Ionizing Radiation Division

Technical Highlights

• Uncertainty Standard on Radiation Processing Dosimetry. The Ionizing Radiation Division's Office of Radiation Measurements has made significant contributions to the development of a new draft E10.01 American Society for Testing and Materials (ASTM) standard on the statement of uncertainty in measurements of absorbed dose in industrial radiation processing. Dose measurements are required to assure that the dose delivered to the product meets the requirements of the process, whether for regulatory needs such as the sterility of medical products or food processing, or for materials' property modification such as polymer cross linking. It is critical that an appropriate statement of the uncertainty in these dose measurements be developed and made consistent with the terminology and basic concepts of ISO/TAG4 Guide to the Expression of Uncertainty in Measurement. The ASTM document will be used as a model for other ASTM documentary standards relating to radiation dosimetry, will help them conform with accepted international guidelines, and will provide a uniform format for the statement of measurement uncertainties. (J.C. Humphreys)
• National Voluntary Laboratory Accreditation Program (NVLAP). NVLAP, in coordination with the Office of Radiation Measurements in the Ionizing Radiation Division, has accredited the first laboratory under the Secondary Calibration Laboratory for Ionizing Radiation LAP. The Food and Drug Administration's Center for Devices and Radiological Health (CDRH) has been accredited in accordance with NIST Special Publication 812, "Criteria for the Operation of Federally Owned Secondary Calibration Laboratories for Ionizing Radiation," to perform calibrations of diagnostic, survey, and reference class instruments for the measurement of x-ray fields. Instruments calibrated by CDRH are used by state radiological inspectors to assure that diagnostic x-ray machines in all hospitals and doctors' offices are operated in a safe and effective manner. (K.W. Inn)
• Workshop on "Measurement Quality Assurance for Ionizing Radiation." The workshop was held at NIST March 16-18, 1993, and was attended by about 170 representatives from private industry, State, contractor, Federal Agency, Armed Service, Defense Department, professional society, and medical sectors. The status of current MQA programs and concepts were reviewed; current government agency policies were presented; potential dosimetry and radioactivity MQA programs were evaluated; and (in-house controls of) reference laboratory quality control practices were shared. The workshop provided the mixed audience with fundamental MQA concepts that could be applied to many different programs. One such concept is the need to demonstrate measurement traceability through performance evaluation tests conducted by NIST. The workshop was organized by the Ionizing Radiation Division of the Physics Laboratory. (K.W. Inn)
• Conference on "Metrology for Environmental Management." The conference was held at NIST April 14-15, 1993, and was attended by more than 130 representatives from DoE, EPA, NRC, DoD, and NIST. Plenary session speakers described metrology needs for their environmental management programs. Four concurrent workshops were conducted after the plenary session to identify specific environmental technology and metrology issues and to formulate approaches for addressing the needs, particularly in the areas of 1) radioactivity measurements; 2) analytical chemical measurements; 3) field contamination sensors and instrumental methods; and 4) waste processing and minimization. The major conclusions of the conference include: 1) partnerships and collaboration of efforts within and between agencies are necessary; and 2) measurements made in support of environmental management should be traceable to the national standards. The conference was co-organized by the NIST Physics Laboratory, the Chemical Science and Technology Laboratory, and the Electronics and Electrical Engineering Laboratory. (K.W. Inn)
• NIST/States Workshop on Procedures for the Sampling and Radioassay of Materials. Analytical methods and procedures driven by regulatory requirements tend to be prescribed and inflexible. However, regulators involved with issues of environmental restoration, and decontamination and decommissioning of operations and buildings are still grappling with approaches to radioactivity analyses. The lack of regulatory policies creates an opportunity for the measure-ment community to have major impact on future formulation of approaches to needed radioactivity measurements. The NIST Office of Radiation Measurements co-sponsored an October 28-31, 1993 workshop on Procedures for the Sampling and Radioassay of Materials with a major stakeholder in radioactivity measurements, the state governments, that was represented by the Conference of Radiation Control Program Directors. Along with NIST and State representatives, other participants included stakeholders from the Environmental Protection Agency, Department of Energy and commercial contractors. The outcome of the workshop will be a first draft of selected procedures for sampling and radioassay that will be recommended to the Radiation Control Program Directors for the 50 states, and updated periodically with technical refinements. (K.W. Inn)

Radiation Interactions and Dosimetry Group

