Ionizing Radiation Division

Technical Highlights

Radiation Interactions and Dosimetry Group

• Energy Deposition and Radiation Quality of Radon and Radon Daughters. The goal of this project is the development of a quantitative description, at the micrometer and nanometer level, of the physical interactions of alpha particles from radon and its daughters with the cells at carcinogenic risk in the lung and bronchial epithelium. Such information is basic to an understanding of the mechanisms of the biological effectiveness of radon and provides input data to biological models of radiation interaction. We have calculated the alpha particle fluence-rate spectra at cell nuclei for cell depths within the range of 5 µm to 60 µm for radon daughters, ²¹⁸Po and ²¹⁴Po, using a source layer thickness of 8 µm and airways of diameters 1.130, 0.651, 0.435, and 0.198 cm (generations 2, 4, 6, and 10 of the Yeh and Schum morphometry). These results were compared to those previously obtained for a source layer thickness of 15 µm. For the rat lung model, we calculated the fluence-rate spectra at cell nuclei for cell depths within the range of 5 µm to 15 µm in airways of diameters of 0.2626, 0.1636, 0.1227, 0.0778, 0.00995 cm (generations 2, 4, 6, 10, and 15). The source layer thickness used (2.6 µm to 1.6 µm) was dependent on the airway generation of interest. Lineal energy distributions were calculated for a target cell nucleus of 5 µm diameter. We have also calculated the fluence-rate and lineal-energy spectra for ²¹⁴Po and ²¹⁸Po in airway generation 4 for a target size of 1 µm to compare with earlier results for a 5 µm target. (L.R. Karam and R.S. Caswell)

• Beta-Ray Dosimetry Calculations. Considerable work has been done on the application of theoretical methods to beta-particle radiation protection practice. As part of the work of an ICRU/ICRP Joint Task Group in updating fluence-to-dose conversion factors used in radiation protection, extensive electron Monte Carlo transport calculations have been done of the depth-dose distribution by electrons incident on phantoms of water, PMMA, and tissue. Tables have been prepared of the personal dose equivalent at depths of 7, 40, 300, and 1000 mg/cm², for electrons incident at energies from 50 keV to 10 MeV and angles from 0° to 89°. These data, along with tables of basic electron penetration data, a review of the physics of electron interactions in matter, and a review of beta-ray transport calculations, have been adopted in an ICRU Report on the Dosimetry of External Beta Rays for Radiation Protection. Work with the NCRP Scientific Subcommittee on hot-particle dosimetry for radiation protection has involved Monte Carlo calculations of the dose distributions from spherical sources of beta and gamma rays, and the development of new point-kernel-based calculations for faster routine estimates for a variety of shapes and arbitrary sizes of emitting particles containing virtually any radionuclide. (S.M. Seltzer)

• Space-Shielding Radiation Dose Calculations. With support from NASA's Life Sciences Biomedical Research Program, a computerized database and code package has been 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. The new code is intended to replace our earlier SHIELDOSE package, which has found wide use in the space-radiation-effects community. The results are based on 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. 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 has been included. The code performs the necessary interpolation over the database and the integration for any specified spectra of incident electrons and protons, giving the distribution in a variety of simple geometries of dose in small detector volumes of Al, graphite, Si, air, bone, CaF₂, LiF, GaAs, SiO₂, tissue, or H₂O. A collaboration is underway to incorporate this new work into an existing commercial package for estimating space radiation effects in any Earth orbit. (S.M. Seltzer)

