The primary mission focus of the Radiation Physics Division (RPD) is ionizing radiation physics metrology. As part of NIST, a national metrology institute (NMI), it is required to establish, disseminate, and ensure the proper utilization of standards in regard to all aspects of ionizing radiation. In addition to the responsibility to maintain state-of-the-art radiation physics metrology, there is a recurring requirement to develop novel standards to meet current and future needs as the state of radiologic sciences advances.
The division prioritizes its efforts based on input from a variety of outside stakeholders, which include the Council on Ionizing Radiation Measurements and Standards (CIRMS). CIRMS is an independent, nonprofit council that draws together experts involved in all aspects of ionizing radiation to discuss, review, and assess developments and needs in this field.1 Drawing on expertise from government and national laboratories, agencies, and departments, from the academic community, and from industry, CIRMS issues periodic reports on the needs in ionizing radiation measurements and standards.2 Such needs are delineated in measurement program descriptions (MPD’s), which indicate the objective, state background information, define needed action items, and estimate resource requirements in terms of personnel and facilities. Through dialog at CIRMS meetings and workshops, the appropriate subcommittees (currently industrial applications and effects, medical, and homeland security/radiation protection) independently determine the needs in their respective communities.
In addition, there is organized input from professional groups such as the Radiological Society of North America (RSNA); Society of Nuclear Medicine and Molecular Imaging (SNMMI); and the American Association of Physicists in Medicine (AAPM). These groups provide clinical input, a real-world perspective that helps maintain relevance of division activities from a clinical and scientific perspective. There is additional strong input from industry, with direct interaction that impacts the quality control and regulatory compliance of American business and supports international leadership in radiation treatment machines, scanners, radioactivity measurement devices, and dosimeters. Through intragovernment committees, there is direct input to specialized radiation and regulatory needs that positively impacts mandated responsibilities of the Food and Drug Administration (FDA), Nuclear Regulatory Commission, Department of Energy (DOE), Department of Homeland Security (DHS), and the National Institutes of Health, among other agencies.
The RPD is also responsible for maintaining liaison with international agencies that have parallel radiation metrology functions in other parts of the world. There is key exchange of both information and standards with groups from the United Kingdom, Japan, and Germany and from other official government bodies on both a routine and emergency basis, to ensure global standardization and uniformity of measurement and reporting of quantitative radiologic activities and radiation use.
Within the RPD there are three groups: the Dosimetry Group; the Neutron Physics Group, and the Radioactivity Group, each with clearly defined core functions.
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1 Council on Ionizing Radiation Measurements and Standards, “Welcome,” http://www.cirms.org, accessed November 24, 2015.
2 The latest needs report was updated following the CIRMS 2015 meeting and can be accessed at the CIRMS website (http://www.cirms.org).
The Dosimetry Group performs work in the following areas: radiation source standardization, chemical interaction studies and reporting, dose from medical imaging procedures, radiation protection dosimetry for the nuclear engineering field, international liaison, determination of biological effects of ionizing irradiation, science and technology of radiation for the DHS, high-dose calibrations, gamma- and x-ray calibrations, and brachytherapy calibrations. A key mission of the Dosimetry Group is to develop and maintain national air kerma and absorbed dose standards (kV and MV x rays, Co-60 and Cs-137 gamma rays, and Sr-90 and Kr-85 beta beams) based on the derived SI unit—the gray—for homeland security, medical, radiation processing, and radiation-protection applications.
The Neutron Physics Group performs work in the following areas: fundamental neutron physics, neutron interferometry, neutron source calibration, neutron imaging, neutron instrumentation calibrations, neutron instrument development, and special calibrations.
The Radioactivity Group performs work in the following areas: basic radionuclide metrology, low-level radionuclide metrology, imaging studies and standards, calibration services, measurement quality assurance, development of nuclear forensic standards and methods, investigation in international and national developments in technology, and production of standard reference materials (SRMs). The key mission of the Radioactivity Group is to develop and maintain state-of-the-art high- and low-level radioactivity detection methodology, including gamma, beta, and alpha counting and spectroscopic capabilities, often based on first-principles methodology that directly measures key radioactive emissions by multiple cross checks and approaches in order to maximize confidence in assay accuracy. The group is also required to develop relevant standards and methods of assay for common radionuclides used in industry and medicine, including calibration services for radioactivity dosimeters utilized in nuclear medicine. The group defines protocols for radioactivity measurement on the national and international level. In addition, other government agencies such as the Nuclear Regulatory Commission and the FDA may mandate linking of radioactivity measurements to NIST-maintained standards.
