Radioactivity Standards:
to develop and provide standards for radioactivity
based on the SI unit, the becquerel, for homeland security, environmental,
medical, and radiation protection applications.
INTENDED OUTCOME AND
BACKGROUND
The Radioactivity Group is responsible for developing metrological
techniques to standardize new radionuclides for research, and for exploring
applications in health care, worker protection, environmental protection, and
national defense. A vigorous program is underway to develop microcalorimetric
methods to measure power from radioactive sources. This work is proceeding in
parallel with liquid-scintillation detector methods based on
triple-to-double-coincidence ratio (TDCR) counting. Accurate activity
measurements over a broad dynamic range are obtained using these two
complementary techniques. For ultra-low-level atom counting, we are continuing
development of resonance ionization mass spectrometry (RIMS), which has
applications in nuclear forensics as well as in environmental radioactivity.
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(© Barry Gardner).
Figure 1. Dr. Leticia Pibida adjusting laser optics for resonance
ionization mass spectrometry (RIMS). |
The Radioactivity Group also provides leadership in a national program for
radiopharmaceutical standards. We provide the national standards for
radionuclides used in 13 million diagnostic procedures and 200,000
therapeutic nuclear medicine procedures annually in the U.S. This work has been
expanded to include radioactivity measurements of brachytherapy sources used to
prevent restenosis following balloon angioplasty.
With heightened concern in the U.S. regarding terrorists' attacks using
radiological or nuclear weapons, we are leading a national effort to develop
standards and protocols for radiation instrumentation for first responders.
Initial efforts are directed towards four areas:
- equipment standards and test methods for hand-held detectors for first
responders;
- a testbed for cargo-container and truck inspections for radiological and
nuclear materials;
- radionuclidic forensic methods based on mass spectrometry; and
- a national quality-assurance system that supports homeland security needs.
Many tens of thousands of low-level radiochemical measurements are made
annually to support environmental remediation and occupational health programs.
The credibility of these measurements has been based on participation in
regulation-driven, performance evaluation programs of limited scope. The
fundamental flaw that the metrology community recognizes is that there is a
lack of direct linkage to national radioactivity standards. This situation is
being addressed in the publication of three ANSI standards. These consensus
standards call for a traceability-testing program that links the quality of
operational measurements to the national standards.
Accomplishments
Cargo-Container Inspection System
The smuggling of materials or weapons in shipping containers represents a clear
threat to the U.S. Major radioisotopes of concern are predominantly the
plutonium and highly enriched (weapons grade) uranium isotopes, as well as some
of their fission products, e.g., 90Sr and 137Cs. These
are the key ingredients of nuclear fuel and nuclear warheads. The threat also
includes radioisotopes primarily used in industrial and medical applications,
such as 60Co, 137Cs, 170Tm, 192Ir,
125I, 99mTc, 241Am, and 226Ra.
We are developing a cargo-container inspection testbed to provide a means for
calibrating detectors used for radioactivity measurements in the field. This
testbed is outdoors to simulate actual operational conditions. Different kinds
of standard radioactive sources will be placed inside a cargo container, so
that different kinds of detectors can be tested and calibrated to optimally
detect and measure the sources. Computer modeling will be used to confirm
changes in effectiveness due to possible geometrical changes of the sources
with respect to the detection system, and to predict the shielding effect of
other materials in the container.
The goal of this work is to develop a standard container and measurement
protocol, so manufacturers can calibrate their detectors, and users can develop
criteria, for radiological detection in ports of entry in the U.S.
Gamma-Ray Spectrometry System
Due to increased concerns about a possible terrorist attack using radioactive
materials in dispersal devices, we have initiated new research in gamma-ray
measurements. Information about the equipment required for these kinds of
measurements is of great importance.
Two new CdZnTe detectors were tested, and their ability to detect gamma-ray
emitting sources was compared with existing High-Purity Germanium and NaI(Tl)
detectors. Because these detectors all can operate at room temperature, they
are attractive candidates for field measurements. The detectors' efficiency,
detection limits, and energy resolution were measured to determine their
performance and the circumstances under which they could be used.