• International Comparison of X-Ray and Gamma-Ray Standards. NIST has compared its x-ray standard, using x rays in the 100 kV to 250 kV energy region, and gamma-ray standards using ⁶⁰Co gamma radiation, with the Italian National Laboratory in Rome (Ente Per Le Nuove Tecnologie, L'Energia E L'Ambiente - ENEA). The agreement was better than ±1 %. NIST also participated in a comparison of ¹³⁷Cs gamma-ray standards with the Bureau International des Poids et Mesures (BIPM) in Paris and with the Austrian National Laboratory using ¹³⁷Cs gamma rays. The agreement with the BIPM was well within 1 %. Although the results from the Austrian National Laboratory were not available at the time of this writing, it is anticipated that the agreement will be within the expected uncertainty of ±1.5 %. The comparisons were conducted through the use of suitable transfer standards. Within the next few months, NIST will participate in a comparison of gamma-ray standards with the BIPM using therapy level ⁶⁰Co gamma rays. These comparisons will serve to corroborate the standards used for therapy (⁶⁰Co) and for radiation protection (¹³⁷Cs). This is the first comparison of its kind between NIST and these three laboratories. (P.J. Lamperti)

• Electron Irradiations for Research and Industry. The principal electron accelerators available for use in the Radiation Interactions and Dosimetry Group are: the 500 keV cascaded rectifier accelerator (CR); the 4 MeV electron Van de Graaff (VDG); and the 7 MeV to 32 MeV Medical-Industrial Radiation Facility (MIRF). These facilities have been utilized in the past year in a number of programs designed to investigate new dosimetry techniques, materials modification, and isotope production. In the area of dosimetry techniques, experiments have been performed at MIRF to develop new radiochromic dye dosimeters embedded in gels for three dimensional dose measurements, three-dimensional EPR imaging in quartz rods, and graphite calorimetry of high-energy electron beams. In addition, this accelerator has been used for the production of radioisotopes of carbon and iodine using direct electron excitation. The Van de Graaff accelerator has been utilized to examine the crosslinking of polymers in alcohol solutions to develop new classes of polymers. It has also been used to investigate the low-energy response of the graphite calorimeter and to investigate the radiation response and damage induced in silicon, gallium arsenide, and indium phosphide solar cells designed for extraterrestrial uses. The 500 keV accelerator has been utilized in a program to develop standard electron beam fields for instrument calibrations. (C.E. Dick, M.R. McClelland, and B.M. Coursey)

• Process Control and Real-Time Monitoring Electrons and Ultraviolet Processing. High-powered electron beams and intense ultraviolet sources are used in a multi-billion-dollar-a-year business for processing many commodities (paints, inks, polymers, coating, insulating and moisture-barrier materials, food containers, shrink-wrap foil, composites, tubing, and films). Quality control of these products depends on standardized radiochromic dosimetry developed and maintained at NIST. Another new important application of electron beams is the continuous on-line sterilization of pouched parenteral (Class III) medical products (e.g., eye ointments, syringe-packaged pharmaceuticals). An invited paper presented at a symposium in collaboration with Energy Sciences, Inc., and Trygon, Inc., describes a new real-time monitor for quality control of such systems to meet FDA regulations. The electronic monitor has been tested at NIST and is tied to reference transfer dosimetry traceable to NIST, to meet strict good-manufacturing practice specifications. (W.L. McLaughlin, J.M. Puhl, J.C. Humphreys, M.L. Walker, and C.E. Dick)

• Quality Control of Industrial Sterilization. Three invited papers were presented in 1994 to the 6th International Kilmer Memorial Conference on Sterilization of Medical Products and to the Health industry Manufacturers Association "2000" conference on radiation sterilization quality control. Dosimetry coordinated by and traceable to NIST through calibration by alanine/EPR and radiochromic/spectrophotometry systems is vital to state-of-the-art manufacture and processing of many of the nations health care products. NIST is also contributing to the Technology's access to affordable digital manufacturing-oriented measurement quality assurance data collection and interpretation systems. The components include improved industrial-scale radiation dosimetry systems, microdensitometers, dedicated analytical equipment, programmable logic controllers and software, computer hardware and communication interface equipment. In particular, new dose-mapping techniques are being developed for industrial radiation process validation qualifications and verification, as stipulated in accredited quality control procedures. (W.L. McLaughlin, M.F. Desrosiers, M.C. Saylor, J.C. Humphreys, and J.M. Puhl)