ASSESSMENT OF TECHNICAL PROGRAMS
Dosimetry Group
Among the recent accomplishments of the Dosimetry Group are the 320-kV W-anode x-ray tube replacement, establishment of an air-kerma standard for electronic brachytherapy using a miniature x-ray tube (50 kV, 300 µA), development of the Lamperti free-air chamber, and implementation of a new Co-60 therapy-level calibration facility with state-of-the-art data acquisition and operating capabilities.
The quality of the work by the group is excellent. It demonstrated competence in maintaining existing radiation dosimetry standards and in developing new ones as needed by the user community. In 2015, a new Co-60 irradiator is being installed to replace an older unit. This work is expected to improve the ability to maintain a number of dosimetric standards essential to the radiation oncology community. According to the Task Group report 51, issued by the American Association of Physicists in Medicine (AAPM),3 therapeutic-level clinical x-ray dosimetric standards are based on Co-60 sources.
The work in the Dosimetry Group appears to be largely in services rather than innovative research. The group does have a list of potential research projects that can be pursued if funding is available.
Research in advanced computing (such as Monte Carlo simulations) could enhance existing capabilities in radiation measurement and dosimetry. It would be advantageous to strengthen the training program that involves students and postdoctoral researchers in each of the critical standards activities and foster a program to share equipment and facilities with users at national laboratories and universities.
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3 Dosimetry of interstitial brachytherapy sources, AAPM Task Group Report 51, reprinted from Medica Physica 22(2).
Neutron Physics Group
In addition to its core activities, there is a unique basic-research program in the fundamental properties of the neutron. This program is synergistic with the core elements, because the stringent demands of the physics measurements call for a steadily advancing art of calibration and instrumentation.
The fundamental neutron science program is carried out at the NIST Center for Neutron Research (NCNR) and in Building 245. With the new guide hall now in full operation, neutron beam intensities are up by a factor of 5 or more. The additional capacity in flux and in the number of available ports represents a substantial improvement. The guide hall is the world’s most modern and places the neutron program in a class with other world-leading reactor neutron programs, such as the one at the Institut Laue-Langevin in Grenoble, the Gatchina reactor in St. Petersburg, Russia, and the high-flux isotope reactor (HFIR) at Oak Ridge.
The basic neutron physics research program by PML scientists at the NCNR has led to advances in metrology. For example, the continuing work to refine the lifetime of the free neutron has led to a recent major advance in neutron flux determination at the 0.1 percent level. This makes possible a new level of precision in neutron cross sections.
The Neutron Physics Group standardizes neutron sources (252Cf, AmBe, etc.) for users around the world in industry, government, and academia. The RaBe photoneutron source at NIST, called NBS-1, is the national reference standard, which has been determined to 0.85 percent.
The Mn bath method is used at NIST and at four other laboratories around the world to calibrate source strengths relative to NBS-1. The Bureau International des Poids et Mesures (BIPM) discontinued its calibration service and sent its three standard sources to NBS-1, and all were found to agree well within the combined uncertainties.
The group also provides spectral reference measurements from a room constructed with boron-loaded walls having a very low return. A scintillation and He-3-based detector has been constructed as a new methodology for higher-energy neutron spectra.
Neutron imaging is another important service provided by the group. It has wide application, in industry particularly; new technologies such as fuel-cell and battery development need it. In the imaging program there are opportunities for new advances in technology, and NIST is an international leader in this area. Phase-contrast methods are used for high-definition density measurement. A unique new instrument that uses Wolter optics to obtain micrometer spatial resolution is under construction and in limited operation; full capability will be in operation by 2018.
The Neutron Physics Group, in cooperation with external collaborators, has been successful in obtaining DOE support for a strong program in fundamental physics. Within the constraints of its limited budget, the Neutron Physics Group has managed to develop new technology in support of the core program—for example, a scintillator-proportional counter array for the measurement of high-energy neutron spectra such as cosmic-ray secondaries.
Radioactivity Group
The work of the Radioactivity Group appears to be largely in services rather than innovative research. The group has a list of potential research projects that can be pursued if funding is available.
Some of the recent accomplishments of the Radioactivity Group include maintaining continuous standardization of approximately 30 distinct radionuclides as primary standards linked to nuclear medicine. A suitable standard for accurate measure of the alpha emitter radium-223, a recently introduced therapeutic radionuclide for prostate cancer treatment, is under development. The group is collaborating with professional societies such as the AAPM in the development and publication of 90Y protocols for radiation brachytherapy. The group’s new measurement of the half-life of 209Po has resulted in substantial revision of the accepted half-life.
