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Radiation Physics Division
The mission of the Radiation Physics Division is to develop, maintain, and disseminate the national measurement standards for ionizing radiation and radioactivity, and methods and models needed to address related applications. The goal of the division is to provide accurate measurement in service to industry and in related basic science essential to its mission as the nation’s precision measurement laboratory. The division provides a unique resource to the United States based on its one-of-a-kind facilities and expertise housed within its component groups—the Dosimetry Group, Neutron Physics Group, and Radioactivity Group.
TECHNICAL QUALITY OF THE WORK
The dosimetry program primarily serves stakeholders in radiation protection, industrial radiation processing, national security, and therapeutic and diagnostic medical physics, fundamentally supporting required calibrations and quality assurance of nationally mandated programs and efforts. A key mission of the dosimetry program is to develop and maintain national air kerma (kinetic energy released per unit mass), air kerma rate, and absorbed-dose calibration standards based on the derived International System of Units (SI) unit—the gray—for homeland security, medical, radiation processing, and radiation protection applications. This is accomplished through the provision of calibration services utilizing the dosimetry of kilovolt (kV) and megavolt (MV) X rays; 137Cs and 60Co gamma rays; 90Sr/Y and 85Kr beta particles; 125I, 103Pd, 131Cs, 192Ir, and 137Cs brachytherapy; and electron beam sources and measurement instruments.
The quality of the work conducted by the Dosimetry Group is excellent, as demonstrated by the streamlined practices for maintaining the existing radiation dosimetry standards while anticipating new approaches to meeting calibration standards in emergent and in-demand fields such as brachytherapy calibration and electron beam calibration. While the activities are predominantly service-oriented, meeting calibration needs for critical programs and activities supported by associated regulations, the group has demonstrated technical accomplishments in designing and building a new therapy-level, gamma-radiation dosimetry calibration facility that improves NIST’s ability to deliver state-of-the-art measurement capabilities for air kerma and absorbed-dose-to-water calibrations to a wide stakeholder base, and in pioneering research and development of new objective methods to assess image quality in security X-ray systems.
The Dosimetry Group has performed detector calibration for homeland security systems, as well as occupational-radiation areas, survey, and personnel dosimetry capabilities using a highly reproducible (~0.1%) 137Cs irradiator source as established by the National Council on Radiation Protection and Measurements (NRCP) Report 160, making this facility critical across multiple national mission areas. The Dosimetry Group is further tasked with fulfilling the American Association of Physicists in Medicine (AAPM) Task Group 56 recommendation that all brachytherapy sources be directly traceable to NIST by automating the Wide-Angle Free-Air Chamber, to measure key quantities in the calibration process of radioactive material in brachytherapy seeds (i.e., air-kerma strength, well ionization chamber response, photon energy fluence spectrum, air-anisotropy ratio, and radiochromic film contact exposure). The culmination of these processes ensures accurate clinical dosimetry while maintaining the dosimetric
traceability of these sources, enabling the three accredited dosimetry calibration laboratories (ADCLs) to maintain secondary, NIST-traceable standards. In the support of medical dosimetry standards, the bilateral benchmark activities between NIST and the Bureau International des Poids et Mesures (BIPM) has ensured harmonization of air kerma and absorbed dose standards with high fidelity, resulting in journal publications. Interest expressed by the group in improving the state of the art in nano-sensor and microchip dosimetry beyond long-established alanine-based, EPR-based (electron paramagnetic resonance–based) methods, resulting in two patents, demonstrates research interests that could beneficially be conducted collaboratively with the NIST-on-a-chip (NOAC) initiative.
The Dosimetry Group primarily works with its large stakeholder community. Alternative mechanisms other than radiological-based calibrations using 137Cs and 60Co could be applied, given movements to non-radiological source calibration in-field, thereby aligning calibration with field practices. Calibration and measurement associated with new modalities in medical physics (e.g., proton and heavy-ion therapy), with associated facilities and staffing expertise, are absent in the established activities of the group, and these capabilities need to be anticipated. While current calibration services harness established techniques, active research engagement is critical to ensure integration of new technologies that will supersede current methods and equipment. In addition to seeking extramural funding, the division could leverage initiatives such as NOAC in, for example, EPR dosimetry research.