• Mammography X-ray Exposure Standards. NIST is constructing a facility that will provide a national standard for mammographic x-rays. The Mammography Quality Standards Act of 1992 (MQSA) requires that the Food and Drug Administration (FDA) implement a federal certification program for each of the approximately ten thousand mammography facilities in the U.S. These inspections should be done with radiation measuring instruments calibrated to x-ray beams commonly found at mammographic facilities. In the past, NIST has offered only beam qualities generated with a tungsten anode and aluminum filtration for the calibrations of mammographic exposure chambers. With the current rate of progress, NIST plans to offer in 1995 calibrations with beam qualities generated with an anode and filtration combination of molybdenum or rhodium, which is representative of clinical mammography x-ray beams. The design of the mammographic calibration facility and the procurement and installation of the equipment was initiated and completed in 1994. A constant potential, highly-stabilized high-voltage (50 kV) generator, two x-ray tubes and a microprocessor based controller, programmable for the tubeheads and generator, have been purchased and installed in the calibration facility. The stationary anode x-ray tubes have targets of molybdenum and rhodium and focal spot sizes appropriate for calibration services. A rail system which allows vertical and horizontal remote movement of calibration instrumentation, a shutter apparatus that permits the translation of both x-ray tubes and motor driven filter and aperture wheels have been installed in the facility. A multitasking computer system will control the calibration process through the use of a real-time interactive spread sheet and graphics interface package. An appropriately designed free-air ionization chamber has been extensively compared to a current NIST standard and evaluated for its appropriateness for measuring mammographic beam qualities. (C.M. Johnson, P.J. Lamperti, and J.H. Sparrow)

• Alanine Dosimeter Response in Proton Therapy Beams. The use of proton beams for radiation therapy of various malignancies is being studied at a number of institutions worldwide. Because of the wide geographic distribution of these facilities and the variety of dosimetry methods in use, there is a need for a mailable dosimetry system that has a uniform response over the energy range used in proton therapy. A comparison was made of the response of alanine dosimeters, thimble ionization chambers, parallel plate ionization chambers, and diodes in the radiation-therapy proton beam at the Harvard Cyclotron in Cambridge, MA. Dosimeters were placed at the downstream end of a water telescope that allowed the residual range of the emerging proton beam to be varied with sub-millimeter precision. Irradiations of alanine dosimeters were then made over a spectrum of proton energies with corresponding residual ranges from approximately 75 mm to 0 mm. With the depth-dose curves normalized at a residual range of approximately 70 mm, the response of the alanine dosimeters was found to be within 5 % of the parallel-plate ionization chamber response over the entirety of the proton beam residual range. When the parallel-plate depth dose data are averaged over 3 mm intervals (corresponding to the alanine dosimeter thickness in the beam direction) the agreement is brought within 3 %. The uniformity of response implies that alanine may be suitable as a transfer dosimeter for intercomparisons between proton therapy facilities. Trial intercomparisons made between proton therapy facilities support this conclusion. (M.F. Desrosiers, D.L. Bensen, K. Gall)

• Multi-Photon Detector Development for Biomedical Applications. This project focuses on evaluating a new device for the measurement of radiolabelled compounds of very low activities. Due to the extremely low residual backgrounds achieved by this system, the Multi-Photon Detector (MPD) can be used in situations where conventional detection systems would be inadequate. This will permit the use of much less radioactivity in biomedical and other studies, thereby reducing radioactive wastes and costs associated with its disposal. The Ionizing Radiation Division is providing electron-capture (EC) radionuclide standards, performing high-pressure liquid chromatographic (HPLC) analyses, performing background measurements, and consulting on other potential biomedical applications of the technique. (L.R. Karam, B.M. Coursey, J.M.R. Hutchinson, I. Sagdeev, and A. Drukier)

