1995/1996 Technical Highlights
- 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
photoeffect, 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. The quality of the work of
the Data Center is reflected in the many requests
for our data from other laboratories and in the
use of our data in engineering and scientific
compendia, books and review articles, and in the
reports and protocols of national and
international standards organizations. The
compilations of the Data Center rely heavily on
the synthesis of available theory to extend the
data and provide for comprehensive coverage over
broad ranges of energy and materials. Thus we
have long been involved in complex computational
analyses and in the development of highly
sophisticated transport-theoretic methods. Our
Monte Carlo transport calculations also are incorporated into some of the most
widely used general-purpose radiation transport codes. (S.M. Seltzer,
J.H. Hubbell, and M.J. Berger)
- Beta-Ray and Hot-Particle Dosimetry Calculations.
Highly refined theoretical methods have
been applied to beta-particle radiation protection
practice. Participating in the work of an ICRU/ICRP
Joint Task Group in updating fluence-to-dose conversion
factors used in radiation protection we have made
extensive electron Monte Carlo transport calculations of the depth-dose
distribution of electrons incident on phantoms of
water, PMMA, and tissue. These data, along with
extensive tables of basic electron penetration
data, a review of the physics of electron interaction
and transport through matter, and a review
of beta-ray transport calculations have been
incorporated into ICRU Report 56, Dosimetry of
External Beta Rays for Radiation Protection. Work
with the NCRP Scientific Subcommittee on hot-particle
dosimetry for radiation protection has
involved extensive Monte Carlo calculations of the
dose distributions from beta and gamma rays
emitted by hot particles, which has led to the
development of new point-kernel-based
calculations for a variety of shapes and arbitrary
sizes of beta- and gamma-ray sources containing
virtually any radionuclide. These new calculations
are being used for estimates of the dose distribution from a number of proposed
brachytherapy sources. (S.M. Seltzer)
- Space-Shielding Radiation Dose Calculations.
With support from NASA's Life Sciences Biomedical Research
Program, a computerized database and code package was developed for the
routine prediction of the absorbed dose from
incident electrons and their secondary bremsstrahlung,
and from incident protons, as functions of the thickness
of aluminum shielding of structures in space. The database is based on
extensive Monte Carlo calculations of the penetration,
scattering and energy loss of electrons in
aluminum slabs, the production of secondary
bremsstrahlung, and the penetration and scattering of
these photons to greater depths. The proton dose distributions were evaluated
in a straight-ahead approximation; a partial study of
the effects of nonelastic nuclear interactions of
the protons with aluminum nuclei and the
transport of nuclear secondaries was included.
The user code performs the necessary interpolation over
the database and the integration for any
specified spectra of incident electrons and
protons, giving the distribution in a variety of
simple geometries of dose in small detector
volumes of various compositions. Numerous
copies of this software have been distributed to
the international space-radiation-effects community,
and a collaboration is underway to incorporate this
new work into an existing commercial
package for estimating space radiation effects in
any earth orbit. (S.M. Seltzer)
- Waste Treatment by Electron Beam. We have
been investigating applications of radiation
(gamma and electron beam) technology for the
reduction and elimination of hazardous waste
materials generated in industry ("chemically
hazardous" waste such as polychlorinated biphenyls).
We are performing mass spectral analyses of PCB-contaminated water models.
Samples are analyzed before and after irradiation
with electrons and gamma radiation to determine
the amount of toxins destroyed, yields and
structures of products formed, and potentially
toxic by-products induced by this emerging
technological application. (L.R. Karam, and W.L. McLaughlin [with
B. Wise and M. Al-Sheikhly, University of Maryland])
- Alanine-EPR Film Dosimeter. Prototypes of a new polymer-based film
dosimeter containing alanine have been manufactured and tested. This
development program, under a CRADA with the W.R. Grace Co., is testing a
range of polymer/alanine ratios for the film dosimeter. These tests were
followed by comparative measurements on films of the same polymer/alanine
ratio manufactured by different methods. The selected film formulation was
mass produced and is undergoing extensive tests of its radiation response
characteristics under a variety of conditions for gamma-rays and electron
beams of different energies. Because films are easier to handle than pellets
(presently the most common alanine dosimeter shape), it is hoped that the
successful use of the thin (50 µm) film version for electron beams
will lead to the development of a more robust film which would be easily
adapted to high-dose gamma-ray processing applications.
(M.F. Desrosiers)
- Radiation Sources. The electron accelerators
continue to be used for a large variety of radiation
interaction studies. The principal sources
currently being employed are the 4 MeV electron
Van de Graaff and the 32 MeV Medical Industrial
Radiation Facility (MIRF). The activities carried
out on these sources will be specified in greater
detail in other areas of this report but will be
mentioned here. The Van de Graaff has been used
to check the radiation response of new and
existing types of diodes and solar cells. These
include silicon on silicon, gallium arsenide on
silicon and gallium arsenide, and indium phosphide
on germanium and indium phosphide types. Additionally, this source has been
used to investigate the shielding properties of various
composites at electron energies between 1 MeV and
2 MeV. The MIRF has been used for a large
variety of programs including: production of
radioactive isotopes of carbon, chlorine, fluorine,
iodine and molybdenum; precipitation of heavy
metals (lead and cadmium) from solution; degradation
of polychlorinated biphenyls (PCBs);
investigation of remote laser telemetering dosimetry;
radiation curing of epoxies; production of
radioactive fullerenes; investigation of radiochromic
dye-gels; and the investigation of scatter
from medical treatment beams. In addition to
these ongoing programs, the beam line for the
production of monoenergetic photon beams from
the interaction of electrons in single crystals is
nearing completion. (C.E. Dick and M.R. McClelland)
- International Comparison of X-ray and Gamma-ray Standards.
