Ionizing Radiation Division

Technical Highlights

• International Comparison of Mammographic X-ray Exposure Standards. NIST completed a comparison of the primary air-kerma standard for mammography, the Attix chamber, with the German primary mammographic standard. This indirect comparison was conducted through the use of NIST reference-class ionization chambers, which were calibrated at NIST and at Physikalisch-Technische Bundesanstalt (PTB), the German standards laboratory. The results from the calibrations at both national laboratories agreed to within 0.5 % for the six different mammographic beam qualities used in this comparison. At the time of this comparison, NIST and PTB were the only primary standards laboratory with mammographic x-ray standards. The Attix chamber was also directly compared with another NIST national standard for low energy x-rays, the Lamperti chamber. The results from the comparison of the two primary standard chambers agreed to within 0.4 %, for the four molybdenum entrance x-ray beam qualities. In preparation for future comparisons of mammographic energy spectra, preliminary mammography spectra have been generated. (C.M. O’Brien, P.J. Lamperti, and J.H. Sparrow)
• Implementation of ISO Bremsstrahlung and NIST Techniques. NIST has continued the efforts to implement the photon techniques specified by the International Organization for Standardization (ISO). In addition to the 41 new ISO beam qualities being developed, NIST is answering the request of the medical community through the development of several new diagnostic x-ray beam qualities. A spectral determination of all new beam qualities will be used to verify the newly established beam parameters. (C.M. O’Brien, P.J. Lamperti and J.T. Weaver)
• MultiPhoton Detector Development for Biomedical Applications. This project in collaboration with BioTraces, Inc., focuses on the continuing development of the MultiPhoton Detector (MPD) for the measurement of radiolabelled compounds of very low specific activities. Due to the extremely low residual backgrounds achieved by this system, the 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. NIST is providing electron-capture (EC) nuclide standards, performing high-pressure liquid chromatographic (HPLC) analyses of ¹²⁵I labeled nucleic acid PCR products and peptides, consulting on other potential biomedical applications of the technique, and performing background measurements. (L.R. Karam and B.M. Coursey)
• Novel Approaches in Nuclear Medicine. We are performing ongoing research projects involving several aspects of nuclear medicine, including investigations of novel delivery methods of radiopharmaceuticals. We have constructed a fullerene production apparatus and have begun incorporation of specific atoms into the fullerene cage. Application of fullerenes as carriers of radioisotopes for use in cancer therapy has been suggested, but has not been studied either theoretically or experimentally.
  