• Novel Approaches in Nuclear Medicine. Among the research projects involving various aspects of nuclear medicine, we are investigating a novel delivery methods of radiopharmaceuticals. We have constructed a fullerene production apparatus and have begun incorporation of specific atoms into the fullerene cage. Fullerenes are molecules comprised entirely of carbon atoms in a closed, cage-like structure and constitute a third form of ordered solid carbon (in addition to graphite and diamond). Due to the hollow structure of fullerenes, atoms can be placed inside the molecular cage, forming an "endofullerene." Application of endofullerenes as carriers of radioisotopes for use in cancer therapy has been suggested, but has not been studied either theoretically or experimentally. Because fullerene cages are capable of physically and chemically isolating the radioisotopes from their associated pharmaceutical, a much greater flexibility in tailoring the radioisotopes for specific tracing or therapeutic applications would be achieved than is possible with currently available radiopharmaceuticals. The main objective of this project is the successful development of radioisotope endofullerenes, or "radiofullerenes," suitable for use in medical diagnosis and cancer therapy (such as those incorporating ²²Na). We are producing, extracting and purifying fullerenes produced under various conditions (in the presence and absence of sodium containing salts) and have been characterizing their structure by mass spectrometry. With the eventual incorporation of radioisotopes, the characterization will be carried out by multi-photon detection (MPD). (L.R. Karam, M.G. Mitch, and B.M. Coursey)

• Development of Instrumentation for Radiation Therapy Applications. One of the challenges in treatment-planning dosimetry is to establish a high-resolution two- or three-dimensional map in tissue for the administered radiation. A new technique in dose mapping is based on radiochromic film. Staff members of the Ionizing Radiation Division are participants of a new AAPM Radiochromic Task Group to develop a protocol for such applications. Under a CRADA with the Photoelectron Corporation of Walton, MA, prototype charge-coupled-device (CCD) microdensitometers are being tested for quantitative mapping of film images created at clinical radiotherapy dose levels. Some key U.S. medical centers are also working with NIST on evaluating the new technology, including Harvard Medical School, Massachusetts General Hospital, the Mayo Clinic, and Georgetown University. Prototypes of several new radiochromic films absorbing at different optical wavelengths have been produced in the U.S., Denmark, and Hungary, tailored specifically for three new high-resolution images are presently being investigated as a means of improving treatment-planning and beam-profile analysis capabilities. (W.L. McLaughlin, C.S. Soares, M.L. Walker, L.R. Karam, J.M. Puhl, and B.M. Coursey)

• Development of Enhanced X-ray Tube Technology. Under a CRADA with the Radiation Interaction and Dosimetry Group, the Rayex Corporation has developed new technology which will significantly increase the maximum x-ray power provided by x-ray tubes. The technology involves a fundamental change in the geometry used in x-ray tube design that was optimized through extensive electron/x-ray Monte Carlo calculations using a world-leading computer code developed at NIST. This enhanced power design promises to increase the x-ray output per unit anode heat load by a factor of approximately two, and thereby permit shorter exposures, smaller focal spots, better image quality, longer tube life, and reduced operating costs. The advantages apply to all types of x-ray tubes, such as those used in mammography, fluoroscopy, CT scanners and x-ray inspection equipment, and should not change the size or cost of the tube. Work is in progress to refine the design and construction of prototype tubes to be used to verify the x-ray power enhancement factors. Success here should lead to rapid commercialization of the technology and have a significant impact on the x-ray tube and apparatus industry, cited in the Department of Commerce U.S. Industrial Outlook 1994 as being the second fastest-growing manufacturing industry from 1987-1994. (J.W. Motz, M.J. Berger, and S.M. Seltzer)

• Use of Radiochromic-Film Dosimetry for Brachytherapy Source Characterization. The high-resolution and high-dose capabilities 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 promising new geometry was tried in 1994, involving wrapping a single layer of film around a 6 mm diameter plastic cylinder in which a seed is placed. This geometry yields information on seed axial and transaxial uniformity, as well as dose rate at a depth of 3 mm in tissue. Results of preliminary measurements will be reported upon at the annual AAPM meeting. (C.G. Soares)