NIST has recently completed a number of comparisons of gamma-ray primary
standards with those of other national and
international standards laboratories. Cobalt-60
air-kerma standards were compared with the
Ente Per Le Nuove Tecnologie, L'Energia E
L'Ambiente (ENEA) in Rome, Italy; agreement was
within 0.1 %. A similar comparison was done for
60Co with the Bureau International des Poids et
Mesures (BIPM) in Paris, France, using the same
transfer standards as was used with the ENEA.
The agreement between the NIST and the BIPM
standards is within 0.2 %. These two comparisons
provide NIST with additional corroboration of
standards used for radiation therapy. In addition,
a comparison of air-kerma standards for 137Cs
gamma-rays has been carried out among six
laboratories: the Bundesamt für Eich- und
Vermessungswesen (BEV), Vienna, Austria; the
BIPM; the Laboratoire Primaire des Rayonnements
Ionisants (LPRI), Gif-sur-Yvette, France;
the NIST; the Österreichisches Forschungszentrum
(OFS), Vienna, Austria; and the Orzagos
Mérésügyi Hivatal (OMH), Budapest, Hungary.
This comparison of the standards of the six
laboratories show an overall agreement of 0.8 %,
a satisfactory level for these standards that are
used primarily for radiation protection applications.
This important work contributes to the
harmonization of international radiation dosimetry.
(P.J. Lamperti)
- Absorbed-Dose Calibrations and Development
of Secondary Standards. Radiation therapy has
been practiced in the U.S. since the turn of the
century. Approximately 500,000 cancer patients
are treated annually in 1300 therapy facilities
using high-energy electron or photon beams from
some 2700 linear accelerators and 60Co
teletherapy units. In 1993, NIST completed
extensive development of a water calorimeter
which can realize the quantity absorbed dose. The
changeover from in-air-kerma calibrations to
in-phantom absorbed dose calibrations is a logical
and necessary evolutionary step in radiation
dosimetry. NIST is collaborating with AAPM TG#1
to develop a secondary standard for use by the
AAPM Accredited Dosimetry Calibration
Laboratories. Characterization of reference
sources and ionization chambers is currently in
progress. Protocols for calibration and round-robin
performance testing are currently being
developed. (J. Shobe and P.J. Lamperti)
- Alanine Dosimeter Response in Proton
Therapy Beams. The use of proton beams for
radiation therapy of various malignancies is being
studied at a number of institutions worldwide.
Because of the wide geographic distribution of
these facilities and the variety of dosimetry
methods in use, there is a need for a reliable
mailable dosimetry system that has a uniform
response over the energy range used in proton
therapy. A comparison was made of the time-dependent
response of alanine dosimeters irradiated in the
radiation-therapy proton beam at the
Harvard Cyclotron in Cambridge, MA. Dosimeters
were placed near the entrance point of the beam
in the phantom and at the Bragg peak. Time-dependent
changes differed slightly between these
two positions. A decrease in intensity (<0.5 %) over
several hours was measured in the Bragg peak
region after which the intensity remained
constant over the next several days; dosimeters
irradiated at the entrance point continued to
decrease from the earliest time of measurement
continuing over the course of a few days (<1 %) before reaching a
constant level. (M.F. Desrosiers, V. Nagy, and K. Gall)
- Mammographic X-Ray Exposure Standards.
The first x-ray calibrations using the NIST
primary air-kerma standard for mammography
were conducted in March of 1996. The parameters
of the new molybdenum and rhodium beam
qualities were verified including a noninvasive
determination of the kVp endpoints. In assistance
to other laboratories, differences in beam
parameters due to changes in filtration thickness
were investigated. Researchers from NIST and the
University of Wisconsin-Accredited Dosimetry
Calibration Laboratory (UW-ADCL) collaborated to
determine appropriate transfer standards for the
mammography energy range. The energy
dependence of numerous ionization chambers
was measured for the NIST molybdenum and
rhodium beam qualities. The preliminary results
of this energy dependence study were used to
characterize the chambers NIST acquired for use
as reference-class transfer standards. These
transfer standards will be used for
intercomparisons with other national laboratories
and ADCLs, as well as for quality assurance of
routine NIST calibrations. Initial energy
dependence studies of the six newly acquired
NIST reference-class mammography transfer
standards were conducted. The data from these
energy dependence studies has been used to
assist with the establishment of guidelines for the
secondary calibration laboratories, in addition to
serving as the initial performance history of the
new facility. (C.M. O'Brien and P.J. Lamperti)
- Scattered Fraction Measurements of High-energy
X-rays. High-energy photon beams from
medical electron linear accelerators, used for
cancer therapy, require radiation shielding of not
only the direct beam but also the radiation
scattered from the patient. A program for the
determination of scattered dose fractions for 6 MeV,
10 MeV, 18 MeV, and 25 MeV photon beams has been taken
using the NIST medical linac in MIRF. The
"patient" is replaced by a cylindrical water
phantom (with radius 12.81 cm) placed at the treatment position in the
radiotherapy x-ray beams. Measurements were made at 200 cm from
the isocenter at angles ranging from 0° to 160°
with respect to the central axis of the incident
beam. The ionization-chamber current measured
at each angle is reported as a fraction of the
ionization current measured at the center of the
phantom. The results of these measurements are
being compared to Monte Carlo calculations
which are often used to determine shielding
requirements in medical radiotherapy facilities.