The neutron physics program is at the forefront of neutron measurement science. It maintains facilities and capabilities that are unique in the United States and, in many cases, on par with the premier neutron institutes around the world. The single-crystal neutron interferometry facility at NIST is the only one in the United States and has achieved the best phase contrast in the world. The neutron/X-ray tomography (NeXT) imaging facility is the world’s best for offering simultaneous tomography with neutrons and X rays. The neutron beam line at the NCNR, used by the Neutron Physics Group, is the preeminent resource for cold-neutron fundamental physics in the United States, and the group is pioneering the measurement of the neutron lifetime using the beam method (to be compared to the bottle method, thereby resolving the neutron lifetime puzzle). The NG-C beamline, after the planned cold0source upgrade in 2023, will increase the neutron flux by a factor of two; it might be the only high-flux beamline available to users in the United States, because the fundamental neutron physics beamline at the Spallation Neutron Source at the Oak Ridge National Laboratory will be dedicated to the nEDM (neutron electric dipole moment) experiment. In addition to the neutron flux calibration services, the group is carrying out a diverse research program, including fundamental physics, nuclear power monitoring, national security, manufacturing, and radiation protection.
The Neutron Physics Group has improved the technique for calibrating neutron flux. The group has been operating a MnSO2 bath to calibrate neutron source emission rate for a variety of radioisotopic sources (with up to 1010/s and 0.8% precision). With it, all U.S. neutron metrology is traceable to a reference source, NBS-1, maintained by NIST. Over the past decade, the Neutron Physics Group developed an Alpha-Gamma device to improve the neutron metrology precision to 0.06%. This expertise in neutron flux calibration directly supports the basic science program to measure the neutron lifetime (and other beta-decay observables) using the cold-neutron beams at the NCNR to test the standard model of particle physics and Big Bang nucleosynthesis. The recent work on the Alpha-Gamma device improves the knowledge of the neutron capture cross-section on 6Li to 0.3% precision, with an ongoing effort to improve the neutron capture cross-section on 235U to 0.2% and other isotopes relevant to nuclear sciences and national security.
The neutron imaging program supports applied research on hydrogen fuel cells, energy storage, and porous media in geology and other civil engineering materials (like concrete). The thermal neutron imaging station (BT2) is fitted with an X-ray system, making it the world’s first such facility. The simultaneous tomography of neutrons and X rays enhances the capabilities to resolve small features, which is critical to studying material degradations and lithium-ion transport in batteries. A new cold neutron imaging instrument, installed in 2015, enables studies of magnetic structures using polarized neutrons and four-dimensional (4D) tomography to reach a spatial resolution on the micron scale. The ongoing effort to develop Wolter optics and a neutron imaging far-field interferometer, with a large
acceptance of neutron flux, will reduce the exposure time by a factor of 1,000 while achieving high resolutions in both space and time.
Neutron interferometry provides a versatile tool to probe new knowledge, including nuclear physics and characterization of quantum materials. It uses the interference of matter-waves of the neutron to probe the four fundamental forces of nature with high precision. Recent work includes high-precision measurements of the scattering length of n-4He (0.06% precision, 2020), the neutron charge radius, and the creation of entangled neutron states (spin and orbital angular momentum) to characterize quantum materials. Ongoing development of far-field interferometry using nanofabricated phase diffraction gratings aims to probe the weak gravitational force to extract the fundamental constant “G” using single neutrons with a precision comparable to the torsion balance–based experiments.
To support the NIST mission relating to neutron calibrations and metrology, the group members are developing new neutron detectors. Recent work includes a photon-assisted neutron detector (PhAND; U.S. patent) and segmented fast-neutron detectors to meet the national needs for fastneutrons dosimetry in homeland securities, underground sciences, and health physics. This expertise in fast-neutron detection enables the Neutron Physics Group to address the needs of a reactor-based neutrino experiment, PROSPECT (Precision Reactor Oscillation and Spectrum experiment), which measures the flux and energy spectrum of neutrinos from the high-flux isotope reactor fission reactor, with a highly enriched uranium fuel, to search for the sterile neutrinos, which are theoretically new particles.
The Radioactivity Group’s key mission is to develop and maintain radiation detection methods, often based on first-principles methodology, that directly measure radioactive emissions. The group is required to develop relevant standards and methods of assay for industrial and medical radioisotopes. The group defines protocols for measurement of radioactivity on the national and international level and for other government agencies such as the Nuclear Regulatory Commission, the Department of Homeland Security, and the Food and Drug Administration, frequently linking radioactivity measurements to NIST standards.