Neutron Interactions and Dosimetry Group

• Neutron Interferometry. The neutron interferometer station at the NIST cold neutron facility became operational in the Spring of 1994. Interference patterns were obtained with two types of interferometers. The interference fringe visibility was greater than 70 %, and phase stability as good as ±5 mrad/day was observed. These unprecedented performance characteristics were achieved by employing advanced vibration isolation and environmental control systems. Initial measurements indicate vibrational isolation to be better than ±0.1 µg, where 1 g is the earth's gravitational acceleration. The positional stability of the setup is about ±2 µm in translation and about ±1 µ radian in rotation. These numbers suggest that NIST interferometer performance is on a par with or better (in some cases by more than an order of magnitude) than other interferometer stations elsewhere. Steps are being implemented to enhance the beam intensity and improve the vibration isolation and the environmental control systems even further. A variety of fundamental physics experiments are expected to be carried out in the near future in collaboration with teams from Universities in the U.S. and Europe. In addition, considerable efforts are being devoted to the exploration of neutron phase contrast imaging, advances in small angle neutron scattering based on neutron optics, and Fourier spectroscopy for surface analysis. (M. Arif, D. Brown, and G. Greene)
• Development of Neutron Spin Filters by Laser Polarization of ³He. The competence project intended to produce polarized neutron beams using a ³He spin filter has seen major advances this year with the successful polarization of a sample of ³He in the guide hall. In addition, the polarization was measured using Nuclear Magnetic Resonance (NMR) and calibrated against the small NMR signal from a sample of pure water. The spin filter is based on the spin-dependent absorption of neutrons by polarized ³He in the reaction ³He(n,p)³H. The polarized ³He is produced either by spin-exchange with laser optically pumped rubidium vapor or by direct optical pumping of metastable ³He. The polarization of the ³He is measured either using NMR, or by measuring the circular polarization of light emitted from an excited state of ³He (the second method only works for the metastable polarization technique). The polarization measured and calibrated this year was produced using the spin-exchange method.
  
  Significant progress has also been made in testing potassium as an alternative to rubidium in the spin exchange process. Immediate plans are to complete the potassium/rubidium comparison, maximize the ³He polarization produced in the spin-exchange setup, and then to test the spin filter on a neutron beam. Progress toward implementing the metastable apparatus includes construction of the vacuum system used to maintain the 1 torr to 2 torr ³He densities needed to sustain a metastable population, and construction and testing of the laser. (A. Thompson, T. Gentile, and M.S. Dewey)
• Determination of the Neutron Lifetime. The neutron lifetime experiment began collecting useful neutron decay events on October 5, 1993 and data taking continued through May 23, 1994. During that period, the proton detector was live for almost 124 days and some 10,500,000 decay events were logged. If this experiment were limited solely by statistics, this would give imply a result with far better than 0.1 % uncertainty. We anticipate, however, that systematic effects will dominate our final error. An important feature in this effort was a multiparametric data acquisition system that allows a detailed study of each event. This will be very useful in reducing systematic effects. Progress is currently underway to analyze the results. (M.S. Dewey, G. Greene, and D. Gilliam)

• Neutron Dosimetry for Reactor Safety Assessment. The experimental efforts in neutron materials dosimetry continued apace at the Californium Neutron Irradiation Facility (CNIF) and at the Materials Dosimetry Reference Facility (MDRF), which is operated at the Phoenix Memorial Laboratory in Ann Arbor in cooperation with the University of Michigan. Work to certify the MDRF as a reference neutron field nears completion. This facility was built by NIST but was characterized and is operated cooperatively by NIST and the University of Michigan. Fluence certification for any irradiation test is the responsibility of NIST. This is accomplished with traceable calibration to a ²⁵²Cf fission spectrum from a neutron source with a NIST calibrated emission rate. NIST is organizing and will carry out round robin irradiations at the MDRF to assess the nuclear industry's ability to accurately measure neutron fluence (E>1 MeV) exposures using the ²³⁷Np(n,f) and ²³⁸U(n,f) reactions. Dosimeters of both isotopes from two different suppliers (Oak Ridge and Geel) will be included in the tests. (E.D. McGarry)