Dose calibrators are used in nuclear medicine tens of thousands of times each day in the United States, and they depend on NIST-determined calibration factors for more than 20 different radionuclides. Some recent examples include 18F, 68Ge, 111In, 123I, 125I, 133Ba, 177Lu, and, as mentioned, 223Ra. Future work focuses on 64Cu and 227Th.
The quality of the work by the Radioactivity Group is excellent. They demonstrated competence in maintaining existing radionuclide standards and in developing new ones identified by the user community. In addition, as part of a grant from the DHS, they have developed key nuclear forensic standards to be used for investigations of stolen radioactive materials destined for use in radiologic devices such as dirty bombs.
PORTFOLIO OF SCIENTIFIC EXPERTISE
Most of the counting equipment is state-of-the-art, and the specialized expertise of the staff is related to the unique experience needed to maintain the standards. Positions of staff members and technicians in the Radioactivity Group who have resigned or retired recently were either filled or reprogrammed. However, if these actions have not resulted in a sufficient number of technicians, this creates an undesirable situation whereby Ph.D.-level staff are required to perform routine sample preparation, including weighing and pipetting of radioactive solutions, packaging, and shipment. The time of these highly trained and dedicated individuals would be better utilized for research into state-of-the-art issues such as radioactivity metrology and dissemination of knowledge on proper use of modern equipment for accurate assay.
Some of the unique equipment can be operated optimally only by a highly expert single individual, and there is relatively little time to train more junior staff in the proper use of certain specialized equipment and counting devices. A stronger training program that involves students and postdoctoral researchers in each of the critical standards activities would help fill the personnel gap that now exists, and it would provide intelligent, willing workers who learn on the job in the unique NIST environment. Another area to explore could be alliances with medical physics training programs at regional universities, to provide internships of varying length that would be sponsored through NIST internal resources. Budget plans that project the expansion of such programs could be considered for the future.
Management can further encourage the pursuit of cutting-edge radioactivity research projects with external funding from DHS, NIH, DOE, and private-sector industry. An example would be the development of positron emission tomography (PET) and computed tomography (CT) and PET/MRI devices in Gaithersburg as part of a secondary standards laboratory devoted to nuclear medicine quantitation. One element of this is the growing emphasis on the theranostics, in which the same radionuclide may be used at the tracer level for dosimetry and detection and, by scaling up to radiotherapeutic doses, could be used for therapy. There will be important standardization issues in the radioactivity measurement as well as tissue dosimetry as the range of dose is expanded over orders of magnitude. A strong presence in this field of the experienced NIST metrology experts would likely give U.S. industry and medicine an important competitive advantage.
Strategic projects that align closely with the priority areas of the PML—in particular, medical imaging—would seem to be a natural component of such an approach, and there is already a good basis for this in terms of expertise and facilities. Enhanced research in areas of collaboration where NIST excels, such as advanced computing (for example, in Monte Carlo simulations), would augment existing capabilities in radiation measurement and detection.
ADEQUACY OF FACILITIES, EQUIPMENT, AND HUMAN RESOURCES
Most of the equipment is state-of-the-art, and staff expertise is related to unique experience needed to maintain the standards. However, in the laboratories of Building 245, where most of the radionuclide standards are prepared for shipment, the rooms are old and lacking in proper heating and ventilation, with consequent inadequate control of humidity and temperature. Neutron source standardization is carried out in a number of shielded, below-grade laboratories in Building 245. Water intrusion during heavy rains has become an issue, because the building is no longer watertight. These events are, at a minimum, distractions to overburdened scientists and in some cases have threatened to flood sources under test. Dispersal of the source material is remotely possible, but the immediate concern is damage and loss of calibration traceability. Repair and renovation of building 245 are urgently needed. These conditions, along with the requirement for researchers to perform technician chores due to a lack of manpower and technical support, are severely compromising the ability of the staff to perform key tasks. The neutron metrology staff has decreased in number to a level causing concern, and efforts are needed to add technical support in this area.
The resilience of the core program against the loss of a single individual is no longer assured. Many core activities are supported by a single scientist, without the assistance of a technician. Scientists are spending time on functions that are inappropriate and wasteful for highly qualified personnel. They must handle packing and shipping, compliance issues, contract generation, purchasing, and budget analysis. This is having an increasingly corrosive effect on the entire core metrology function in the division, and especially on the neutron physics program.
The fundamental neutron research program has a high profile in the U.S. nuclear physics program, as delineated in the long-range plans periodically released under the auspices of DOE and NSF, most recently in 2015. This profile has allowed NIST to recruit outstanding talent for the neutron program. These individuals are also responsible for the neutron metrology services, and they carry them out with the same goal of leadership and excellence.