• Medical Industrial Radiation Facility. The latest addition to the radiation research facilities of the group is MIRF (Medical-Industrial Radiation Facility). This facility is based on a 32 MeV electron linac acquired from the Radiation Therapy Center of Yale University Hospital where it was decommissioned in 1992. The accelerator was dismantled and moved to NIST by NIST personnel and has been reassembled in Room A005 of the Radiation Physics Building. The accelerator and associated radiation shielding are completely installed and beam acceptance tests have begun. At present, an electron beam has been generated at the entrance to the beam handling system and measurements of the klystron output power indicate that sufficient rf power is available to support maximum energy and beam power. The accelerator will provide electron energies from 7 MeV to 32 MeV at an average beam current of up to 0.1 mA. In addition, bremsstrahlung-generated photon beams will also be provided by the bombardment of suitable targets. An experimental area is in preparation containing a linear translation and rotation table for positioning experimental apparatus (water phantoms, calorimeters, samples, etc.) in the radiation field. (C.E. Dick, M.R. McClelland, and B.M. Coursey)
• A National Standard Mammographic X-Ray Range. The current NIST x-ray beam qualities offered for the calibration of ionization chambers used for mammographic dose measurements are generated with a tungsten anode and aluminum filtration. However clinical mammography x-ray beams are generated with a molybdenum anode and molybdenum filtration. Spectral differences between the tungsten and the molybdenum produced beams can result in errors of dose measurements and poor quality assurance in mammography. A national standard for mammographic beams is deemed appropriate to properly conduct a federal certification program, which has been required by the Mammography Quality Assurance Act of 1992. A facility which will provide molybdenum x-ray beam qualities is in progress. The design of the facility and the selection of the equipment is intended to facilitate collaborations with the USFDA's Center for Devices and Radiological Health (CDRH). An appropriately designed free-air ionization chamber is to be employed as the national standard. Comparisons between the tungsten and the molybdenum generated x rays are to be made with the chamber. The correction factors for the chamber are to be determined so calibrations of reference ionization chambers can be performed for the CDRH. The intention is to develop a transfer standard for use in research and clinical settings and to determine the necessary protocol for testing such transfer ionization chambers on the new mammographic x-ray range. (C.M. Johnson, P.J. Lamperti, J.H. Sparrow, C.G. Soares, M.D. Walker, and B.M. Coursey)
• Graphite Calorimetry for High-Energy Electron Beams. The increasing use of accelerator-produced high-energy electron beams in the 4 MeV to 12 MeV energy range to cure coatings and composites, sterilize medical devices and instruments, induce color centers in gem stones, disinfest certain foods of pathogenic microorganisms, and extend the shelf life of fruits requires accurate dosimetry for process validation. Novel calorimeters have been designed that allow the direct measurement (in real-time) of the radiation dose delivered by electron beams and the calibration of films for routine use with these accelerators. The calorimeters consist of arrays of graphite disks into which temperature-sensing thermistors have been embedded. One of the calorimeters has an interchangeable phantom that allows calibration of passive dosimeters such as radiochromic films and alanine. The calorimeters have now been tested with a com-mercial linear accelerator operating at 12 MeV. (M.L. Walker, J.C. Humphreys, and W.L. McLaughlin)
• High-Dose Dosimetry for Materials Damage Studies. The accurate measurement of very large radiation doses (10⁵ Gy to 10⁷ Gy) is essential in the evaluation of key materials subjected over long periods to ionizing radiation fields. These materials include superconducting magnet components, electronics devices and insulating materials, composites, polymers, resins, adhesives, gaskets, sealants, lubricants, fiber optics constituents, and solid-state devices. Many are irradiated at both elevated and cryogenic temperatures, which is a challenge for achieving correct dosimetry results. It has been possible to meet these aims by the use of optical-quality (pure) alkali halide crystals (e.g., LiF) by analysis of the accumulation of radiation-induced color centers, which involves applying visible and infrared spectrophotometry. A newly discovered color center, having an absorption band centered at 790 nm, is formed in LiF by doses > 10⁶ Gy. These color centers may be annealed by high-temperature radiant heat (550 °C), so that the dosimeters can be reused. (W.L. McLaughlin and J.M. Puhl)
• Kinetics Studies of the Mechanisms of Radiochromism. The radiation chemistry of leuco dye systems and polymers is under investigation. The coloration by ionizing radiation of triphenylmethane leuco dyes, tetrazolium compounds, and crystalline oligomers (e.g., polydiacetylenes) generally proceeds by pseudo-first order and subsequent second-order reactions, with relatively low rate constants and high activation energies. In addition, the radiation chemical yield of stable chromophores is relatively small, but with a large dynamic range, thus providing useful high-dose dosimeters with linear response. Moreover, the materials can be tailored to match both biological tissues and key solid-state devices (semiconductor components), and therefore have broad medical and industrial utility, as well as high-resolution imaging capabilities. (W.L. McLaughlin, M. Al-Sheikhly, A. Kov cs, and D.F. Lewis)