• Radiation Protection Dosimetry. A series of measurements has been made of the neutron response as a function of angle for the Navy albedo dosimeter. The results are in general agreement with results obtained earlier, for a different dosimeter type, at Oak Ridge, and follow the general trend suggested by the ICRU. The results, however, showed an asymmetry which is not completelyunderstood, and is being further investigated. A small ³He proportional counter was used as a surrogate for an albedo dosimeter to make detailed measurements as a function of distance for both bare and moderated californium, with the counter bare, completely Cd-covered, and with Cd just covering the front. These three configurations are meant to correspond to the location of the cadmium (or lack of cadmium) on the Navy dosimeters, the so-called Hankins dosimeter, and the so-called conventional albedo dosimeter, respectively. The data are being analyzed in terms of an effective depth in the phantom, and to get a better understanding of the effect of the phantom on room scatter. (R. Schwartz, B.A. Torres, and P. Ghilardi)

• Absolute Determination of Neutron Flux. In the determination of the neutron flux or fluence rate for the neutron lifetime experiment or for dosimetry applications in standard neutron fields, absolute charged particle counting with an accurately defined solid angle is frequently required. A blind comparison of our defined-solid-angle counting capability with that of the Radioactivity Group of the Institute for Reference Materials and Measurements (IRMM) in Geel, Belgium has been carried out, by circulation of a specially prepared set of ²³³U alpha sources, with activities of nominally 6000 Bq, 11,000 Bq, and 17,000 Bq. The set of ²³³U deposits was prepared by the IRMM Sample Preparation Group under the direction of Jean Pauwels. Dr. Pauwels is also serving as the referee for the blind comparison. The NIST Radioactivity Group also counted samples of all three activity levels using a 2π gas counter. Our results were reported to Pauwels toward the end of 1994. He will now resolve any differences in reporting protocol and then open all of the results to all of the participants. A technical paper describing the comparison will be prepared in the coming year. (D. Gilliam, J. Nico, and C. Winfrey)

• Neutron Cross Section Standards. A very active extramural experimental program has been pursued to develop accurate cross section standards for U.S. programs in nuclear energy, safeguards, waste containment, radiation therapy, and personnel dosimetry. These measurement programs at the Ohio University Tandem Accelerator Facility, the LAMPF and LANSCE facilities at Los Alamos, and the ORELA facility at Oak Ridge are addressing important discrepancies which still remain to be resolved in even the best established cross section standards for some energy regions. The investigations this year have included studies of the angular dependence of elastic scattering from protons at 10 MeV, the ²³⁷Np(n,f) cross section in the range 10 eV to 1 MeV, the ¹⁰B(n,α₁γ) cross section from 10 keV to 1 MeV, the ¹⁰B and ¹¹B total cross sections between 20 keV and 20 MeV, neutron-induced charged-particle production measurements for oxygen and nitrogen from 1 MeV to 40 MeV (which are of interest in radiation protection and therapy), and the fission cross sections for ²³²Pa and ²³⁸Np (which are of interest in the burnup of nuclear waste). (O.A. Wasson, A. Carlson, and R. Schrack)

Radioactivity Group

• Low-level Radioactivity Standards. The Radioactivity Group has begun the certification of the eighth natural-matrix radioactivity SRM, Ashed Bone. The standard will be used by the internal dosimetry community to verify and validate their analytical methodologies for the determination of ⁹⁰Sr, ²¹⁰Pb, ^228,230,232Th, ^234,235,238U, ^{238,239+240,241}Pu, and ²⁴¹Am in autopsy samples. Such studies are critical for the refinement of biokinetic models that track the distribution and fate of radionuclides in the long-term biological sinks (bones, liver, and lung) and determine risk assessment of radiation-related disease. (Z.-C. Lin, J.W.L. Thomas, and K.G.W. Inn)

