radiopharmaceutical manufacturers, commercial suppliers of
calibration sources and services, and the nuclear-power industry. Industrial
steering committees guided the work of four research associates in cooperative
testing programs. (3) Over 112 calibrations of customer sources were
provided. (L.L. Lucas, J.C. Cessna, and L.R. Karam)
- Glow-Discharge Resonance Ionization Mass Spectrometry (GDRIMS).
GDRIMS was successfully tested for the first time with cw lasers during this period. Three papers
summarizing the work have been submitted for publication or are in preparation.
Cesium was used to test the system. At present it is estimated that the system
can be used to determine trace amounts of radioactivity at normal environmental
levels directly, without chemical analysis, with half-lives in the
105 year range. This would include the important radionuclide
239Pu at somewhat elevated levels. It is anticipated that
improvements in the near future could improve this by 2 to 3 orders of
magnitude. This new capability was established through several steps taken this
year. The regime in the glow discharge for which atomization, which is needed
for selective laser ionization, dominates over ionization was investigated and
parameter choices were made for which the efficiency is increased and the
background is reduced. Generally this occurred in the pressure range of
120 mTorr. Also, the optimal sizes and shapes of the exit aperture and
repeller were tested for maximum signal strength and ion background rejection,
and a new diode laser was installed to perform the excitation step. This
allowed us to examine different ionization schemes and optimize them for
efficiency. Depletion measurements which involve optical pumping into the
6p 6 2P3/2(F=5) state resulted in the
enhancement of the selectivity on the order of three orders of magnitude and
measurements were performed to reduce Doppler broadening by careful alignment
of the lasers with respect to atomic beam. The overall efficiency, that is the
number of ions detected divided by the number of atoms emitted, was measured to
be 10-8 while the selectivity (the detectable signal of a nuclide
divided by the background tail interference from an adjacent nuclide) was
estimated to be 1011. Finally, the 135Cs/137Cs
ratio of a NIST standard was measured in collaboration with Pacific Northwest
National Laboratory over a two-week period using their system that uses a
graphite furnace in place of the glow discharge. We conclude that we are now in
a position to predict the operation of the system when applied to a particular
radionuclide. (L. Pibida, L.R. Karam, and
J.M.R. Hutchinson)
- NIST Radiochemistry Intercomparison Program (NRIP). The NIST
Radiochemistry Intercomparison Program NRIP marks the successful completion of
the second year of its measurement traceability program for low-level
environmental radioactivity measurements. Four rounds of testing were completed
(five total matrices were offered: water, air-filters, synthetic urine, and
synthetic feces). The analyte list was expanded for year two to include
90Sr, 234U, 238U, 238Pu, and
241Am, at 0.03 to 0.3 Bq per sample. Participation in the
program has expanded from six to include: Environmental Evaluation Group
(Carlsbad, NM), Westinghouse WID (Carlsbad, NM), Oak Ridge Associated
Universities (ORAU), Carlsbad Environmental Monitoring and Research Center
(Carlsbad, NM), Sandia National Laboratory (Albuquerque, NM), Los Alamos
National Laboratory (Los Alamos, NM), National Air and Radiation Environmental
Laboratory (Montgomery, AL), Oak Ridge National Laboratory (Oak Ridge, TN),
Bechtel Nevada Test Site (Mercury, NV), Lockheed Martin Idaho Technologies Co.
(Idaho Falls, ID). The program is vital for relating low-level radioanalytical
measurements to the National Standards.
The program has been implemented to meet the demand for traceability as defined
under ANSI N42.23. This voluntary standard defines a hierarchy of traceability
with an unbroken linkage from service laboratories to NIST. Under the standard,
the testing materials must be (1) of appropriate matrices (i.e., matrix
categories commonly analyzed by the laboratory), and (2) of appropriate
(ambient) activity concentration ranges for these matrices. Currently NRIP is
the only traceability program providing performance evaluation materials at
environmental levels. In year two, acceptance criteria as defined in ANSI
N42.22 "Traceability of Radioactive Sources to the National Institute of
Standards and Technology and Associated Instrument Quality Control" were
adopted by the NRIP group. The ANSI N42.22 standard defines the requirements
for traceability for radioactivity-source manufacturers. The NRIP group has
adopted the performance testing criteria of the standard, which states that --
for traceability -- the difference between reported results and the NIST
certified values must be less than three times the total propagated uncertainty
for the measurement. The standard establishes further the quantity "three
times the total propagated uncertainty" as the "Traceability
Limit" -- the limit to which a source manufacturer may claim traceability
to NIST. Following each test, participating laboratories are issued Reports of
Traceability, which reflect both the relative bias (compared to the NIST values)
and the Traceability Limit of the measurements as defined in ANSI N42.22.