Secondary neutron measurements have been
made at the same positions for 18 MeV and 25 MeV
beams utilizing thermoluminescence dosimeters
(TLDs) placed in a 12.7 cm polyethylene
moderating sphere. The results of this work is
being used by an AAPM task group assigned to
reappraise the design and evaluation of structural shielding for medical
facilities that use high-energy x rays. (J. Shobe)
- 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. The procedure of angioplasty is performed over 300,000
times in the U.S. each year, and in about 40 % of the cases restenosis
occurs, requiring another treatment. Research has shown that a dose of about
10 Gy, delivered to the wall of the blood vessel after the angioplasty has
been performed, is effective in inhibiting restenosis. 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 is in progress. As an example,
spatial distributions of absorbed dose rate in tissue-equivalent plastic are
shown in Fig. 1 for the plane, parallel to the train, at a distance of
approximately 2 mm from the encapsulating surface. The plot on the left
is for a train of 5 seeds with poor uniformity, that on the right for
9 seeds with good uniformity. Work for Novoste Corporation led to their
being the first company allowed by the U.S. Food and Drug Administration (FDA)
to have their system used to perform clinical trials. Collaborations were also
begun between NIST and NeoCardia for dosimetry of a 32P wire,
Isotopen-Technik of Germany for the dosimetry of a 90Y wire, and
with Washington Hospital Center for dosimetry of various sources, including a
miniaturized x-ray generating intravascular device. (C.G. Soares)
Figure 1. Dose distributions from "trains" of
90Sr/ 90Y seeds being developed for intravascular
brachytherapy.
- Novel Approaches in Nuclear Medicine. We are
performing ongoing research projects involving
several aspects of nuclear medicine, including
investigations of novel methods of delivering
radiopharmaceuticals. We have constructed a
fullerene production apparatus and have begun
incorporating specific atoms into the fullerene
cage. Application of fullerenes as carriers of
radioisotopes for use in cancer therapy has been
suggested, but has not been studied either
theoretically or experimentally. Since fullerene
cages are capable of physically and chemically
isolating radioisotopes from their associated
pharmaceutical, a flexibility in choosing radioisotopes
for specific tracing or therapeutic applications, not
possible with currently available radiopharmaceuticals, would
be possible. The successful production of radiofullerenes for
cancer therapy applications would constitute a
significant advance in the field of radiotherapy.
The main objective of this project is the successful
development of radioactive fullerenes suitable for
use in medical imaging such as technetium and
radioactive carbon (11C). We have used the MIRF
electron accelerator to convert non-radioactive 12C
to radioactive 11C in C60 fullerenes obtained
commercially, as well as C60 and larger species
purified in our facility. Yield and purity of 11C
heterofullerenes have been determined by high pressure liquid chromatography
(HPLC), multiphoton detector (MPD), liquid scintillation counting (LSC), thin
layer chromatography (TLC), and Fuji phosphoimaging. In addition, we have
produced, extracted and purified by HPLC fullerenes produced in the presence
and absence of radioactive 99mTc and have quantified the
amount of radioisotope encapsulated by MPD, LSC and Fuji imaging (after
separation on TLC plates). Work has begun on encapsulation of iodine.
(L.R. Karam and B.M. Coursey [with M.G. Mitch, Univ. Maryland;
L. Rodríguez-Rosado and L. Laureano-Pérez, SURF])
- Development of Radiosensitive Dosimetric Gels. We have prepared and
done preliminary studies of radiochromic gels (gelatin containing the
radiochromic dye 3,5-triphenyl-2H-tetrazolium chloride), showing the
dose-coloration response to gamma irradiation. Based on those results, we have
subjected similarly prepared gels to electron beam irradiation (at the MIRF)
and have observed a comparable response. Gels were also provided to other
laboratories for comparison in therapy type beams. (L.R. Karam,
W.L. McLaughlin, P.J. Lamperti, and B.M. Coursey [with
L. Francis, SURF; R. Schultz, M. Maryanski, and M. Ranade,
MGS Research])
- Wide-Angle Free-Air Chamber for 125I.
The wide-angle free-air chamber (WAFAC) will serve as the
NIST standard for air-kerma-strength measurements for 125I
brachytherapy seeds and perhaps other low-energy photon emitters. The
chamber and associated fixtures are in the
process of being moved to another room. The
mechanical support mechanism, source holder,
and filter holder are being redesigned at this time.