• 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 800 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)

• ²⁰⁹Po Decay and a Delayed Isomeric State in ²⁰⁵Pb. Solution standards of polonium isotopes have a great popularity in laboratories throughout the world, and are among the most frequently requested radioactivity standards. They are used primarily as calibration standards for alpha-emission rate measurements, and as low-level tracers and separation yield monitors in radiochemical procedures that are employed with environmental and geophysical samples. Assays of ²¹⁰Po, for example, are of interest not only for monitoring the nuclear fuel cycle, but also for investigating the geochemistry and radiohydrology of the uranium-radium decay chain in nature. Carrier-free ²⁰⁹Po solution standards have recently been prepared and calibrated, and will be disseminated by NIST as Standard Reference Material SRM 4326. During the course of the ²⁰⁹Po calibrations, some initial difficulties and inexplicable early findings led to a major re-evaluation of the ²⁰⁹Po decay scheme, and to a systematic evaluation of the calibration methodology which was based on liquid scintillation (LS) spectrometry. Polonium-209 decays principally by alpha-particle emission to ²⁰⁵Pb, and has a weak electron capture (EC) branch decay to ²⁰⁹Bi. The relative branching ratios for these two decay modes has been determined. In addition, evidence emerged for the existence of a low-energy delayed isomeric state in ²⁰⁵Pb. The state is sufficiently long-lived to confound routine 4πα LS assays of ²⁰⁹Po. This work and findings resulted in the completion of three journal articles. Identification and evidence for the ²⁰⁵Pb isomeric state, and its implications for routine measurements of ²⁰⁹Po, are presented in an article that is in press in Applied Radiation Isotopes. (R. Collé, F.J. Schima, Zhichao Lin, P.A. Hodge, J.W.L. Thomas, J.M.R. Hutchinson, and B.M. Coursey)

• ²²²Rn (radon) Half-Life. A precise and uniformly accepted value of the half-life for the radioactive decay of ²²²Rn is of interest and important in a variety of disciplines ranging from studies in global atmospheric modelling and the geophysical sciences to indoor air quality and concern over radon's potential human health hazard. Two studies on the radon half-life have been completed in the past year. The first study is a very precise determination of the half-life by 4παβ liquid scintillation measurements that resulted in a value (3.8224 days) having a relative combined standard uncertainty of 0.05 percent. The second study consisted of a critical review and evaluation of 17 independent determinations of the half-life made over the past 90 years by many different measurement methodologies. A ²²²Rn half-life value of 3.8232 days was recommended. This value has an estimated relative combined standard uncertainty of 0.01 percent. (R. Collé)

• 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) The new polyethylene-encapsulated ²²⁶Ra/²²²Rn-emanation standards were issued as SRM 4968. This work was completed after the result of a "crash" program to develop a new capsule sealing procedure (following inadequate performance with an earlier capsule prototype), and to perform a complete recalibration (and confirmation) of the previously obtained emanation fractions for the earlier prototype. Several papers (in a series) on the development, calibration, and applications of the capsule standards are in varying stages of publication; (2) Efforts to test and evaluate the long-term performance of the emanation capsules, as well as to demonstrate new applications, have been initiated; (3) Analyses of five years of data on the long-term performance of the radon-in-water standard generator was completed; (4) 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; (5) The collaborative work on the international marine-atmospheric ²²²Rn measurement intercomparison conducted in Bermuda in late 1990 was at last completed. Two papers, one describing the methodology for standardized sample additions and the other on the intercomparison results, were submitted to the Journal of Geophysical Research; and (6) A new collaboration with the Atmospheric Sciences Research Center (located at Moffett Field, CA and operated by the State University of New York at Albany) was initiated. This work involves performing tests and measurement intercomparisons that will be employed in providing calibrations for a new series of atmospheric radon altitude profiles. Such profile data are of critical importance in all global transport models. (R. Collé, J.T. Cessna, P.A. Hodge, J.M.R. Hutchinson, and M.P. Unterweger)