The test matrices and analytes for each fiscal year are to be determined at
the upcoming Annual Conference on Bioassay, Analytical and Environmental
Radiochemistry. Continued growth in the number of participating laboratories
is anticipated from the commercial national and international communities.
(K.G.W. Inn, C. McMahon, Z.Y. Wu, and Z.C. Lin).
- Development of New Standards and Metrology for Measuring Low-Level
Radionuclides in Bioassay Materials. The program is to develop the
capabilities in preparation of in-vitro and in-vivo radiobioassay
standards required by the ANSI 13.30 standard. The program focuses on the
preparation and verification techniques needed in developing ultra-low level
reference materials for the assessment of quality of radionuclide measurement
in urine, feces, lung, and BOMAB
and anthropomorphic phantoms. A wide array of nuclear detection methods
including mass spectrometry, liquid scintillator, proportional counter,
solid-state alpha detectors, HPGC
detectors and MCNP
have been applied to the measurements. (Z.Y. Wu and
K.G.W. Inn)
- Development of Low-Level Radioactivity Natural Matrix Standard Reference
Material. The program provides various standard reference materials that
closely matched the matrix and activity of radionuclides presented in different
environmental samples. Seven such SRMs are currently used by environmental,
bioassay, and oceanographic communities in their radiochemical method
evaluation and validation. The SRMs provide a basis for measurement
comparability between laboratories. Current projects include Ashed Bone, Rocky
Flats Soil-2, and Ocean Shellfish. Each of these SRMs provides the radiochemist
with different analytical challenges. (Z.C. Lin, C. MacMahon,
K.G.W. Inn, and Z.Y. Wu)
- Traceability and Standards for the Environment. As part of our
efforts to provide traceability and standards for environmental remediation and
waste handling, we have provided field sampling, on-site assessment, internal
QC PE in-vitro radiobioassay
materials, and assessment of transuranic (TRU) waste drum assay traceability
to Westinghouse Waste Isolation Pilot Plant (WIPP) and technical radiobioassay
monitoring laboratory service program with on-site assessment, PE traceability
testing, calibration of in-house reference material to ORNL. In addition, we provide
technical radiobioassay reference laboratory service programs for RESL
and PNNL with
on-site assessment, PE
traceability testing, and calibrations of in-house reference material. We also
provide technical support for
EPA/NVLAP
efforts to accredit external radiochemistry Performance Training (PT)
suppliers. We have provided CEMRC
technical training for spiking soil in preparation for seeking Reference
Laboratory status for CAO.
Our NIST Radiochemistry Intercomparison Program (NRIP) provides 12 national
laboratory, university, and federal agency laboratories with low-level
radionuclide PE programs for radiobioassay, soil, air filter, and aqueous
matrices. (K.G.W. Inn, C. McMahon, Z.C. Lin, and
J. Wu)
- Radioanalytical Traceability. Over the past several years, NIST has
been working with the American National Standards Institute’s nuclear
instrumentation N42 and N13 committees to establish widely accepted criteria
for radioanalytical traceability. Three standards have been published:
1) ANSI N42.23 (Measurement and Associated Instrumentation Quality
Assurance for Radioassay Laboratories), 2) ANSI N42.22 (Traceability of
Radioactive Sources to the National Institute of Standards and Technology and
Associated Instrument Quality Control), and 3) ANSI N13.30 (Performance
Criteria for Radiobioassay). Each of these standards were developed through
consensus participation among industrial, commercial, utility, federal, state,
and national laboratory representatives to strengthen the credibility of
national radioanalytical programs. ANSI N42.23 envisions the accreditation of
a small group of program-specific reference laboratories that participate
directly in a traceability-testing program with NIST, technical document
reviews and on-site assessments. These reference laboratories would act as
intermediaries in the establishment of the traceability testing link to the
service laboratories through Performance Testing (PT) programs. One current
driving force to establish reference laboratories is the privatization of the
EPA Drinking Water Crosscheck Program that will require the NVLAP accreditation
of PT Providers for drinking water laboratories. The traceability testing
criteria for radiobioassay PT reference laboratories defined by ANSI N13.30 are
similar. A third sector that will be establishing traceability testing for its
reference laboratories is DOE-EM for its environmental remediation work. The
ANSI N42.22 standard provides additional and more specific criteria for source
manufacturers, including those reference laboratories that will be producing PT
materials. In addition to the criteria for quality assurance, facilities and
equipment, and certificates, ANSI N42.22 provides very specific criteria for
the acceptance of testing results:
|VR - VN| < 3 ×
(σN2 +
σR2)1/2
where the absolute value of the bias between the reported value, VR,
and the NIST value, VN, shall be less than or equal to three times
the total propagated NIST uncertainty, σN, and the reported
uncertainty, σR.