Redesign of the WAFAC support mechanism will
allow for greater flexibility in determining the
WAFAC parameters. We anticipate that redesign
of the source support mechanism will reduce
uncertainties due to source holder scatter and
attenuation. In addition, work is in progress to
automate the measurement and analysis procedure,
including a motorized filter/shutter
wheel. Locating the WAFAC in a separate facility
will provide an opportunity for a dedicated
measurement facility. (P.J. Lamperti, J.T. Weaver, and
J. Shobe)
- Neutron Interferometry and Optics. Since
September 1996, the Neutron Interferometry and
Optics Facility (NIOF) in the NIST Cold Neutron
Guide Hall has been fully operational as a
national users' facility, and it has a crowded
schedule of experiments. One third of the beam
time is reserved for NIST staff research, focused
primarily on development of new techniques for
materials research. The remainder of the beam
time is allotted to outside users by an advisory
committee, based on the merit of research
proposals submitted. Experiments have already
been done involving researchers from the
University of Missouri-Columbia, Exxon, the
Hahn-Meitner Institute (Berlin), and the Atom
Institute (Vienna). These experiments have given
preliminary results in phase-contrast imaging,
high-resolution two-dimensional absorption
imaging, neutron scattering length measurements,
and testing of very large scale interferometer
crystals. Most of these experiments are
expected to be concluded in the next few months
after receiving additional beam time.
The very keen interest of the national and
international user community in this new facility
arises from its unique capabilities. Two new large
interferometers of NIST design have been
fabricated and are being used to carry out experiments.
These interferometers have novel design
features allowing elimination of second-order
beam contamination and operation at low neutron
energies (~4 meV) which are advantageous for
materials science and solid state physics
research. These are the first neutron
interferometers in the world that have been
successfully operated at such low neutron energies.
When operated at slightly higher energies,
one of these interferometers achieved fringe
visibility as high as 88 % at the best spot and had
a large area with fringe visibility exceeding 70 %
everywhere within. At 4 meV the fringe visibility is
about 40 % at the best spots. Phase stability is of
the order of a few milliradians per 24 hours. The
special foundation under the facility, robust
vibration isolation systems, an active position
stabilization system, and the special
characteristics of the new NIST-designed
interferometer crystals all contribute toward the
achievement of these unprecedented performance characteristics.
In addition, we have initiated the development of a large scale (~20 cm
diameter) neutron radiography/tomography capability. (M. Arif,
D. Jacobson, A. Thompson, and T. Gentile)
- Development of Neutron Spin Filters by Laser
Polarization of 3He. The developmental program
to produce polarized neutron beams using a 3He
spin filter at NIST has seen major advances
during the last two years. This method should
yield efficient, broadband neutron polarizers that
will have several advantages over conventional
polarizers, both for condensed matter studies and
for fundamental physics. In addition, the
technology for polarizing the 3He gas is applicable
to the newly emerging medical field of polarized
gas magnetic resonance imaging (MRI), as well as
applications in basic nuclear physics.
The spin filter is based on the spin-dependent
absorption of neutrons by polarized 3He in the
reaction 3He(n,p)3H. We have parallel programs to
produce polarized 3He either by (1) spin-exchange
with optically pumped rubidium vapor or (2) direct optical pumping
of metastable 3He followed by mechanical or cryogenic
compression of the low pressure gas. Progress in each program will
be discussed separately below.
Spin-Exchange Based Spin Filter: Milestones
passed in the last two years include optimization
of 3He polarization using the Ti:Sapphire laser,
conversion to use of inexpensive diode laser
arrays for optical pumping of Rb, construction of
a cell filling station, production of cells with
properties required by a real polarizer, and initial
testing of cells on a neutron beam. Work has
begun on a medical imaging spin-off of this technique.
A 3He polarization of 25 % was measured in
the spin-exchange setup early in 1995 using the
Ti:Sapphire laser. Later in 1995 we measured
15 % polarization using a laser diode array. The
diode laser requires higher density 3He for optimal
performance, so we have constructed a cell filling
system and a new oven to better match the
properties of the diode laser. The filling system
was commissioned in April of this year, and we
have produced multiple cells with long lifetimes
(some near 200 hours). In the summer of 1996 we
tested some of the cells on the polarized neutron
beam at the end of NG-6 at the Cold Neutron
Research Facility. Results from these tests are
guiding the design of improved cells and the
choice of cell materials. The new materials have
been ordered and we expect to have a viable spin
filter cell within the next few months.
In April 1996 we were contacted by a
researcher at the University of Pennsylvania
regarding MRI of lungs using polarized 3He. In
collaboration with the Medical Imaging Group at
Pennsylvania, we have produced multiple cells
containing roughly one liter of polarized 3He, and
produced initial images of a volunteer's lungs,
which are almost impossible to image using traditional MRI techniques.
Metastability-Exchange Based Spin Filter: In
the metastable method, the gas is polarized at a
pressure of ~133 Pa, and then must be compressed
up to a pressure of ~100 kPa for use as a neutron
polarizer. Maintaining the polarization during
compression is a technical challenge, but has
been achieved by a group in Mainz, Germany
using a two-stage piston compressor. We are
developing a similar compressor in collaboration
with Indiana University. Our collaborators at
Indiana have designed this apparatus, and NIST
is assisting with the construction of their optical
pumping system. In addition, we are investigating
an alternate compression apparatus based on
modification of a commercial diaphragm pump.