• Stereotactic Radiosurgical Treatment Planning. The development of radiochromic film for mapping electron and photon beams in stereotactic radio- surgery is showing profound growth. A joint publication with the Mayo Clinic and the Universities of Kentucky and Pittsburgh on "Gamma-Knife" treatment planning of deep-seated intracranial lesions is now in press (Medical Physics). The Mayo Clinic's Department of Radiation Oncology is beginning a collaboration with NIST and has assigned the thesis work of a Ph.D. candidate to this program. This will entail a three-year joint effort to improve clinical results by enhancement of quality control through better radiation measurements. In addition, the American Association of Physics in Medicine (AAPM) has formally approved the formation of a "Radiochromic Task Group" toward the use of NIST-developed radiation treatment planning applications, the group consisting of national leaders in this field, including three group staff members from NIST. (W.L. McLaughlin, C.G. Soares, R. Blackwell, and D.F. Lewis)
• Blood Irradiation Quality Control. In a collaboration with the Biomedical Research and Development Laboratory of the American Red Cross and with ISP Technology (a subsidiary of GAF Chemical Corporation), radiation measurement methods are being developed for maintaining quality assurance in the irradiation of blood-bank transfusion supplies. Such irradiation is sanctioned by the Food and Drug Administration as the method of choice to diminish the chance of transfusion-induced diseases involving lymphocyte division that may occur following prosthetic and medical grafting procedures. The radiation dose and dose distributions that may occur from x- and gamma-ray sources must be measured accurately for meeting regulatory rule-making requirements. The collaboration involves the development of several candidate dosimetry systems, including radiochromic films, and alanine/ESR and lithium fluoride/thermoluminescence devices. The radiochromic and alanine dosimeters are especially promising as small, tissue-simulating measurement tools, having relatively high precision and accuracy. (W.L. McLaughlin, M.F. Desrosiers, D.L. Bensen, J.M. Puhl, and D.F. Lewis)
• Energy Deposition and Radiation Quality of Radon and Radon Daughters. We have been using a computer code derived from the "analytic method" code for neutrons to model the interaction of alpha particles deposited along the human bronchial airway (generations 2, 4, 6, and 10) by radon daughters ²¹⁴Po and ²¹⁸Po. Using the CRAY supercomputer, we have calculated the fluence-rate and lineal energy spectra for the human lung model (generations 2, 4, 6 and 10). Calculations are run according to various input parameters such as target cell depth (5 µm to 60 µm in 5 µm increments), diameter of the lung airway (from 1.130 cm to 0.198 cm), and mucus layer thickness (8 µm or 15 µm). We have also run calculations to compare results obtained for different target sizes (5 µm or 1 µm). Results are also dependent on the penetration ability (stopping power and energy) of the specific alpha particle. We are collaborating with Dr. Werner Hofmann (University of Salzburg, Austria) to apply the results of these calculations to biological models and to suggest the biological consequences of radon and alpha particle exposure. (L.R. Karam and R.S. Caswell)
• Alanine-Electron Paramagnetic Resonance Industrial Dosimetry. A new government/industry collaboration plans to improve radiation exposure measurements in areas such as medical therapy and industrial processing. Victoreen Inc., of Cleveland, Ohio, recently signed a cooperative research and development agreement with NIST to develop dosimeters based on the amino acid alanine. When mixed with a polymer binder, alanine dosimeters can be made in the shape of pellets, films, cylinders and cables. Because they are very stable, they can be archived as a permanent record of radiation exposure. NIST and Victoreen intend to develop alanine dosimeters for the lower radiation levels used in radiotherapy as well as the higher levels in product sterilization, pest control, food preservation and polymer curing. This effort is intended to develop inexpensive, massproduced alanine dosimeters for medical and industrial users of radiation in the United States and abroad. In a second agreement NIST and Micro-Now Instrument Company Inc., of Skokie, IL, are working together on an automated system for interpreting the amount of radiation absorbed by alanine dosimeters. Micro-Now recently signed a cooperative research and development agreement with NIST to develop software for an automated dosimetry reader. (M.F. Desrosiers, D.L. Bensen, and J.M. Puhl)
• Alanine-Cotton Fiber Dosimeters. In collaboration with the University of Maryland, College Park, the amino acid, alanine (a dosimeter material), was incorporated into cotton cloth fibers. Stable alanine radicals were formed and detected in irradiated alanine-impregnated fibers. The alanine signal was proportional to the alanine content (maximum of 10 percent by weight) and the lower limit of detection was 10 Gy. The ultimate goal of this project (sponsored by the Defense Nuclear Agency) is to produce emergency personnel dosimeters that could be attached or incorporated into uniforms. (M.F. Desrosiers and F.G. Le)