Furthermore, the right hand side of the equation defines the "traceability
limit" to which a source manufacturer could claim traceability. It is
anticipated that traceability testing for the reference laboratories supporting
these four national sectors will be initiated by the end of 1999.
(Z.C. Lin, K.G.W. Inn, J. Wu, and C. McMahon)
- Re-Calibration of the Mixed Gaseous Radioactive Measurement System.
The p-type intrinsic Ge detector was calibrated for activity of the Nobel gas
radioisotopes 85Kr, 127Xe and 133Xe. The
latter two gaseous radioisotopes were activity calibrated by internal gas
counting of gas samples that were quantitatively related to nominal 33 mL
glass spherical sources. These spherical sources were then measured on the Ge
detector, and a response factor determined. The 85Kr activity
standard K300 was concurrently measured to re-evaluate that response factor.
This measurement system is used in the NIST-NEI traceability program for these
radioactivities. (F.J. Schima and M.P. Unterweger)
- Calibration by Gamma-ray Counting. The gamma-ray detector systems
continue to be used to standardize solution and gas sources for the NEI
measurement assurance programs. Efficiency curves for several energy regions
for three of the Group’s gamma-ray detectors are being reinvestigated and, when
needed, redetermined. (F.J. Schima, L.R. Karam, L. Pibida,
J.T. Cessna, and B.E. Zimmerman)
- Upgrade of Gamma-ray Spectroscopy Facilities. The Radioactivity
Group is upgrading its gamma-ray spectroscopy facilities in order to allow them
to keep pace with the growing demand for source analysis using this technique.
The non-networked, UNIX-based data acquisition system currently in use is being
converted to PC-based systems, which are on the NIST network. Some of the
analysis software is being ported to the Windows environment, while other
programs will be replaced by commercially available versions currently being
evaluated. The detectors themselves are being moved to a larger laboratory to
minimize interferences between detectors and to allow for the installation of
additional detectors. These changes and subsequent recalibrations are being
carried out in such a way as to not disrupt normal operations.
(B.E. Zimmerman, L.R. Karam, J.T. Cessna, and
L. Pibida)
- Phosphor Plate Imaging for Standards Development. The SPP imaging plate detector,
with pixel sizes as small as 25 µm x 25 µm, was
originally developed for diagnostic radiography and they are sensitive to all
types of ionizing radiation. Its usage ranges from the field of biomedical
research to material science and now is being applied for environmental
radiation monitoring purposes. A typical SPP plate stores a radiation image as
a distribution of F-centers in a thin coating of photostimulable phosphor
(BaFBr:Eu2+). The latent image is read out by measuring the
intensity of fluorescence stimulated by scanning a He-Ne laser beam over the
surface of the plate for a few minutes. We have been conducting measurements
using a Fuji BAS 2000 SPP system to determine its applicable measurement
limits for various types of radioactivity and to suggest operation procedures
tailored to different types of radiation and radioactivity levels. These
measurements are designed to realize the potential of this SPP system as a
radiation imaging device, a quantitative measurement instrument and a research
tool to design counters for ultra low level radioactivity measurement required
by the electronics and aerospace industries. (Y.T. Cheng, L.R. Karam,
and M.P. Unterweger)
- Application of SPP to Cell-Radiolabelling Efficiency. In
collaboration with members of CSTL, and as part
of a contract effort for the Air Force, we have studied the application of SPP
to monitor the efficiency of bacterial cell labelling with radioactive isotopes.