• Glow-Discharge Resonance Ionization Mass Spectrometry. Work continued on the development of a glow-discharge initiated mass spectrometer system which would permit the direct compositional analysis of soils and sediments for radioactive and non-radioactive trace elements. For effective radioassay, a sensitivity in the range of 10^-13 is useful for most environmental contaminants. Plans were moved forward and calculations performed toward incorporating a continuous wave Titanium-sapphire (Ti-Saph) laser into the system to perform initial highly selective Z discrimination before isotopic selection in the mass spectrometer. The Ti-Saph laser has been ordered and initial work has been performed on the preparation of appropriate samples which are made from Rocky Flats soil and powdered silver. (J.M.R. Hutchinson)

• Imaging Plate Technology. Work has been performed using an imaging plate which stores a radiation image as a distribution of F-centers in a photostimulable phosphor. It has been initially used to examine radioactivity sources for homogeneity and other characteristics. The plate has 100 µm resolution and extremely low background count rates. In theory it can detect one alpha particle. Our investigations included the evaluation of its use for examining and quantifying radioactivity in the natural matrix standards. Alpha-particle-emitting sources with varying alpha energies were used to irradiate the plate. Graded absorbing layers were inserted between source and plate to discriminate between the various energies. Slopes of the count rates as a function of energy thickness demonstrated that alpha energies with differences greater than a few hundred keV could be distinguished from each other. (J.M.R. Hutchinson)

• Atmospheric Krypton-85. Due to concerns about DOT requirements for labeling radioactive material, a measurement of ⁸⁵Kr in ultra-high purity krypton gas has been undertaken. Mr. G.E. Schmauch, an engineering safety adviser with Air Products, Inc., is the contact person and has obtained 25 liters (STP) of krypton gas (99.995 % purity) which was condensed from the atmosphere in May-June of 1994 in the Ohio region of the USA. Quantitative samples of the gas are being counted in the NIST internal gas counting system. Preliminary results indicate ⁸⁵Kr concentration in the atmosphere, late spring 1994, in the air over Ohio, to be approximately 300 Bq per cubic meter. (F.J. Schima)

• Strontium-89. Strontium-89 radioactivity is of great medical importance. It is available as a commercial radiopharmaceutical called "Metastron" and NIST has a CRADA with Capintec, Inc., which manufactures dose calibrators used in the administration of this material. Measurement problems have been encountered because of a by-product radioactivity ⁸⁵Sr that is characteristically produced in the commercial material. The Radioactivity Group has worked on the production of ⁸⁹Sr by ⁸⁹Y (n,p) reaction. This production mechanism should, in principle, yield highly pure samples of ⁸⁹Sr. Such material from the Obninsk fast-flux reactor in Russia has indicated that ⁸⁵Sr/⁸⁹Sr is less than 0.005 % or a factor of 30 times better than the commercial product. This should allow for more precise values of the half-life and probability per decay of the weak 909 keV gamma-ray emission. Additionally, a much more reliable dose calibrator calibration appears possible. (F.J. Schima)
• Calibration of Large-Area Alpha and Beta Sources. Calibrations of the 2πβ emission rates of several large area sources are under investigation for use in the calibration of β field monitors. The effects of β-backscattering are under investigation in order to provide accurate values for the activity of these sources. The calibration and characterization of large-area ²³⁸Pu and ²³⁹Pu alpha sources are continuing and their incorporation into the U.S. Army and Air Force field monitor calibration systems are proceeding. Also, planchet-sized sources of thorium and depleted uranium are also being calibrated. (M.P. Unterweger)

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