• Radiopharmaceutical Dose Assessment. Internally-administered radiopharmaceuticals offer much promise for improving the quality and effectiveness of certain therapies. Measurement of the absorbed dose distribution in the target organ is a major challenge to theoreticians and an important step in future drug design. We have successfully used electron paramagnetic resonance (EPR) spectrometry to map the absorbed dose in a humerus from a beagle treated with ¹⁶⁶Ho for bone marrow ablation. Human trials using ¹⁶⁶Ho are in progress at the M.D. Anderson Cancer Center in Houston, TX. We hope to modify the EPR method such that it can be applied to the measurement of absorbed dose in bone biopsies from treated patients. (M.F. Desrosiers, F.G. Le, and P. Fattibene)
• Retrospective Assessment of Human Exposures. EPR dosimetry will be used to provide a historical account of internal radiation doses to exposed individuals and populations (e.g., radium dial painters). The project (sponsored by the Department of Energy) involves Harwell International (UK) and the U.S. Transuranium and Uranium Registry. The collaboration plans to use archived bone tissue and compare autoradiographs depicting radionuclide distribution to EPR dose maps. For most radionuclides and their progeny, limited knowledge is available on the transport and redistribution through bone remodeling. The development of microdosimetry for bone tissue should produce significant benefits to radiation risk assessment. (M.F. Desrosiers)
• Energy-Dependence Study of EPR Dosimetry of Cortical Bone and Tooth Enamel. A careful experimental and theoretical investigation was made of the EPR signal per unit absorbed dose in cortical bone and tooth enamel to determine if previous reports of non-proportionality (of up to factors of two) at low photon energies are correct. This work makes use of the results of the calculations of photon energy-absorption coefficients, along with photon Monte Carlo calculations to accurately assess the absorbed dose in the irradiated samples. No significant energy dependence was found, implying that previous studies were flawed due to poor estimates of actual dose to bone. This important result removes a previously reported impediment which would limit the application of EPR dosimetry to an expanding catalog of radiological problems. (M.F. Desrosiers, F.G. Le, and S.M. Seltzer)
• Dosimetry for Insect Population Control. The radiation sterilization of insect pests such as the Mediterranean fruit fly (Medfly) is an active program of the Animal and Plant Health Inspection Service (APHIS) of the U.S. Department of Agriculture (USDA). The insects are irradiated by gamma-ray radiation in the pupae stage with a sufficient dose to assure sterility but not hinder motility or viability of the insect when it becomes an adult. NIST provided an important contribution to the quality control of this process involving irradiators in Guatemala and Hawaii by mapping the radiation field in a canister as it underwent a standard irradiation cycle. This was accomplished by using circles of radiochromic film spaced at intervals in an irradiation canister with cardboard to simulate the average density of the pupae. (M.L. Walker, W.L. McLaughlin, and J.M. Puhl)
• Use of Radiochromic Film Dosimetry for Brachytherapy Source Characterization. The high resolution capability coupled with the relative insensitivity of the radiochromic dye film system suggested its use for characterizing the small, high-dose rate sources used in brachytherapy. Both ¹²⁵I and ¹⁹²Ir sources are being studied. Films were irradiated in various geometries and read with a high-resolution scanning densitometer. A study of theradiochromic film response as a function photon energy was performed, which is a necessary preliminary to convert film response to absorbed dose. A comparison has been made between the measured seed air-kerma strength and the film dose interpretation from a contact exposure averaged over the seed axis. Preliminary results of this comparison indicate that the film can be used to predict the air-kerma strength to within an expanded uncertainty (2σ) of ±5 percent, independent of ¹²⁵I seed type. These results were presented at the Annual Meeting of the American Association of Physicists in Medicine (AAPM) in Washington, DC, August 1993. (C.G. Soares)
• Low-Energy Photon Dosimetry with Radiochromic Film. In response to a problem presented by a industrial customer, NIST developed a novel technique for assessing dose rate from low-energy x-ray (< 50 kV) beams operating at high currents in close-to-source geometries. The high dose rates and tight spacing preclude the use of traditional ionometric methods for dose-rate determination. Instead, the use of radiochromic film was proposed. The film was calibrated at NIST in an x-ray beam whose spectrum approximated those used by the customer. The spectra employed by the customer were estimated from the x-ray set parameters supplied, and differences in the film response between the NIST calibration and the customer irradiations were accounted for through a knowledge of the energy dependence of the film response. The expanded uncertainty (2σ) of the method was estimated at less than 20 percent. Possible future applications include dose reconstruction from an accidental exposure from a diffractometer. (C.G. Soares and P.J. Lamperti)