Success in this alternate approach could yield a
compact, simple apparatus that would allow very
economical development of the metastable method
for neutron polarizers and other applications. The
apparatus for optical pumping of 3He at low pressure has been
constructed and 80 % polarization has been obtained. (A. Thompson,
T. Gentile, G. Jones, and F. Wietfeldt)
- The Neutron Lifetime and Asymmetries of the
Weak Interaction. The NG6 End Station in the
Cold Neutron Guide Hall is operated as a national
users' facility for fundamental neutron physics,
under the supervision of the same advisory
committee mentioned above in regard to the NIOF.
Experiments at this beam station are focused on
testing aspects of the standard model of particle
interactions which are accessible from precise
measurements of neutron interactions and decay.
For a number of reasons, including two
longer-than-expected shutdowns of the NIST
research reactor, the neutron lifetime experiment
had to be taken off-line temporarily to allow beam
time for two other weak interaction experiments
which were ready to run. The neutron lifetime
experiment at NIST collected data during two
separate periods, 10/93-5/94 and 11/95-5/96.
This difficult experiment which had run with
some success in Grenoble previously, has had
more serious difficulties in the attempts to
improve on or even reproduce the Grenoble
results at NIST. Investigations of several possible
causes of these difficulties are in progress off-line
while the initial runs are made for the two other
experiments: Parity-Non-conserving Spin Rotation
in Liquid Helium and the emiT Experiment, a
search for time-reversal asymmetry in neutron beta decay.
Parity-Non-conserving, Neutron Spin Rotation
in Liquid Helium: This experiment concerning
details of parity violation is primarily the work of
a team from the University of Washington, Seattle
(D. Markoff, S. Penn, B. Heckel, and E.
Adelberger). NIST physicists have been involved in
preparation of the supermirror polarizer and
analyzer for this experiment, as well as other
aspects of beamline preparation and shielding;
the polarization achieved was 96 %. This
experiment collected useful data intermittently
during 9/96-11/96, but suffered frequent outages
for recovery from cryostat problems. The data
obtained will be marginal statistically for setting
a significant bound on the spin rotation per unit
length of travel. A future run with an improved cryostat is expected.
The emiT experiment (time reversal) is a very large
collaboration involving physicists and engineers
from the Univ. of Washington, the Lawrence
Berkeley Laboratory, the Univ. of Michigan, Los
Alamos National Laboratory, and NIST. This
experiment uses the same supermirror polarizer
arrangement as the spin rotation experiment. The
experiment will run for about 20 weeks beginning in December of 1996.
Two more experiments besides conclusion of the beam-type neutron lifetime
experiment are in the planning stages for 1997. An ultracold neutron,
bottle-type lifetime experiment with collaborators at Harvard University is in
preparation, with an initial run expected in 1997. A repeat of a Russian
spin-antineutrino asymmetry experiment is also likely to be allocated beam
time. NIST staff participated in an initial run of this experiment in Grenoble
in the summer of 1996. (F. Wietfeldt, J. Nico, D. Gilliam,
J. Adams, and G. Jones)
- Neutron Dosimetry for Reactor Safety Assessment.
Through a cooperative agreement with the
Office of Nuclear Regulatory Research, NIST
provides measurement assurance services and
consultation related to neutron dosimetry and
nuclear reactor safety. After many years of NIST
consultation with experts from industry and
national laboratories, the Draft Regulatory Guide
DG-1053 (formerly DG-1025), Calculational and
Dosimetry Methods for Determining Pressure
Vessel Neutron Fluence, was issued in essentially
final form with very little objection from the major
reactor vendors or regulated utilities. NIST will
continue to support the measurement assurance
steps called for in this guide to keep the national
measurement system on track in this field.
Consultation issues have begun to go from the
generic case to plant specific cases, as aging
nuclear electric power plants are nearing levels of
cumulative neutron radiation exposure which
could be of concern regarding the risk of brittle
fracture of a pressure vessel under certain
extreme conditions. NIST also has carried out and
published research on fission neutron transport
through thick steel sections to test the relevant
nuclear data files employed in calculations for assessment of reactor safety.
(J. Adams, E.D. McGarry, J. Nico, and J. Grundl)
- Nuclear Cross Section Standards. Nuclear
cross section data standards are important for
nuclear reactor safety and performance calculations,
development of fusion energy, understanding accelerator
neutron source dosimetry, and basic studies in astrophysics. The NIST
program in support of these nuclear data standards
has been substantially reduced in scope in
recent years, and the emphasis of the remaining
program has shifted from measurements to
evaluation and international coordination of
standards efforts. NIST performs its role in
motivating and coordinating new standards
measurements by examining the standards data
base, pursuing the extension of the standards
over a larger energy range, and leading efforts
directed toward new evaluations of the standards.