Since the advent of the nuclear era, the use of radioactive isotopes has brought tremendous benefits to society in a multitude of ways, from expanding our knowledge of the world around us to improving the lives of millions of people through medical diagnoses and treatment. The use of radioactive isotopes has allowed researchers to answer a myriad of questions about wide-ranging subjects, from the behaviors of ancient peoples to the movement of carbon through the environment. The use of radioactive materials by industry allows the analysis of material densities, the inspection of critical systems, product sterilization, and oil and gas exploration. In health care, radioisotopes have been instrumental in diagnosing and treating disease, improving the lives of millions of people. One isotope alone (technetium-99m) is used in over 16 million people per year in the United States as an imaging agent.
TECHNICAL EXPERTISE OF THE STAFF
The staff of the Dosimetry Group is distributed among core expertise areas in accelerator physics, computational dosimetry, high-dose dosimetry, X-ray dosimetry, gamma-ray dosimetry, calorimetry/photonics, and brachytherapy.
The scientific expertise in the Neutron Physics Group is evidenced by its success in several areas of activities. The area of calibration and metrology includes neutron source calibrations, absolute neutron fluence, standard cross-sections for thermal neutrons, and high-dose irradiation with neutrons. The area of basic physics research includes fundamental neutron physics, interferometry, and reactor neutrinos. The area of applied research includes neutron imaging, polarized 3He spin filters, neutron detection, dosimetry, and spectroscopy. The caliber and expertise of the staff in all activities and at all levels is outstanding. Four of the staff have been recognized as fellows of the APS. Senior staff members are leading the standardization of precision measurements in neutron sciences in the United States.
There is strong synergy among the groups in calibrations and metrology, basic sciences, and applied research, with the staff supporting multiple areas of the program. In particular, the neutron
imaging program supports many leading industrial research and development program, s as evidenced by the impressive numbers (approximately) of unique experiments (416), patents (40), papers (200), and Ph.D. dissertations (66) since its inception. The basic research program on beta-decay physics involves a handful of difficult measurements; each takes multiple years of data collection to acquire the counting statistics and to perform many checks on systematic effects, which also requires running times of comparable lengths. The interferometry program has developed a versatile tool that produces significant scientific results. Several high-impact publications, many with external university collaborators, showcase the unique niche of this approach. The 3He laboratory continues to support the fundamental neutron physics and neutron scattering programs in the NCNR by providing state-of-the-art large vapor cells to polarize cold neutrons.
The Radioactivity Group leads the nation in the development of new methods and instruments for determining the absolute activity of a radionuclide or mixture of radionuclides. The group is publishing and disseminating information on new approaches for determining the absolute activity of a radionuclide, and the group is fulfilling its mission of advancing measurement science and providing standards for the use of radioactive materials in medicine and industry.
The Radioactivity Group continues to develop new methods for creating primary radioactive standards. This includes live-timed 4παβ-γ anti-coincidence counting and Monte Carlo simulation analysis. The group is part of a NIST Innovations in Measurement Science–funded proposal to develop leading-edge technology that can be used to determine the absolute activity of a mixture of radionuclides using 4π decay energy spectrometry with ultracold transition edge sensors. In the area of medical applications, the group developed a methodology for calibrating Ge-68 activity content in a commercially available calibration source that is traceable to a national standard for positron emission tomography isotopes, and it developed standards for the complex decay chain targeted-alpha therapy radioisotopes Ra-223 and Ra-224.
ADEQUACY OF RESOURCES
Maintenance of facilities and infrastructure is a necessary foundation for mission accomplishment, as exemplified by the modernization project for Building 245, one of the world’s premier laboratories for ionizing radiation measurement science and research. Recent infrastructure upgrades have increased efficiency of operations, with fewer interruptions in calibration service work due to environmental conditions (e.g., temperature and humidity stability in air kerma–based standards and calibrations). Investment in critical facility improvements in X-ray calibration, relocation of the 60Co source for medical dosimetry standards, building a new high-dose-rate brachytherapy vault, and upgrading to 10 MeV the electron beam accelerator used in industrial irradiation processes are all demonstrated commitments to facility modernization and sustainability. Movement to new H-wing laboratories dedicated to general dosimetry calibration activities is under way; these activities include photon and electron, brachytherapy, and radiation-processing dosimetry facilities and instruments, collocated with mechanical and electronics shops.