  The main objective of this project is the successful development of radioactive fullerenes (radiofullerenes) suitable for use in medical imaging such as radioactive iodine (¹²⁶I). We have used the MIRF (Medical and Industrial Radiation Facility) electron beam to convert non-radioactive ¹²⁷I to radioactive ¹²⁶I in larger fullerenes purified in our facility. Yield and purity of ¹²⁶I endofullerenes have been determined by high-pressure liquid chromatography (HPLC), MPD, liquid scintillation counting (LSC), thin-layer chromatography (TLC), and Fuji phosphoimaging and are continuing development of modified TLC and HPLC solvent parameters for ¹²⁶I encapsulated within fullerenes. This work with ¹²⁶I endofullerenes will provide further data in protocol development to eventually apply to encapsulation of the more medically common ¹²⁵I. We have begun a calibration of Fuji phosphoimaging plates with MPD using irradiated KI (¹²⁶I). (L.R. Karam, M.G. Mitch, B.M. Coursey, and G. Galegos).
• Waste Treatment by Electron Beam. We have been investigating applications of radiation (gamma and E-beam) technology for the reduction and elimination of hazardous waste materials such as those generated in industry ("chemically hazardous" waste such as polychlorinated biphenyls). We are performing mass-spectral analyses of polychlorinated biphenyls (PCB)-contaminated water models. Samples are analyzed before and after irradiation with electrons and gamma radiation to determine the amount of toxins destroyed, yields and structures of products formed, and potentially toxic by-products induced by this emerging technological application. (L.R. Karam, W.L. McLaughlin, and M. Al-Sheikhly)
• Development of Dental Polymers. Current directions in dentistry have focused a great deal of attention of the development of nontoxic polymers that can be used for dental reconstruction and tooth protection. Since a major criterion of such a polymer is that it be free of impurities (to maintain structural integrity and reduce toxicity), compositional elucidation of potential polymers must be performed. In association with the American Dental Association, we have been using mass spectrometry to determine the structures and purities of various dental polymer systems and investigating the use of ionizing radiation as a possible agent in the modification of such polymers. (L.R. Karam and M. Farahani)
• Development of Radiosensitive Dosimetric Gels. We had obtained prepared Bang^TM gel calibration samples from MGS, Inc. and irradiated them in a water phantom using a ⁶⁰Co source (vertical beam) with dose rate 0.719 13 Gy/min to determine dose-yield calibration curves. After irradiation, the gels (known doses, controls, and "unknowns," the received doses of which were known to us but not to MGS personnel) were returned to MGS for analysis and calibration curve development. Such an approach has allowed MGS to demonstrate the feasibility of determining the absorbed dose to polymer and radiochromic gels, held in containers that simulate ionization chambers, by optical absorption measurements and tomographic optical scanning. (L.R. Karam, P. Lamperti, R. Schultz, M. Maryanski, and M. Ranade)
• Approaches in Intravascular Brachytherapy. Approximately 40 % of patients undergoing balloon angioplasties each year experience restenosis, a renarrowing of the arterial wall. Researchers have determined that the application of radiation to the site of the angioplasty immediately after the procedure can reduce the restenosis rate by 40 % or more. We have developed a system of entrapping ^99mTc within liposomes of various charged (positive, negative or neutral) membranes to be used as a possible technetium packaging unit for intravascular irradiation after balloon angioplasty (using the Infiltratorÿ perfusion catheter system developed by Interventional Technologies, Inc.). After preliminary labeling of liposomes showed the feasibility of this approach, labeling of liposomes with ^99mTc at the Washington Hospital Center (WHC) were performed. Labeled liposomes were then injected into pig models and a persistence study was conducted at artery sites (both with and without injury). Imaging of excised arteries (with free ^99mTc and ^99mTc labeled liposomes) showed that ^99mTc labeled liposomes persisted longer at the application site than free ^99mTc. (L.R. Karam, B.E. Zimmerman, [R. Waksman and R. Chan, WHC])
• Absorbed-Dose to Water Calibrations and Development of Secondary Standards. In 1993, NIST completed extensive development of a water calorimeter which can realize the quantity absorbed dose to water. The changeover from in-air-kerma calibrations to in phantom absorbed dose to water calibrations is a logical and necessary evolutionary step in radiation dosimetry for teletherapy purposes. NIST is interacting with the American Association of Physicists in Medicine (AAPM) Task Group 1 to develop a secondary standard for use by the AAPM Accredited Dosimetry Calibration Laboratories. Characterization of reference sources and ionization chambers is currently in progress. Protocols for calibration and round-robin performance testing are currently being developed. (J.T. Weaver, P. Lamperti, and J. Shobe)