These efforts have taken place largely through
participation in the Cross Section Evaluation
Working Group and our international involvement
through the Nuclear Energy Agency Nuclear
Science Committee and the International Nuclear
Data Committee. Continuing experimental
collaborations include an effort at the Ohio
University Tandem Accelerator to refine the
angular distribution data for the very important hydrogen scattering cross
section standard. (A. Carlson and R. Schrack)
- Dissemination of National Standards of
Radioactivity. The Radioactivity Group disseminated
the national standards of radioactivity
mainly through the following three activities:
(1) Over 700 radioactivity standard reference
materials (SRMs) were provided. (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 60 calibrations of customer sources were provided.
(L.L. Lucas, J.C. Cessna, and J.M.R. Hutchinson)
- Glow-Discharge Resonance Ionization Mass
Spectrometry. Work continued on the development
of a glow-discharge initiated mass spectrometer system
which would permit the direct
compositional analysis of soils and sediments for
radioactive and non-radioactive trace elements.
For effective radioassay, a sensitivity in the range
of 10-13 is useful for most environmental
contaminants. A continuous wave titanium-sapphire
(Ti-Saph) laser was incorporated into the
system to perform initial highly selective
Z discrimination before isotopic selection in
the mass spectrometer. Preliminary tests of
this system have been performed. (J.M.R. Hutchinson)
- Natural Matrix Standards. In October 1977, the
International Committee for Radionuclide
Metrology sponsored a symposium at Woods Hole,
Massachusetts to define needs and initiate
programs to develop natural matrix radioactivity
standards. The term "natural matrix standard"
(NMS) refers to a standard of radioactivity which
is homogeneously contained in a matrix, such as
soil or vegetation, in the same chemical forms as
are found in the environment. Since that time six
NMSs have been issued: River Sediment, Human
Lung, Human Liver, Rock Flats Soil #1,
Freshwater Lake Sediment, and Peruvian Soil.
Three more are under development as follows:
- Over the past decades, on the order of 1015 Bq nuclear
waste have been stored in the oceans. Potential contamination of the oceans
from leaking nuclear waste has caused worldwide concern. Because the
determination of low-level radioactivity in ocean sediment is a difficult
technical task, a basis for measurement quality assurance, methods
verification, and data comparability is needed. The NIST ocean sediment
Standard Reference Material (SRM 4355) is being developed using a
composite of 1 % contaminated Irish Sea sediment and 99 % Chesapeake
Bay sediment by weight. Twelve radionuclides including 40K,
90Sr, 137Cs, 226Ra, 228Th,
230Th, 232Th, 234U, 235U,
238Pu, and 239-240Pu were certified. The mean values
were reported for an additional 10 uncertified radionuclides: 129I,
155Eu, 210Po, 210Pb, 212Pb,
214Pb, 214Bi, 228Ra, 237Np, and
241Am. The standard reference material in unit quantities of
about 100 g each will be available by the end of 1996.
- The second standard for ocean studies is dried shellfish flesh. Shellfish
is a material widely assayed for radionuclide content because it is a
bioaccumulator and is part of the human diet. Approximately 350 kg of
dried shellfish from the western Pacific Ocean, Irish Sea and White Sea
are now available for processing. Interlaboratory comparisons are being
planned with international expert laboratories for 1997.
- Bone as a sink of a number of long-lived
radionuclides is a key organ for biokinetics model
development and dosimetry studies. Development
of a bone standard for the bone-seeking
radionuclides is one of the most important tasks
in ensuring quality control in bone sample
analysis and for providing a common basis for
data comparison and evaluation. The development
of a bone ash standard for low-level radioactivity
measurements provides a great analytical
challenge in radiochemistry because of its high
calcium and phosphate content. The standard is
being characterized for 90Sr, 210Pb and U, Th and
Pu among international radiobioassay expert
laboratories. (K.G.W. Inn and Z. Lin)
- Chemical Speciation of Environmental Radioactivity.
The primary concerns associated with
radionuclides in the environment are: (1) migration
through natural systems; and (2) bioavailability
via the food chain. The problem is that
ionizing radiation in sufficient doses can affect a
variety of processes in higher organisms. Since
the mobility and bioavailability of radioactive
elements in the environment must be dependent
upon the element's chemical speciation, the
characterization of the element's physicochemical
forms in soils and sediments is a key factor in
understanding and predicting the migrational
behavior of trace metals and radionuclides. One
widely used empirical approach for describing
speciation is the application of sequential-chemical extractions.
NIST is now exploring the possibility of
certifying Standard Reference Materials (SRMs)
for soils and sediments by "fraction" as well as for
"total" concentration as indicators of the
bioavailability of radionuclides in the environment.
A statistical, experimentally designed,
sequential leaching, radiochemical separating,
and low-level beta-particle counting procedure
has been designed and will be carried out in
collaboration with Florida State University to
establish a reference method. The seven-step
extraction procedure will be optimized for Pu, U,
and 90Sr from the following fractions:
exchangeables, carbonates, reducibles, organics, iron and
manganese oxides, acid leachables, and silicates.