The Building 245 modernization project has imposed demands on division finances and resources, which has detracted from staff time and regular duties (e.g., moving equipment and escorting construction workers). It will be necessary to anticipate the impacts on resources, staffing, and budget to mitigate interruption of activities during (1) the migration to facilities in the new H-wing; (2) the move to a renovated laboratory in the existing building for the EPR dosimetry facility; (3) renovation in place for the A-wing medical industrial radiation facility (MIRF) and 10 MeV industrial accelerator, scheduled in the 2022-2024 timeframe; and (4) renovation of the B-wing 137Cs and 60Co radiation protection, accelerator, and high-dose calibration facilities in a currently undetermined timeframe.
The Neutron Physics Group occupies two research buildings. In the NCNR, it is running eight neutron beam lines and the thermal column; in Building 245, there are several laboratories, including the Mn (manganese) bath room, the polarized 3He laboratory, a low-scatter room for dosimetry, and a
machine shop. The decaying infrastructure of the neutron calibration and setup laboratories in Building 245 is being renovated. The renovation, however, presents a major challenge to all operations, with an uncertain timescale for completion. In the NCNR, the high-flux neutron beam lines, the imaging stations, and the interferometry facilities are the best in the world. However, the COVID-19 lockdown in 2020, followed by a recent safety incident in the NIST reactor, triggered by an over-temperature of one fuel rod, prevented the reactor from coming online and had disruptive impacts on the neutron physics programs. Delays associated with the planned cold source upgrade in 2023 will inevitably reduce the scientific productivity of the group.
In the Dosimetry Group often one dedicated staff member is assigned to each of the core areas; mitigating this reliance on a single individual could improve capacity for performing mission-critical work and additional research. The training program that involves students and postdoctoral researchers in each of the critical standards activities could be strengthened. Fostering a program to share equipment and facilities with users at national laboratories and universities would be useful. It will be important to ensure sustainability of succession planning and knowledge retention by prioritizing allocation of resources in a manner that appropriately addresses the requirements of human capital development and infrastructure improvements.
Research staff in the Neutron Physics Group are world-leading experts in neutron measurement sciences. However, considering the large scope of work, loss of staff, and difficulty hiring replacements, there will be a challenge in sustaining an adequate level of staffing. The group lost several senior staff members recently due to departure and retirement, but there is no plan to replace them, due to budgetary constraints. Understaffing has significantly increased the service-base workload of current staff in order to meet the NIST mission. As such, Ph.D.-level scientists are deprived of time and opportunities for doing innovative research. Many recent and current postdoctoral researchers have demonstrated credentials to do outstanding science; however, the group reported that there is apparently no funding available to retain and promote them at NIST. Currently, the National Academies/National Research Council (NRC) postdoctoral program is the main channel for the group to recruit postdoctoral fellows, but there is significant competition among the sciences represented by the applicants and the selection committees. The understaffing situation remains the major risk to the program to meet its core missions while pushing the advancement of neutron metrology. Without increasing the number of staff, NIST risks losing its leadership role and its competitiveness with other neutron facilities in the United States and around the world.
The Radioactivity Group has successfully moved multiple detection systems to the new laboratories in the H-wing. The group has demonstrated that these unique instruments with over 40 years of operational history are operating as well after the move. The group has also demonstrated reduced ambient backgrounds for many of the instruments in the new laboratories. The staff for many of the activities within the group are served by single researchers with very little backup. Because of this, the Radioactivity Group is at risk of depleting its expertise through an ageing workforce and staff departures.
EFFECTIVENESS OF DISSEMINATION OF OUTPUTS
Since 2018, the Dosimetry Group has published 10 manuscripts in peer-reviewed journals and proceedings, including two publications with the Radioactivity Group, and selected publications with international counterparts. Some staff are active in professional societies, notably the AAPM, but there has been limited dissemination of capabilities and results in the scientific community. Expanded engagement with professional societies would promote scientific exchanges and enhance state-of-the-art practices in areas including, but not limited to, Monte Carlo simulation tools, nuclear data science, and detection science. Expanded engagement with university partners, stakeholder programs, and international counterparts resulting in joint publication would also be beneficial. Engagement with professional societies could help promote participation in the Council on Ionizing Radiation Measurements and Standards by students, postdoctoral researchers, and faculty.