• Radiation-Therapy Dose Mapping with a New X-Ray Imaging Camera. Under a CRADA, the Rayex Corporation and NIST have developed an x-ray imaging camera with the potential of simultaneously measuring in real time the profiles of tissue density and of radiation dose in a patient undergoing radiation therapy. Nothing is presently available with which an on-line, real-time, non-invasive measurement of the actual dose delivered can be made (and possibly used to control the dose delivery). The camera consists of an array of 100 thin x-ray detectors positioned near the patient. The digital signals from x rays in the radiation beam are processed to perform one-sided computed tomography. Calibrations with a variety of phantom structures and with various beam modalities and energies are crucial to accurate performance. These will be carried out at NIST facilities in collaborative studies with medical physicists from Johns Hopkins Medical Institutes. (C.E. Dick, M.R. McClelland, S.M. Seltzer, and J.W. Motz)
• Reference Dosimetry for the Intercomparison of Proton Beams in Radiation Therapy. Cancer therapy studies using high-energy proton beams are underway in medical centers in the United States, including the Harvard Cyclotron and Loma Linda University in California, and in Russian medical centers, including one at the Institute for Theoretical and Experimental Physics (ITEP) in Moscow. NIST is collaborating with the Radiation Research Program at NCI in developing new dosimetry systems to facilitate intercomparisons between these laboratories. Alanine-EPR and radiochromic films are being used for measuring detailed proton dose and depth-dose curves in high-energy proton beams at the two U.S. accelerators and at two facilities in Russia. Dose measurements and depth-dose profiles obtained from alanine samples read out at the NIST EPR facility agreed with the ITEP values to within ±5 percent of the ITEP values for samples irradiated to 100 Gy with 200 MeV protons. The initial results were presented by Dr. Zink (NCI) at the Washington meeting of the American Association of Physicists in Medicine. (D.F. Nichiporov, B.M. Coursey, M.F. Desrosiers, J.M. Puhl, W.L. McLaughlin, and S. Zink)
• Calculation of Energy Deposition by Therapy Proton Beams. Proton beams, because of their highly focused pattern of energy deposition, are increasingly employed for radiation therapy in treatment of certain tumors where it is particularly important to spare adjoining critical structures. With support from the National Cancer Institute, our program for the development and utilization of a state-of-the-art Monte Carlo calculation of the spatial pattern of absorbed dose and proton fluence spectra has continued. The work includes the development of extensive data on production cross sections for secondary heavy charged particles in nonelastic interactions by the protons with constituent nuclei in water and tissue, a calculation of the slowing down of these particles to assess their contributions to LET distributions, and an attempt to predict their role in the relative biological effectiveness of proton therapy beams. (S.M. Seltzer and M.J. Berger)
• Space-Shielding Radiation Dose Calculations. With support from NASA's Life Sciences Biomedical Research Program, a computerized database and code package is being 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. This work represents a thorough updating, extension, and refinement of our earlier SHIELDOSE package, which has found wide use in the space-radiation-effects community. The preparation of the database has involved 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 have been evaluated in a straight-ahead approximation; a study of the effects of nonelastic nuclear interactions of the protons with aluminum nuclei and the transport of nuclear secondaries has been included. (S.M. Seltzer)
• Monte Carlo and Analytic Methods in the Transport of Electrons, Neutrons, and Alpha Particles. At the request of the U.S. Department of Energy we have carried out a review of Monte Carlo and analytic methods used in radiation transport calculations for electrons, neutrons and alpha particles. These methods are used for macroscopic calculations, and for calculations on the micrometer and nanometer scales. The results of the study were presented at a DoE Workshop on Computational Approaches in Molecular Radiation Biology: Monte Carlo Methods, in Irvine, California, April 26-29, 1993. (R.S. Caswell and S.M. Seltzer)

Neutron Interactions and Dosimetry Group

• Neutron Interferometry. The initial phase of construction of the NIST neutron interferometry facility at the Cold Neutron Research Facility (CNRF) was completed at the end of 1993. The two stage, positionally stabilized, multi-degree of freedom vibration isolation system became fully operational. The long term positional stability is better than 1 µm. The thermal and acoustic laboratory enclosure has also been completed. The multi-crystal focusing mono-chrometer has been installed and aligned. An interferometer support system, fabricated at the University of Missouri has been integrated into the beam line. With the completion of construction, it is anticipated that the facility will be-come fully operational in early 1994 and a user program will be started in late 1994. This new facility will open up many possibilities for both applied research and fundamental physics research. The applications may include materials studies based on neutron phase topography and studies of hydrogen impurities and precipitates in metals. The fundamental physics possibilities include tests of fundamental symmetries and tests of the foundations of quantum mechanics. (M. Arif, D. Brown, G. Greene, and M.S. Dewey)