We have measured the activity of 32P-labelled bacillus on air
sampling filters with the SPP system, obtaining results 2 orders of magnitude
more sensitive than previously used methods. (L.R. Karam with
C. O’Connell and H. Rodriguez of CSTL)
- Novel Approaches in Nuclear Medicine. We continue several ongoing
research projects involving various aspects of nuclear medicine, including
investigations of novel delivery methods of radiopharmaceuticals. Using our
previously constructed fullerene production apparatus, we have developed
extensive protocols for the incorporation of 125I inside the
fullerene cage. Since fullerene cages are capable of physically and chemically
isolating radioisotopes from their associated pharmaceutical, the incorporation
of 125I would allow for a more stable carbon-carbon bond in a
radiopharmaceutical. Using modified (for safety) protocols developed for the
incorporation of non-radioactive iodine in fullerenes and detection methods use
for measuring the electron-beam activated 126I; we have demonstrated
the feasibility of incorporating the medically common isotope 125I
in fullerenes of different sizes. Yield and purity of 125I
endofullerenes have been determined by high-pressure liquid chromatography
(HPLC), MultiPhoton Detection and Fuji phosphoimaging. We have used exhaustive
procedures to determine whether or not the iodine is located within the cage
cavity, by heating and trapping volatiles of iodinated samples to remove any
iodine trapped within the soot of particulates or attached to the outside of
fullerenes ("exofullerenes") with subsequent extraction, purification
and isolation of the volatile species. (L.R. Karam, M.G. Mitch, and
B.M. Coursey)
- Counting Yields for Beta and Alpha Particle Sources. Using the
results of Monte Carlo calculations, the counting yields of beta- and
alpha-particle sources have been determined and tabulated. The counting yield
is defined as the fraction of the emitted particles that emerge from the source
and are counted by a 2π detector, and takes into account backscattering and
self-absorption in the source. The results apply to sources consisting of a
thin layer of radioactive material placed on top of, or distributed in a top
layer of, a thick metal backing. (M.J. Berger, L.R. Karam,
M.P. Unterweger, and J.M.R. Hutchinson)
- Holmium-166m. 166mHo is a long-lived radionuclide (the
half-life is 1200 years) that emits a large number of gamma rays with
energies from 80 keV to more than 1400 keV. The large number of gamma
rays, the wide energy range, and the long half life make 166mHo a
very desirable gamma-ray source for determining the detection efficiency of
germanium detectors and for monitoring their long-term stability. High-purity
stable 165Ho was neutron irradiated to produce 166mHo,
which is now being calibrated in terms of activity. The gamma-ray emission
probabilities and their uncertainties are also being evaluated. SRM 4274,
166mHo solution, will be issued in FY 2000. (L.L. Lucas,
F.J. Schima, and B.E. Zimmerman)
- Standardization of 177Lu. Accurate measurements of the
amount of radioactivity present in radiopharmaceuticals rely on standards
developed by the Radioactivity Group at NIST. These standards are critical for
ensuring accurate measurements of administered dose, determining reaction
yields, and for normalizing dosimetry measurements. Moreover, a radioactivity
measurement standard for new radiopharmaceuticals is required by the United
States Food and Drug Administration prior to giving approval to the drug for
human use. One such radionuclide that shows great promise for use in
radiotherapy against cancer is the rare earth isotope 177Lu
(t1/2 = 6.7 d). The Radioactivity Group has recently
developed a new standard for this radionuclide, working in collaboration with
Mallinckrodt, Inc.
The radioactivity in each of several solutions submitted by Mallinckrodt was
calibrated using 4πβ liquid scintillation (LS) counting, with
confirmatory measurements made with "4π" γ-ray
spectrometry. The expanded (k=2) uncertainty on the activity
measurements was 0.6 %. Impurity measurements were carried out using
HPGe γ-ray spectrometry
and indicated that the only impurity present was the long-lived (160.4 d)
177mLu isomer at an activity level of approximately 0.03 % that
of the 177Lu, as of the reference time. Measurements were then
carried out to determine a calibration factor for the NIST 4πγ
ionization chamber that can be used to make rapid activity determinations of
additional solutions that may be submitted. (B.E. Zimmerman)
- Development of "Transfer Standards" for Solutions of
186Re. Rhenium-186 is currently being investigated for a wide
variety of applications in nuclear medicine. Currently, there is no recommended
dial setting for measuring 186Re in a Capintec dose calibrator,
which is the de facto standard measurement device for performing
radioassays in the clinical and manufacturing settings. Furthermore,
Mallinckrodt, Inc. is investigating the use of conical v-vials for shipping
products containing 186Re solutions for use in a new intravascular
brachytherapy device. It is hoped that dose calibrators can be used to perform
assays of the solutions in the v-vials as part of their quality assurance
procedures. Attenuation of radiation emitted from radioactive solutions plays a
vital role in predicting the response of ionization chambers ("dose
calibrators") to these sources. This is especially true when the solution
is contained in conical v-vials, in which neither the solution height nor glass
thickness vary linearly with volume. For this reason, we have undertaken an
investigation to empirically determine the dose calibrator settings for
measuring 186Re solutions in both the standard NIST geometry and in
conical v-vials with variable volumes. (B.E. Zimmerman)
- Calibration of High-level 125I Solution Sources for Use in
Intracavitary Brachytherapy. The American Cancer Society projects 18,000
U.S. patients will be diagnosed each year with malignant brain tumors and
nearly all experience tumor re-growth after initial treatment. Most tumors
recur within a short period of time and more than 80 % are located within
two centimeters of the original cancer site. Proxima Therapeutics, Inc. (PTI)
is developing a device that it hopes will decrease the incidence of recurrence.
