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 develops and improves the metrological techniques used for the standardization of radionuclides. Our mission is to develop,
maintain, and disseminate radioactivity standards, develop and apply radioactivity measurement techniques, and engage in research to meet the
requirements for new standards. We carry out a wide range of programs in low-level standards for environmental measurements and monitoring, nuclear
medicine, and radiological instrumentation used for security.
We continue to lead the national effort, in collaboration with the Department of Homeland Security (DHS), to develop standards and protocols for
radiation instrumentation for early and emergency responders. We are spearheading the development of ANSI standards and testing protocols for
spectroscopic portal monitors, neutron detectors, x-ray and high energy gamma-ray interrogation methods, x-ray imaging, data formats for
instrumentation data output, and training standards for responders. The Group has been heavily involved in the testing and data analysis of
advanced spectroscopic portal monitors (with DHS/DNDO) at the Nevada Test Site, and is involved with the testing of large-area NaI crystals
to be used in these spectroscopic portal monitors.
The Group continues to lead an internationally recognized program for standards in nuclear medicine, providing the national standards for
radionuclides used in 13 million diagnostic procedures and 200,000 therapeutic nuclear medicine procedures annually in the U.S. Work is currently
being carried out on the standardization of two alpha-emitting radionuclides, 211At and 223Ra, that are being investigated for use in targeted
radiotherapy against two different forms of cancer. A new initiative, aimed at establishing standards and measurement support to improve accuracy
and consistency in quantitative Positron Emission Tomography/X-ray Computed Tomography (PET/CT) and Single-Photon Emission Computed Tomography (SPECT)
imaging, has also been started.
The Group’s environmental program leads the community in low-level and natural matrix material measurements and standardization, and continues to be
heavily involved in the worldwide measurement of environmental-level radionuclide dispersal and contamination through a large number of international
intercomparisons, traceability programs, and SRMs. A Radioanalytical Emergency Procedures Manual Database has been developed to assist organizations
preparing for emergency response.
Accomplishments
New Approaches in Radionuclide Metrology
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Figure 8. nIST TDCR system, open view showing the photomultiplier tubes.
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The NIST Triple-to-Double Coincidence Ratio (TDCR) spectrometer has been rebuilt and, as part of the testing phase, standard solutions of 3H were
measured to assess its performance. An agreement of 0.3 % between the TDCR and certified massic activities (activity per unit mass) was obtained
without any attempt to optimize the value of the ionization quench factor. As a second test, the TDCR was used in experiments leading to the
standardization of 63Ni. Part of this study involved a direct comparison with Le Laboratoire National Henri Becquerel (LNHB) using the same solution.
The NIST TDCR result for the massic activity was in excellent (0.3 %) agreement with the value reported by LNHB, which used three different
TDCR spectrometers to obtain their result.
A short study involving measurements of the electron capture nuclide 55Fe was somewhat less satisfying, with the NIST TDCR and CIEMAT/NIST results
agreeing only to within 4 %, although no effort was made during these experiments to optimize the ionization quench factor. Despite the lack of
agreement in the experimental results for 55Fe, a comparison of calculated efficiencies as a function of the coincidence ratio using NIST-developed
software and a program written at LNHB agreed to within 0.01 %.
Due to the complexity of the efficiency calculation arising from the need to account for atomic rearrangements and competition between x-ray and
Auger electron emission, a separate program needs to be developed for each electron emitter being studied. Programs are under development to analyze
data from the decay of the 223Ra decay chain, 68Ge/68Ga (mixed electron capture, positron annihilation), and the 210Pb decay chain (beta, conversion
electron, alpha). Experiments are currently underway to standardize 90Sr/90Y using the TDCR spectrometer. Initial results indicate excellent agreement
between the TDCR and CIEMAT/NIST.
Primary Standardization for a New SRM and International Measurement Comparisons
A primary standardization of 55Fe by isothermal microcalorimetry, initiated in 2004, was completed in 2006. Determinations of the activity for
nuclides that decay by pure, low-Z (atomic number) electron capture (EC) to the ground state of their daughters are amongst the most difficult
within the realm of radionuclidic standardization. The present work on this difficult-to-measure nuclide is a return by NIST to the use of calorimetry
for primary radionuclidic standardizations. While calorimetry was a classical radio-nuclidic measurement method used by the NIST Radioactivity Group
from the early 1950s through 1975 for primary standardizations, it was not used for the past 30 years or so. In the past 5 years, calorimetry was
used by NIST to perform calibrations of brachytherapy sources of 32P, 90Sr, and 103Pd. A solid 30
GBq source of 55Fe was prepared and gravimetrically linked to a 55Fe master solution.