The Neutron Physics Group’s training of postdoctoral researchers and students, both within and outside NIST, is highly effective. The group maintains strong collaborations with outside institutions. The group has been hosting a large number of graduate students and postdoctoral researchers, academic users, and industrial users from the isotope production industry, fuel cell and energy storage industry, national laboratories, and the Navy. In recent years, the group hosted summer schools on fundamental neutron physics and the 9th Workshop on Neutron Wavelength Dependent Imaging in 2017. The dissemination of the group’s program is effective, as evidenced by its 27 publications since 2018. The interferometry team has produced 22 publications since 2015. The imaging team, benefitting from a large number of external collaborations, has published almost 99 papers since 2015.
In addition to its basic-science activities, the major way in which the work of the Radioactivity Group is disseminated is through the development and availability of standards and standard reference materials. From 2009 to 2021, the group completed 30 new standard reference materials and recertified 22 standard reference materials. The Radioactivity Group participated in international comparisons of standards for Cu-64, F-18, Ba-133, Ra-224, tc-99m, and tc-99. From 2017 to 2021, the group published 37 papers in the peer-reviewed literature. The majority of these papers appeared in the premier journals for this type of work.
GENERAL CONCLUSIONS AND RECOMMENDATIONS
The Dosimetry Group has made major contributions to improved precision of dosimetry metrics through Monte Carlo computation improvements, nuclear-data science, and detection science, which in aggregate have greatly contributed to the precision of dosimetry measurement. Expanded dissemination of this knowledge outside NIST, through greater engagement with university partners, and international stakeholders, would accelerate widespread application of this outstanding technology.
RECOMMENDATION: The Dosimetry Group should expand dissemination of its knowledge of radiation metrics through greater engagement with university partners, and international stakeholders.
Staffing of the critical service and research activities within the Dosimetry Group is very top heavy, with senior scientists compelled to take on support functions to sustain key activities. It would be advantageous to strengthen the student and postdoctoral training functions to help mitigate this problem. It would also be useful to give consideration to analyzing and identifying resource allocation priorities in a manner that ensures the sustainability of succession planning and knowledge retention, while balancing the needs of human capital development and infrastructure improvements.
The Neutron Physics Group is integral to the Radiation Physics Division through their work on neutron calibrations and metrology. Within the group, there is a virtuous cycle connecting the basic research with neutrons to the applied research that often benefits from the operationalized products of basic research (such as polarized 3He spin filters) and to the core NIST mission in metrology and calibration. Each of these applications benefits from the synergy of technology and staff across the focus areas.
The group’s neutron imaging program provides a set of unique capabilities to multiple applications as evidenced by numbers of unique experiments, patents, papers, and RD100 2018 (neutron X-ray imaging), awards. Neutron interferometry has transitioned from an artifact of quantum mechanics to a versatile tool addressing issues, including nuclear physics and characterization of quantum materials. The group is publishing significant papers, many with external collaborators. The beam line used by the Neutron Physics Group is the preeminent resource for cold-neutron fundamental physics in the United States (based on overall results), and the group is the world leader in the beam measurement of neutron lifetime.
The Neutron Physics Group has made very few new hires over a number of years. The group is small for the scope of work for which it is responsible, and this constitutes a risk for a national resource essential to maintaining the state of the art in neutron-related measurement and applications.
RECOMMENDATION: The Radiation Physics Division should examine the ratio of the Neutron Physics Groups’ staffing to its workload and consider means of achieving and maintaining an effective ratio by increasing staff if appropriate.
The Radioactivity Group leads the nation in the development of new methods and instruments for determining the absolute activity of a radionuclide or mixture of radionuclides. The group is publishing and disseminating information on new approaches for determining the absolute activity of a radionuclide. The group is fulfilling its mission of advancing measurement science and providing standards for the use of radioactive materials in medicine and industry. However, the Radioactivity Group is at risk of depleting its expertise due to an ageing workforce and staff departures.
RECOMMENDATION: The Radiation Physics Davison should address predicted staff departures as part of its staffing plan.