• The Material Dosimetry Reference Facility (MDRF). This high-intensity reference neutron field for industrial materials dosimetry is in full operation at the Ford Nuclear Reactor, as a collaborative effort between NIST and the University of Michigan. Certified neutron fluences for calibration irradiations at MDRF are established on the basis of neutron fluence transfer from the NIST Californium Neutron Irradiation Facility. The MDRF was designed to produce a spectrum matching the spectrum found at the inside face of the pressure vessel of a commercial pressurized water reactor. Proper interpretation of MDRF calibrations requires accurate calculation and integral detector validation of the neutron spectrum. Considerable effort was spent this year in improving measurements and calculations. Using the DORT two-dimensional discrete ordinates code, detailed calculations were made of the sensitivity to the source spectrum, the core enrichment, the effect of cadmium, and the modeling of the ¹⁰B cylindrical sleeve surrounding the MDRF. Calculated spectra, derived cross sections, and integral detector characterization measurement results were presented at the Eighth ASTM-EURATOM Symposium on Reactor Dosimetry (ASTM STP 1228, 1994). The first irradiation for a customer was a test of helium accumulation fluence monitors for Rockwell International. These dosimeters are employed by Rockwell in both fission reactor and fusion reactor applications. (J. Grundl, C. Eisenhauer, and E.D. McGarry)
• Determination of the Neutron Lifetime. The beta decay lifetime of the free neutron is a fundamental quantity of great importance in a wide range of investigations including particle physics, nuclear physics, astrophysics and cosmology. For the past several years, NIST Physics Laboratory personnel have been engaged in a series of measurements designed to improve our knowledge of the neutron lifetime. In the past year, a sophisticated neutron decay detector, installed at the Cold Neutron Research Facility, has been used to measure the absolute decay rate of neutrons in a low energy neutron beam. In the latter half of 1993 approximately three million neutron decay events were observed. The measurement is continuing and a result is anticipated in 1994. This work is sup-ported in part by the Department of Energy. (M.S. Dewey, G. Greene, and D. Gilliam)
• Laser Polarization of Neutrons. Polarized neutrons, which can be produced with a ³He spin filter, are of fundamental importance both in materials science and in the study of fundamental symmetries. Significant progress has been made during 1993 on construction of a ³He neutron spin filter based on spin exchange with optically pumped rubidium. Major milestones include installation of an environmental enclosure to be used for initial testing of the spin filter, delivery and testing of the laser to be used to polarize the ³He, production of an initial set of ³He cells, construction and testing of the magnetic field system which will be used to orient and measure the ³He polarization, design and partial fabrication of the apparatus for mounting ³He cells and measuring nuclear magnetic resonance signals from the polarized ³He, and partial fabrication of a gas-handling system to be used to fill ³He cells. (A. Thompson, G. Greene, and M.S. Dewey)
• Search for Time Reversal Invariance Violation in Neutron Beta Decay. Following the successful proof of principle test carried out in late 1992, considerable effort was directed towards the final design for the complete detector. The detector must be capable of simultaneously measuring, with high efficiency, the direction of emission of both the proton and electron created in the decay of a polarized neutron. This design is now complete and construction is expected to begin early in 1994. This work is a collaboration among NIST, Los Alamos National Laboratory, University of California (Berkeley), and the University of Michigan. (G. Greene, A. Thompson, and M.S. Dewey)
• Absolute Determination of Neutron Flux. A test of a totally-absorbing ¹⁰B prompt gamma detector for determination of neutron flux (neutrons/second) has been carried out by comparison with a conventional "thin" detector whose calibration is dependent on isotope dilution mass spectrometric (IDMS) assay of ¹⁰B and ⁶Li deposits. This test was carried out in the very well-characterized monoenergetic neutron beam at the interferometer station in the CNRF. The uncertainty of the "black" ¹⁰B detector measurement was estimated to be ±0.14 percent (1σ). The dominating uncertainty in the "thin" detector measurement was that of the isotopic deposit mass, of the order of ±0.3 percent. This comparison gives an independent confirmation of the "black" detector efficiency at the level of uncertainty of the next best detector available. Further development of a second "black" detector based on calorimetry is in progress through collaboration with Indiana University, with the goal of achieving confirmation of the calibration at an accuracy comparable to that of the ¹⁰B detector. (J. Richardson, W.M. Snow, K. Coakley, and D. Gilliam)
• Isotopic Mass Standards Development. Well-characterized isotopic standards are frequently required for measurement of neutron fluences and densities in radiation protection, regulation of the nuclear power industry, and fundamental neutron physics research. Under the terms of a formal collaboration agreement with Institute of Reference Materials and Measurements (IRMM) in Belgium, new sets of ultra-high quality ¹⁰B and ⁶Li isotopic deposits have been prepared and compared by thermal neutron reaction rate observations. We are in possession of about one third of these deposits now, IRMM is keeping about the same number, and the remainder are being destructively analyzed by isotope dilution mass spectrometry (IDMS) at IRMM. The results for the lithium IDMS have been reported to us. The boron IDMS is in progress, including measurements by both the conventional method and the potentially more accurate gas source method. NIST staff played major roles in developing new evaporator hardware for the sample preparation and in the thermal neutron reaction rate comparisons of the samples. The IRMM staff have taken the higher accuracy of the "black" detectors as a challenge and are making a very serious effort to match or beat that level of accuracy by improvements in the IDMS. (D. Gilliam [NIST] and J. Pauwels, A. Lamberty, and R. Scott [IRMM])