This new treatment modality involves the implantation of a small balloon in the
cavity that remains after the tumor is removed. The balloon is subsequently
filled with a solution containing radioactive 125I. The short range
of the low-energy radiation from the 125I kills the remaining cancer
cells while sparing healthy tissue. This particular approach has the advantage
over external beam therapy in that multiple treatments can be performed to
ensure complete eradication of the cancer cells. The high dose delivered to
healthy brain tissue by external beams usually precludes such multiple
treatments. One of the important issues in treatment planning is accurate
calculation of the dose being delivered to the patient, which requires accurate
standards of the nuclide being used. NIST is developing "transfer
standards" in the form of calibration factors for measuring high-level
(> 4 GBq) solutions in conical v-vials using commercially
available re-entrant ionization chambers. (B.E. Zimmerman and
J.T. Cessna)
- Quantitative Destructive Assay Technique For Radioactive Coronary
Stents. Nearly one-half million coronary angioplasty procedures are
performed every year in the United States. In up to 50 % of these cases,
the patient will experience restenosis, or re-closing, of the artery walls. One
method used to prevent this phenomenon is the permanent placement of a metal
stent that expands to form scaffolding to hold the artery walls open.
Unfortunately, this procedure has not proven effective in every case, and a
re-examination of the site will find scar tissue growing through the stent.
Another method employed is intravascular brachytherapy, where the area is
subjected to a dose of radiation at a level known to inhibit proliferative
cells, such as scar tissue. Recently a Belmont, California company, IsoStent,
has combined these two methods by developing a stainless steel stent containing
the beta emitter 32P. The Radioactivity Group has developed
quantitative destructive assay techniques for this type of source.
(J.T. Cessna)
- Calibration of 32P "Hot Wall"
Angioplasty-Balloon-Catheter Sources by Destructive Radionuclidic Assays. A
quantitative, destructive-analysis procedure was devised for assaying the
32P activity content of "hot-wall" angioplasty-balloon
catheters. These sources are developed by Radiance Medical Systems, Inc.
(Irvine, CA), are intended for use in the prophylactic inhibition of restenosis
following balloon angioplasty in heart-disease patients. They consist of a thin
source of 32P which is incorporated directly into the balloon wall
of the angioplasty catheter. As a pure beta-particle emitter, the
32P content can not be assayed by nondestructive means since it has
no distinctive, external, radioactive signature. The assay was based on
performing a physicochemical digestion of the balloon catheter to extract the
32P activity followed by liquid scintillation (LS) spectrometry of
the resultant solutions. The calibration results were also used to link
Monte-Carlo-based theoretic modeling of the absorbed dose spatial distributions
to radiochromic-film dosimetric measurements that were performed by the
Dosimetry Group. (R. Collé)
- Re-Evaluations and Upgrades of the 222Rn
Pulse-Ionization-Chamber Measurement System. The national standard for
radon measurements is embodied in a primary radon measurement system that has
been maintained for nearly sixty years to accurately measure radon
(222Rn) against international and national radium (226Ra)
standards. All of the radon measurements made at NIST, and the radon transfer
standards and calibration services provided by NIST, are directly related to
this national radon standard. This primary radon measurement system consists of
pulse ionization chambers and ancillary gas handling and gas purification
equipment. The system was last modernized (with replacement ionization chambers)
nearly a decade ago and several shortcomings in its performance have recently
become evident. As a result, a major re-evaluation and upgrade of the system is
underway. This work includes conducting extensive systematic evaluations of the
system’s performance under a variety of operating and sample conditions,
re-designing the operating protocols, replacing the data acquisition and
analysis hardware, and re-writing the software codes.
(R. Collé, J.T. Cessna,
M.P. Unterweger, P. Hodge, and L. Karam)
- Standardization of New Tritiated-water Standards. The calibration of
the new tritiated-water standard, SRM 4927F, by internal-gas counting and
liquid scintillation counting has been completed. This standard and the lower
activity SRM 4926E and SRM 4361C will be used extensively worldwide.
The standards and their predecessors will continue to be used as the basis for
a uniform measurement scale for environmental tritium studies. The NIST
tritiated-water standards are also used extensively in liquid scintillation
counting. A remeasurement of the half-life of tritium has also been done and a
reevaluation of the half-life using all reported values is in progress. An
accurate value for the half-life is very important in extending the useful
lifetime of the tritium standards. (L.L. Lucas and
M.P. Unterweger)
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