Four experimental conditions (reagent concentration,
pH, duration of extraction period, and
temperature of reaction) were identified as
potentially significant parameters. The study will
begin by optimizing the method using SRM 4357
(Ocean Sediment). (K.G.W. Inn and M. Schultz)
- Photonuclear Produced Radioactivities. The
MIRF facility has been used to produce positron
emitting radioactivities. Techniques have been
developed to characterize those sources. Efficiencies
for positron emission, which have been
stopped and annihilated, have been developed
and compared, when appropriate, with associated
gamma-ray emission from the decay. Radioactivities
characterized this last year included 126I,
18F and 34mCl. The last radionuclide is of
particular interest as a high gamma-ray energy
emission rate standard for Ge detector calibration.
34mCl has a 3303.6 keV gamma-ray with a
probability per decay of 0.123. This abundant
emission and readily prepared radioactivity allows
the extension of the current efficiency curve to that energy.
(F.J. Schima)
- 63Ni Standardization and Decay Studies. Standard
solutions of 63Ni have recently been prepared and were disseminated
as NIST SRM 4226C. The solutions were calibrated by
4πβ liquid scintillation (LS) spectrometry with 3H-standard
efficiency tracing. This radionuclide is of interest to the nuclear reactor
community because it is often found in reactor environments as the neutron
activation product of nickel present in the steel used in construction of those
facilities. Certain physical properties, namely a low β-
energy (66.945 ± 0.004 keV) and a relatively long half-life
(101.1 ± 1.4 a) also make it attractive for radionuclidic
metrology studies, as it tends to be a more sensitive indicator of effects in
measurement technique and procedures than would a radionuclide with a higher
β- energy (stated uncertainties are standard uncertainties).
This SRM is gravimetrically related to two others previously prepared by
NIST/NBS (SRM 4226, prepared in 1968 and calibrated by microcalorimetry,
and SRM 4226B, prepared in 1984 and calibrated with 4πβ LS
spectrometry with 3H-standard efficiency tracing). This allows a
comparison to be made between the three sources as a check of both measurement
consistency andsolution stability. After adjusting the data from the 1968 and
1984 measurements to include the latest available nuclear data for both
63Ni and 3H, we found the three measurements to be in
agreement to within 0.3 %. Because of this remarkable consistency over a
27 year span, the three data sets provided enough data to make the
first-ever determination by radioactive decay of the 63Ni half-life.
Using these data, we determined the half-life to be
101.06 ± 1.97 a. A critical review of measurements of the
63Ni half-life was performed, resulting in a new recommended value
of 101.1 ± 1.4 a. A series of articles addressing all of these
topics has been published (see Appendix A). (B.E. Zimmerman and
R. Collé)
- Final Results for the International Intercomparison
of Marine-Atmospheric Radon Measurements.
The importance of various kinds of high
quality radon measurement data to the world's
atmospheric transport modelers was identified in
the preceding highlight. In 1991-1992, NIST
conducted an in situ calibration and
intercomparison exercise for marine atmospheric radon
measurements. The participating laboratories
have been responsible for perhaps 95 % of the
available surface-level measurements gathered
around the globe over the last decade. The results
of this intercomparison exercise have at last been
fully published in a series of articles that
appeared in the Journal of Geophysical Research
and Journal of Research of NIST. The
intercomparison utilized a common standardized,
in situ, reference basis (provided by NIST) that
could be directly related to U.S. national, and
internationally, recognized, 226Ra and 222Rn
standards, and evaluated the performance of all
principal instruments that are used to measure
radon activity concentrations for marine-atmospheric
studies. The findings will assist
various users in the global modeling community
in applying the available and future radon
measurement data bases in a more reliable and
effective manner. The work went beyond serving
the needs of just this particular intercomparison.
It also demonstrated the broader utility of the
developed procedures, i.e., the calibration protocol
and the methodology for providing in situ
standardized samples. Most environmental
measurement intercomparisons of field instruments
in actual use merely rely on evaluating the
relative performance of the participants, or some
comparison to the pooled results. This exercise
demonstrated, for the very first time, the
capability of providing a standardized reference
basis even for such low-level, field-measurement
intercomparisons. The developed methodologies
could be adopted with slight modifications to
cover other radon concentration ranges and other
applications, and could be employed in many
other types of radon environmental field-measurement
intercomparisons. (R. Collé)
- International Intercomparison of 63Ni
and 55Fe. The Radioactivity Group recently participated in
an international measurement intercomparison
for 63Ni and 55Fe, which was conducted amongst
principal national radionuclidic metrology
laboratories. The intercomparison was sponsored
by EUROMET, and was primarily intended to
evaluate the capabilities of liquid scintillation
spectrometry techniques for assays of nuclides that decay by low-energy
β- emission
(like 63Ni) and by low-Z (atomic number) electron capture
(like 55Fe). Preliminary results from this
intercomparison reveal an excellent agreement for
63Ni between the NIST finding and those from
other participating laboratories. The results for
55Fe suggest that we need to conduct rigorous,
systematic evaluations of our LS capabilities in
assaying radionuclides that decay by low-Z
electron capture. (R. Collé and B.E. Zimmerman)
- Liquid Scintillation Spectrometry Intercomparison
of Tritiated Water Standards. Radioactivity standards
of tritiated water (3H2O)
disseminated by the LPRI and the NIST, the
national radionuclidic metrology and standardization
laboratories of France and U.S.A., respectively, have been intercompared
by liquid scintillation (LS) spectrometry. The ratio of the
certified massic activities for the two standards
was compared to that obtained from direct
measurements on matched sets of LS cocktails
prepared from the standards. Seven experimental
trials (involving a total of 21 counting sources for
each standard) were performed for the
comparison. The trials were performed under a
wide range of experimental conditions, including
use of two different LS spectrometers and three
series of LS cocktail compositions (with
systematically varied 3H detection efficiencies).