• Response of Albedo Neutron Dosimeters as a Function of Angle of Incidence. The International Commission on Radiation Units and Measurements (ICRU), in Reports 39 and 47, suggests that neutron personnel dosimeters should have a particular, non-isotropic, response as a function of angle of incidence. We have started a measurement and calculation program to determine the angular response for several different types of dosimeters, for both bare and D₂O-moderated californium neutron sources. The work is to be done in collaboration with the Battelle Pacific Northwest Laboratory, and part of the work is to be used to be used as material for an M.S. thesis by a graduate student from Georgetown University. Preliminary measurement and calculation are in very good agreement for the actual dosimeters, and in reasonable agreement with a calculation from the Physikalisch-Technische Bundesanstalt in Germany, for the ideal dosimeter for measurement of the personal dose equivalent as defined in ICRU 47. (R. Schwartz, E. Boswell, C. Eisenhauer, and B.A. Torres [Georgetown University])
• Neutron Reaction Cross Section Measurements. Improvement in the accuracy of the neutron reaction cross section for ¹⁰B is one of the most important nuclear data needs for neutron energies below 20 MeV. We have completed new collaborative measurements of the total cross section in the 0.2 MeV to 20 MeV neutron energy region using the Oak Ridge National Laboratory Electron Linear Accelerator (ORELA) pulsed neutron facility. Since the accuracy of previous measurements was limited by the properties of the boron sample, we have extensively characterized the new isotopically enriched boron samples using the analytical capabilities at NIST. Significant departures (several percent) from the most recent international evaluation were observed for energies less than 1.5 MeV. The results of our previous measurements of the ¹⁰B(n,αγ) reaction in the 0.2 MeV to 4 MeV neutron energy region have been published in Nuclear Science and Engineering. During the past year we have extended these gamma-ray production cross section measurements to lower neutron energies for normalization purposes. An international interlaboratory working group endorsed by the Nuclear Energy Agency Nuclear Science Committee, chaired by A.D. Carlson, coordinates the efforts to improve this important basic nuclear data standard. (R. Schrack, O. Wasson, and A. Carlson)
• Fission Cross Section Data. Collaborative measurements were made this year of the neutron-induced fission cross sections of ²³²Pa and ²³⁸Np which are important in the design of systems for the transmutation of nuclear waste, a major national problem. Measurements had never been made on these nuclei before due to their short half lives of 1.3 and 2.1 days, respectively. These data were obtained at the Los Alamos National Laboratory Neutron Scattering Center (LANSCE). These challenging, pioneering measurements required timely coordination to produce the actinide isotopes in an accelerator beam, chemically separate the material and deposit it on the electrodes of the fission chamber, and carry out the cross section measurements in the neutron beam before the samples decayed away. Also the decay of these nuclei produces contaminants which complicate the analysis of the fission measurements. Preliminary analysis of the measurements has revealed resonance structure which will improve the optimization of the ATW (Accelerator Transmutation of Waste) design. Analysis of the large amount of experimental data obtained in previous fission measurements for the nuclides ²³³U, ²³⁶U, ²³⁸U, ²³⁷Np, and ²³⁹Pu continues. These very accurate data were obtained relative to the ²³⁵U(n,f) and ¹⁰B(n,α) neutron cross section standards. The measurements provide information required to resolve problems with fission dosimetry standards needed for international nuclear data. The ²³⁶U work, which was accepted for publication in the journal Nuclear Physics and was the subject of a NIST highlight (in August), resolves a problem with the under-standing of the systematics of nuclear fission. The analysis of the data on ²³⁷Np revealed that the ENDF/B-VI evaluation of this fission cross section is in error by a factor of 3 in the energy region from 10 eV to 2000 eV. A paper on this subject was presented at the Eighth ASTM-Euratom Symposium on Reactor Dosimetry in September. (A. Carlson)
• Interlaboratory Measurements of Charged-Particle Production. The neutron-induced charged-particle production cross sections for oxygen and nitrogen are important for the determination of kerma factors for medical therapy, radiation protection, and space shielding in addition to neutron source calibrations in manganese sulfate baths. International collaborative measurements on these materials in the neutron energy region from 1 MeV to 40 MeV continued using the neutron time-of-flight beamline at the Los Alamos National Laboratory. Nitrogen samples were procured for the 1993 measurement season during which the detector system was optimized for alpha particle measurement. The particle identification system provided determination of charged-particle energy as a function of incident neutron energy at four angles. The analysis of the substantial data collected is in progress while plans are underway to optimize the detectors for hydrogen detection for measurements in 1994. (O. Wasson and A. Carlson)