The source was used to obtain an accurately determined power measurement using the NIST dual-cell isothermal calorimeter.
The power measurement was converted into a 55Fe activity through the use of a conventional average energy per decay estimate.
This activity was linked to the master solution, which had an assigned (k = 1) uncertainty of 0.39 %.
This standardization was used as the basis for calibrating a new 55Fe solution standard (SRM 4929F) as well as for measurements of a BIPM-distributed
55Fe solution that was part of an international measurement comparison. Although the results on the international intercomparison are not yet compiled
and released by the BIPM, the NIST results are in very good agreement with several other national metrology laboratories.
Standards for Medical Imaging
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Figure 9. Typical phantom used in clinical quality assurance testing. The red sphere inside the phantom is a
NIST prototype demonstrating how calibrated radioactive inserts can be prepared.
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NIST has established a satellite facility at Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tennessee, to promote measurement accuracy for nuclear
medicine imaging. NIST scientists will use the NIST/ORNL Nuclear Medicine Calibration Laboratory to prepare and measure radioactivity standards used
for Positron Emission Tomography (PET), a noninvasive technique that helps doctors diagnose diseases (such as cancer), plan medical treatment, and
measure the efficacy of therapies. An estimated one million PET procedures were performed in 2004, a number expected to reach three million annually
by 2010.
The NIST program needs to be carried out regionally because the short half-lives of most PET radiopharmaceuticals prevent shipment of standard test
samples over long distances. Radioactive 18F, for example, decays to half its total radioactivity in about two hours. Therefore, NIST is locating its
satellite laboratories at key sites near manufacturers’ distribution networks. The sites are selected based on location, capability, and reputation.
ORNL has extensive expertise in radiation measurements, already operates a measurement traceability and testing program with NIST, and is licensed and
capable of accepting, handling, and shipping radioactive materials. Initial calibrations of a 18FFDG solution performed in May 2006 led to
the determination of a calibration factor for dose calibrators as are used in the clinical setting. An intercomparison exercise is planned for the
near future.
Our close collaboration with the user community will lead to the development of improved national standards to evaluate the accuracy and precision of
critical and increasingly used medical diagnostic procedures. For example, we are developing realistic and well-characterized phantoms (models) of specific
organs and other body parts for the evaluation and calibration of whole body imaging technologies, particularly those relying on the distribution of
radioactivity (such as PET). Our work in developing a calibration for 68Ge, which can be used as a longer lived radionuclide for PET instrument
calibrations, also contributes towards this goal. Such developments will assure clinical reproducibility, the comparability of clinical results from
different sites, and the correlation of measurements with biological parameters.
Emergency Preparedness for Radiological Incidents
A survey of the national radionuclide metrology community indicated a serious need for performance testing exercises to assess the capability of
laboratories to respond quickly to an unexpected release of radioactivity into the environment. Radiological emergency response capabilities had
previously focused on gross alpha and beta screening and gamma-ray measurements. However, as the 2006 210Po poisoning incident in the UK had shown,
having the capabilities to conduct radioanalytical measurements for specific alpha- (and beta-) particle emitting radionuclides is also crucial.
The NRIP’06 emergency preparedness exercise resulted in radioanalytical measurement results reported within eight hours of sample receipt for
90Sr, 230Th, 234U, 238U, 238Pu, 239Pu, and 241Am in urine, soil, and water.
Agreement of reported radiochemical results with NIST certified values ranged from 1.4 % to 180 %. While the results varied from laboratory to
laboratory and among radionuclides, the information obtained by the participating laboratories will be helpful for method improvement and improved
preparedness for radiological emergency incidences.
The Group is also developing a Radio-analytical Emergency Procedures Manual Database (REPMD) to assist organizations preparing for emergency response.
This will collect existing procedures from reliable sources into a guide to be accessed and searched by laboratories seeking appropriate methods for
sampling, screening, surveying, and making rapid radioanalytical measurements of food, biological, and environmental materials. A first-stage
prototype, demonstrating the ability to handle PDF documents describing radioanalytical methods and to simplify the method-collection process, has
already been delivered.
First strategic focus |
Second strategic focus |
Third strategic focus
"Technical Activities 2005-2007" - Table of Contents |