The measured energy resolution of the CdZnTe detector is better than that of
the NaI(Tl) detector, but its efficiency is lower, mainly due to the smaller
crystals that are presently available in the market. On the other hand, its
small size can be an advantage for measurements in confined and narrow places,
and it has low power consumption. It may be the best choice for monitoring
relatively high-level source movement or dispersal.
Microcalorimetry for Absolute Radioactivity Standardizations
A new, primary standardization capability has been established at NIST to
provide standardizations for rather large, GBq-range, brachytherapy sources. In
addition to the dual-compensated, cryogenic calorimeter operating at
≈ 8 K, a
commercial "isothermal microcalorimeter" has been adapted and
evaluated for use in performing classical, calorimetric-based standardizations.
This dual-cell, near-isothermal (heat flow) calorimeter operates at
near-ambient temperatures, utilizes specially-fabricated source-holder cells
that are used to maximize the energy absorption of the ionizing radiation, and
incorporates resistance heaters within these measurement cells to obtain
very-accurately-determined, independent, power calibrations.
Evaluations were initially performed on two different types of intravascular
brachytherapy sources containing nuclides that decay by pure
β emission, viz.,
(1) a stainless-steel-jacketed 90Sr-90Y source with
a highly-refractory, ceramic-like, inner matrix, and (2) a
"hot-wall" balloon-catheter source that consists of a thin film of
32P enveloped between polyethylene walls. The measured thermal power
was related to source activities through the use of calculated average energies
per decay, and was compared against known source activities determined from
previous radioanalytical destructive assays. Monte Carlo calculations for the
energy deposition in the measurement cells were used to correct for power
losses due to escaping ionizing radiation.
This verification work clearly demonstrated, quantitatively, that the
“isothermal” microcalorimeter was sufficient for performing primary calibrations.
New Technique for Absolute Standardization of Radionuclides
Beta-emitting radionuclides make up the majority of nuclides used in nuclear
medicine. Because of its high detection efficiency, liquid scintillation
counting (LSC) is the preferred method for calibrating these radionuclides.
Traditional applications of LSC require the use of tracers, such as
3H, and a calculational model to determine the detection efficiency
for the nuclide under investigation. The Triple-to-Double Coincidence Ratio
(TDCR) method is a new, quasi-absolute, LSC technique that can determine the
counting efficiency for a radionuclide, without the use of an external tracing
standard.
We have constructed a TDCR spectrometer with three phototubes and the
electronics to process 2- and 3-fold coincidences of detected photons emitted
from the liquid scintillator medium. We measured its operating characteristics
using solutions of NIST SRMs for 3H (tritiated water) and
63Ni. The TDCR-measured activities agree with previously certified
values to within 0.04 % and 0.2 %. We now can calibrate radionuclides
encountered in nuclear medicine with shorter sample preparation times and with
less, long-lived, radioactive waste.
Workshop on Standards for Important Radiotherapy Nuclide
(© Denease Anderson).
Figure 2. Michelle Millican preparing radioactivity standards for a
radiotherapy nuclide. |
The American Cancer Society estimates that there will be a total of
64,000 new cases of lymphoma in the U.S. this year, with an expected
five-year survival rate (for non-Hodgkin’s lymphoma) of 52 %. A new drug
for treatment of this disease is ZevalinTM, which is a monoclonal
antibody labeled with the radionuclide 90Y. While 90Y has
many properties that make it suitable for use in radiotherapy, those same
properties present a number of challenges regarding its measurement.
Anticipating imminent FDA approval of ZevalinTM and an exploding
need for standards and measurement quality assurance for other
radiopharmaceuticals using this nuclide, NIST held a workshop in December 2001
to address the measurement issues. Invited speakers included representatives
from NIH, radiopharmaceutical manufacturers, radiopharmacies, isotope
producers, government regulators, and NIST.
Among the concerns expressed by the 46 participants were the need for more
frequent distribution of standards for 90Y (currently once a year),
a desire for NIST to issue 90Y solution standards in clinically
useful geometries instead of the standard NIST 5 mL glass ampoules, and
the need to establish measurement traceability for an estimated
450 radiopharmacies. NIST is working with radiopharmacies, the FDA, the
Society of Nuclear Medicine, the American Pharmaceutical Association, and
others to organize and implement a program that meets these needs.
First strategic focus |
Second strategic focus |
Third strategic focus
"Technical Activities 2002" - Table of Contents |