Radioactivity Group

• Dissemination of National Standards of Radionuclide Activity. Continuing primary functions of the Radioactivity Group are the supplying of special SRMs anchored on the national standards and the checking of measurement traceability of subsidiary organizations. Over 700 SRMs were distributed in the past fiscal year, and over 220 certificates of traceability were issued 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 two research associates in cooperative testing programs. (L.L. Lucas, J.M.R. Hutchinson, and F.J. Schima)
• Radon Measurements Standards Program. The primary objectives of this program are to maintain the national standards for ²²⁶Ra and ²²²Rn, to develop new transfer standards and measurement applications, and to disseminate standards and provide other mechanisms for insuring the quality of radon measurements. Highlights of this year's accomplishments include: (1) Work on the new polyethylene-encapsulated ²²⁶Ra/²²²Rn-emanation standards was completed and they will be issued as SRM 4968 in December 1993. In the past year, the older version capsules which utilized mechanical seals were found to be inadequate over long times. A "crash" program was initiated and conducted to develop a new all-polyethylene heat sealing procedure for the capsules, and to perform a complete recalibration (and confirmation) of the previously obtained calibration factors for the compression-seal capsules. Two papers describing the development and calibration of the standards and another paper on their application for calibrating electret monitors (the most widely used radon measurement methodology) are near completion; (2) Several collaborations with other laboratories, e.g., with the DoE Environmental Measurements Laboratory, on testing and demonstrating the utility of the capsule standards were completed; (3) Under an interagency agreement, the proficiency/traceability testing of the two EPA laboratories that conduct the national radon measurement proficiency testing program for commercial radon measurement vendors is an ongoing program that ensures the relatability of EPA to U.S. national standards; and (4) The collaborative work on the international marine-atmosphere ²²²Rn measurement intercomparison conducted in Bermuda in late 1990, the first intercomparison of its kind to use standardized sample additions that could be referenced to NIST standards was completed. Drafts of two papers are in a final commentary period among all of the participating laboratories. (R. Collé‚, J.M.R. Hutchinson, J.T. Cessna, P.A. Hodge, and M.P. Unterweger)
• ³⁶Cl/Cl AMS Standards: Dilution Verifications. A consortium of accelerator-mass-spectrometry (AMS) laboratories recently prepared a series of ³⁶Cl/Cl isotopic ratio AMS standards by an eight-step serial gravimetric dilution scheme. Of the resulting nine solutions used to prepare the standards, only the latter six could be assayed by AMS to confirm the gravimetric dilution factors. Extensive, exhaustive, and difficult low-level radioactivity measurements were performed on the first four solutions to verify the first three dilution factors. The measurements included use of a novel technique -- assays on precipitated salt samples -- developed for this work. A paper describing the studies and their findings has been published. (R. Collé and J.W. Thomas)
• Glow-Discharge Resonance Ionization Spectrometry. A glow discharge cell has been coupled to a resonance ionization mass spectrometer system (RIMS) and the production of ions and atoms demonstrated. This source allows the direct analysis of solid materials and represents a first step in our goal to achieve direct compositional analysis of environmental materials without chemical processing. Potential advantages of the glow discharge source over other atomization sources are its simplicity, its applicability to direct analysis of conductors with wide coverage of the periodic table, and its sensitivity.

  The glow discharge source consists of a cylindrical stainless steel cavity held at approximately 67 Pa (0.5 torr) of argon with a rod of sample material inserted from one end. A plasma is developed at the inside end of the rod by impressing a 1000 V potential drop between the cavity wall and the rod. Neutral atoms are sputtered from the sample by bombardment of ions within an argon ion plasma, after which they are selectively ionized by means of carefully tuned lasers, and then mass analyzed in a double focussing mass spectrometer.

  A sample containing 95 percent aluminum and 5 percent iron has been examined. The ion source was operated in two modes (i) using glow discharge ions and (ii) using RIMS ions. The glow discharge ions gave a clean spectrum but with an enhanced peak at mass 54 corresponding to aluminum interference. The interference was completely removed in the RIMS spectrum because of the selectivity of the laser.

  Work is continuing toward improvement of the selectivity by various means and also the sensitivity by dramatically increasing the duty cycle with high frequency and c.w. lasers. (J.M.R. Hutchinson)

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