• Beta-Particle-Emitting Ophthalmic Applicator Calibration Service. With the advent of the calibration service for ophthalmic applicators at the University of Wisconsin (UWI) Accredited Dosimetry Calibration Laboratory (ADCL), the routine role of NIST in these calibrations has diminished considerably. Of the four sources calibrated in 1997, one was for the UWI ADCL to calibrate their system. NIST's role in this field will henceforth be more geared towards providing transfer standard sources and fields both to secondary laboratories and source manufacturers. Towards this end, we acquired over ten used ophthalmic applicators at no charge, and calibrated two for resale to source manufacturers as transfer standards. The major international intercomparison of ophthalmic applicator dosimetry was completed, which included the dosimetry of both curved and flat sources of ⁹⁰Sr+⁹⁰Y and ¹⁰⁶Ru+¹⁰⁶Rh. The results of this comparison will be available at the beginning of 1998 and will be included in the International Commission on Radiation Units and Measurements (ICRU) report on beta particles for medical applications. (C.G. Soares)
• High-Energy Electron and Photon Measurement-Assurance Service. Users of medical linear accelerators for cancer teletherapy need assurance that their machines are delivering the required radiation dosage. To meet this need, for the past 30 years NIST has offered a measurement assurance service that involves mailing to the users passive dosimeters to be irradiated in a prescribed geometry and returned to NIST for evaluation of the absorbed dose delivered. Up until now, ferrous-ferric (Fricke) dosimeters have been used for this service. There were two mailings of Fricke dosimeters in 1997, involving 20 participants, and a total of 55 dose interpretations. Two batches of dosimeters were prepared, and alanine dosimeters were also included as part of the shipments. The successful results of the alanine will allow us to replace the currently used Fricke dosimeter system entirely in future mailings with the alanine/EPR system. (C.G. Soares, J.M. Puhl, and M.F. Desrosiers)
• Calibration of Beta-Particle Sources and Instruments for Radiation Protection. A calibration service for protection-level beta-particle sources and instrumentation has been in place for several years. The measurement system is automated and capable of measuring extremely low absorbed-dose rates. In 1997, the system was used to calibrate one protection-level beta-particle extrapolation ionization chamber. In addition, the system was used to irradiate whole-body skin dosimeters for four customers. (C.G. Soares)
• Implementation of ISO Bremsstrahlung Techniques. As part of an effort to improve measurement quality assurance at Department of Energy (DoE) laboratories, NIST has been funded to implement the bremsstrahlung photon techniques specified by the International Organization for Standardization (ISO). In 1997, effort continued towards the characterization of the ISO beams in terms of measured spectra. (C.G. Soares, P. Lamperti, J.T. Weaver, and C.M. O'Brien)
• Intravascular Brachytherapy Source Dosimetry. The use of beta-particle emitting brachytherapy sources for the prevention of restenosis (re-closing) of coronary blood vessels after angioplasty continues to be actively explored. The procedure of angioplasty is performed over 400,000 times in the U.S. each year, and in about 40 % of the cases, restenosis occurs, requiring another treatment. Research has shown that a dose of about 10 Gy, delivered to the wall of the blood vessel after the angioplasty has been performed, is effective in inhibiting restenosis. NIST has taken an early and leading role in the calibration of the sources used for this therapy, employing the NIST extrapolation chamber equipped with a 1 mm-diameter collecting electrode to measure dose rate at a depth of 2 mm in water-equivalent plastic. These measurements are confirmed using radiochromic dye film, which is also used to characterize sources in the cylindrical geometry for transaxial uniformity. In addition irradiation of planar sheets of film at various depths in water-equivalent plastic were used to construct data sets which can be used to predict the dose rate at arbitrary locations around the sources using a modified form of the AAPM Task Group 43 Protocol. A publication describing this work has been submitted to the journal Medical Physics. Work for Novoste Corporation led to their being the first company allowed by the FDA to have their system used to perform clinical trials, and NIST calibrated over 60 source trains for the Phase II trials begun in July. Collaborations were also continued between NIST and NeoCardia for dosimetry of a ³²P wire, Isotopen-Technik of Germany for the dosimetry of a ⁹⁰Y wire, and with Washington Hospital Center for dosimetry of various sources, including a miniaturized x-ray generating intravascular brachytherapy device. Collaborations were also begun with Nucletron for ¹⁸⁸W/Re wire sources. Funding was received from the National Advanced Manufacturing Testbed (NAMT) Program at NIST for this work, and an international workshop on brachytherapy source manufacturing was organized and hosted by NIST in October. (C.G. Soares)