The results exhibited an apparent mean disagreement between
standards of less than 0.4 % on a relative basis. For contrast, the
relative combined standard uncertainty on the massic
activity ratio for the two standards, as obtained
from their respective certified uncertainty assessments,
is about 0.7 %. A paper on these results
was published in Applied Radiation Isotopes.
(B.E. Zimmerman and R. Collé)
- Development of Standard for the Palliative
Therapy Radionuclide 117mSn. As part of an
increasingly active program to develop national
standards for radionuclides of interest to the
nuclear medicine community, this group has
recently performed a calibration of 117mSn, which
is currently under study for use in palliative
therapy for pain associated with metastatic bone
cancer. The calibration was performed using three techniques:
γ-ray spectrometry with HPGe detectors, γ-ray
spectrometry with a 4π 30 cm
NaI(Tl) system, and 4πβ liquid scintillation (LS)
spectrometry. Data were obtained using HPGe
spectrometry to confirm the probability per decay
of the major emissions. A procedure for the direct
standardization of this isomeric radioactivity
based on sum coincidence peaks is underway.
This procedure would allow calibration of 117mSn
sources using an HPGe detector with adequate resolution. Data were also
obtained for the re-determination of the half-life using all three
detection systems and included an additional
measurement using the NIST ionization chamber.
The half-life was found to be 19.98 ± 0.04 d (standard
uncertainty), the weighted average of the LS, HPGe, NaI(Tl), and
ionization chamber measurements. This value is
3 % higher than the ENSDF-recommended value,
which is based upon a single measurement. Our evaluation of all
117mSn half-life measurements, including our new data, indicates
that the ENSDF recommendation is an outlier.
This radionuclide is particularly exciting
because of the greater uptake of 117mSn(4+) DTPA
in bone tissue relative to the marrow. Compared
to other commonly-used bone palliation
radionuclides such as 89Sr, 32P,
and 186Re, there is as much as a 4-fold increase in the
ratio of bone-surface dose to bone-marrow dose with the use of
117mSn(4+)DTPA. This suggests that a much higher
dose can be given to the patient before marrow
toxicity levels are reached, possibly leading to the
ability to treat the metastases themselves. An
additional advantage in using 117mSn is the
presence of a 159 keV γ-ray, which allows the
uptake and distribution of the radionuclide to
studied with conventional imaging devices. (B.E. Zimmerman,
J.T. Cessna, F.J. Schima, and M.P. Unterweger)
- Calibration of Large-Area Beta Sources. Calibrations of the
2πβ emission rates and measurements of homogeneity of several large
area sources have been completed. The effects of β-backscattering
are under investigation in order to provide accurate values of activities
of these sources for use in calibrating β field monitors. These
investigations include comparison of these measurements with Monte Carlo
calculations performed by Martin Berger. In addition, a method has been
developed to estimate the effective source thickness, an important parameter
in relating the measured rate to the activity. (M.P. Unterweger and
P. Hodge)
- Iodine-129. 129I is a very long-lived (the half life
is 15 million years) fission product that can be significantly
concentrated by some organisms. Hence there is interest in monitoring this
radionuclide in food and in the environment. Isotopically-enriched
129I was obtained from the Oak Ridge National Laboratory. This
material contains approximately atom fraction of 96 % 129I. The
activity was calibrated by 4πβ (LS)-γ-anticoincidence
counting and the material is now available as Standard Reference Material
(SRM) 4949C. (L.L. Lucas)
- Iron-55. 55Fe is a radionuclide with a half life of
2.9 years that is produced with great efficiency whenever iron is
irradiated with neutrons. It is a very common byproduct of reactor operation.
55Fe decays by electron capture and emits only low-energy
(5 keV) x rays. Hence, its calibration is more difficult than most
of the radionuclides for which standards are issued. The massic activity
is being measured in the NIST 4π(e+X)-γ-anticoincidence counting
system using 54Mn as the efficiency-tracing radionuclide. The
half-life data are being reevaluated and SRM 4929E will be issued shortly.
(L.L. Lucas)
- Cesium-137. 137Cs is a long-lived (the half life is
30 years) fission product that is used as a gamma-ray source for
irradiation and detector calibration and as a fission monitor for nuclear
fuel. For this reason, SRM 4233 and its subsequent reissues have been
certified in terms of both activity and number of 137Cs atoms. The
137Cs massic activity was measured in the NIST
4π(e+X)-γ-anticoincidence counting system using 134Cs as
the efficiency-tracing radionuclide. The massic number of 137Cs
atoms was measured by isotope dilution mass spectrometry using ultra-pure
133Cs as the diluting isotope. The latest reissue of this standard
is now available as SRM 4233D. (L.L. Lucas)
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