Nuclear medicine is more and more reliant on a combination (fusion) of instruments whereby a CT scanner and a positron emission tomograph are combined into a single gantry for imaging use. This approach to nuclear medicine is essentially quantitative; accuracy and precision of measurement would be greatly benefited by involvement of the Radioactivity Group in developing standardized phantoms and appropriate radioactivity standards. As part of the future relevance of NIST to medical practice, a PET/CT scanner was purchased and installed in the Radioactivity Group. Unfortunately, funding was insufficient for a proper maintenance contract, and at the time of this review, the PET/CT had been out of action for several weeks. Attention to proper maintenance and support for such a valuable program is important. Management could further encourage conjoint programs to share equipment and facilities with users at national laboratories and universities, to enlist whenever possible outside facilities to bolster the NIST mission.
The positions of staff members in the Dosimetry Group who have retired recently have been filled or reprogrammed. However, some of the critical standards for diagnostic x rays, Cs-137/Co-60, and high-dose industrial applications are maintained by a single staff member. At least one critical staff member is close to retirement, and if the position is not filled with a qualified individual, the quality of the program will suffer.
DISSEMINATION OF OUTPUTS
Members of the Dosimetry Group are active in various professional societies—for example, the AAPM, the American National Standards Institute (ANSI), and the Health Physics Society (HPS)—which helps to disseminate the work at NIST. The group also interacts with CIRMS, DOE, and the FDA. Members of the Dosimetry Group also publish in journals The group supports three AAPM-accredited
dosimetry calibration laboratories as well as other secondary calibration laboratories through its calibration services.
For the Neutron Physics Group, the program output is disseminated in many ways and is effective. In addition to published research papers and conference presentations, the calibrations and the distribution of standard reference materials are universally recognizable products. Many guest researchers come to NIST to take part in the fundamental neutron physics program and to collaborate with personnel in the Radiation Physics Division. There is also abundant cross-fertilization between disciplines because so many disparate sciences are practiced at the NCNR. An example is the development of the small-angle neutron scattering (SANS) device at the NCNR, with polarized neutrons for the study of magnetic ordering in materials. A major step toward this new capability came from the helium-3 polarizers and analyzers first devised for fundamental neutron physics by an RPD scientist.
The Radioactivity Group does an excellent job in disseminating its work through interactions with CIRMS, AAPM, SNMMI, DOE and FDA and in providing calibration standards for many common radionuclides used in nuclear medicine. In some cases they interact with groups who provide a secondary standard service that is traceable back to NIST. The advantage of this approach is that a very a large demand for radioactivity measurement standards can be met by expanding through secondary standards that will still relate to the NIST-prepared primary standards.
Members of the Radioactivity Group are active in various professional societies such as AAPM, SNMMI, and HPS, which helps disseminate the work of NIST, and they also publish journal papers. These services include the dissemination of technical reports such as Technical report series 454, Quality Assurance for Radioactivity Measurement in Nuclear Medicine.
Unfortunately, there are examples where dissemination of proper radioactivity standards has not been able to proceed as rapidly as required owing to a lack of funds. Because medical imaging and nuclear medicine increasingly depend on short-lived radionuclides such as F-18 (110-minute half-life) or C-11 (20-minute half-life) there is recognition that specialized and accurately calibrated detection devices need to be sent between institutions in the United States to serve as the standard of measurement, since it is physically impossible to ship the radionuclides themselves.
One such device uses the triple-to-double coincidence ratio (TDCR) method. As explained on the website of the Laboratoire National Henri Becquerel,2
. . . TDCR is an absolute activity measurement method specially developed for pure beta- and pure EC-emitters activity determination, in which the detection efficiency is calculated from a physical and statistical model of the photon distribution emitted by the scintillating source. As the signals delivered by the photodetectors are affected by the thermal noise of the photocathode, a coincidence method is used to remove that noise. There is not enough information in a twophotodetector system to determine the experimental detection efficiency without an additional reference source. The TDCR method uses a three-photomultiplier detector, allowing the observation of 3 kinds of double coincidences (2 photodetectors) and triple coincidences (3 photodetectors).
The Portable TDCR is a portable version of one of the primary measurement systems for activity measurements of beta emitters, positron emitters, and electron-capture decay radionuclides. There is concern that, because of the lack of manpower for calibration and testing of these TDCR devices at NIST within the Radioactivity group, U.S. hospitals have difficulty taking proper advantage of these specialized devices for on-site measurement to establish proper radioactivity standards. Meanwhile, foreign standards laboratories such as the Physikalisch-Technische Bundesanstalt (PTB), the European Association of Nuclear Medicine (EANM), and others, funded through a European Union investment in metrology, have working instruments.