• Beta-Ray, Hot-Particle and Intravascular Brachytherapy Dosimetry Calculations. Highly refined theoretical methods have been applied to beta-particle radiation protection practice. As part of the work of an International Commission on Radiation Units and Measurements/International Commission on Radiation Protection (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, polymethylmethacrylate (PMMA), and tissue. These data, along with extensive tables of basic electron penetration data, a review of the physics of electron interaction and transport through matter, and a review of beta-ray transport calculations have been incorporated into ICRU Report 56, Dosimetry of External Beta Rays for Radiation Protection. Work with the National Council on Radiation Protection and Measurements (NCRP) scientific subcommittee on hot-particle dosimetry for radiation protection has involved extensive Monte Carlo calculations of the dose distributions from beta and gamma rays emitted by hot particles, which has led to the development of new point-kernel-based calculations for fast routine estimates for a variety of shapes and arbitrary sizes of beta- and gamma-ray sources containing virtually any radionuclide. This work is part of a new NCRP Report, Limits for Exposure to "Hot Particles." Extensive Monte Carlo calculations and these new point-kernel calculations are being used to study expected dose distributions and to develop correction factors for calibration measurements for a number of intravascular brachytherapy sources being introduced for human trials. (S.M. Seltzer)
• Environmental Effects on Dosimeter Response. A new temperature- and humidity-control system for environmental studies on dosimeters (e.g., radiochromic films, polymeric and solid-state detectors, alanine) has been developed and installed for use with our Gammacell irradiators. This will allow comprehensive studies of some of the most troublesome sources of uncertainty on transfer dosimeters during irradiation and storage, namely temperature and relative humidity dependence. The temperatures can be controlled to within ±1.0 ºC over a range of -40 ºC to 200 ºC and relative humidity between 0 and 100 % (±3 % r.h.). Environmental studies are well underway of the response of alanine pellets used for moderate-to-high dose transfer dosimetry. (M.F. Desrosiers, V. Nagy, and J.M. Puhl)
• Electron Paramagnetic Resonance (EPR) Spectrometer Response Reference System. The accuracy and precision of the alanine transfer dosimetry system would be improved by a reference sample which would track, and be used to correct for, variations in the EPR system response. After extensive testing of many different organic and inorganic materials, it was found that the material that best corrected for random spectrometer fluctuations was synthetic ruby crystal. The prototype ruby reference was able to reduce common fluctuations as high as 1 % and greater to a level of approximately 0.03 % for high-intensity EPR dosimeters and 0.1 % to 0.2 % for lower-intensity dosimeter signals. Thus, ruby effectively removes the random spectrometer fluctuations from the sample signal measurement. Under an existing CRADA agreement, Bruker Instruments has designed, built and delivered a ruby reference device that can be mounted on the EPR cavity of the NIST spectrometer. Efforts are under way to test modifications in the data acquisition protocol to include the ruby reference signals in the measurement process. (M.F. Desrosiers and V. Nagy)
• Alanine-EPR Film Dosimeter. Prototypes of a new polymer-based film dosimeter containing alanine have been manufactured and tested. This development program, under a CRADA with the W.R. Grace Co., is testing a range of polymer/alanine ratios for the film dosimeter. These tests were followed by comparative measurements on films of the same polymer/alanine ratio manufactured by different methods. The selected film formulation was mass-produced and is undergoing extensive tests of its radiation response characteristics under a variety of conditions for gamma-rays and electron beams of different energies. Due to the relative ease of handling for films over pellets (presently the most common alanine dosimeter shape), it is hoped that the successful use of the thin (50 µm) film version for electron beams will lead to the development of a more robust film which would be easily adapted to high-dose gamma-ray processing applications. (M.F. Desrosiers)
• Neutron Interferometry and Optics Facility (NIOF). In this year we have begun to get very significant results from several years of developmental efforts at the NIOF.

  A measurement of the scattering length for silicon has been completed, achieving an accuracy level ±0.005 % (68 % confidence level) which is a factor of five better than the previous best measurement. This accuracy was possible because of the superior performance of the NIST interferometer and environmental control systems, and because of an innovative wavelength-independent measurement technique perfected by A. Ioffe and a Czech collaborator, M. Vrana. A report of this work has been accepted for publication.

  High-quality phase-contrast images of two test specimens have been obtained. Images of an aluminum step wedge and an aluminum/copper test piece were made to verify the performance of our new setup for phase-contrast imaging.

  An initial measurement of the ²⁰⁸Pb scattering length has been completed, and additional work is in progress to evaluate the uncertainties in the sample density and homogeneity. This particular isotope is "doubly magic;" i.e., it has closed shells for both protons and neutrons. Thus the strong nuclear interactions of ²⁰⁸Pb are suppressed like the chemical reactions of an inert gas with closed electron shells. The result is that the electric field of the ²⁰⁸Pb nucleus reacts in a less complicated way with the internal electric charge distribution of the neutron, and the scattering length measurement can give definitive information about that charge distribution.

  A major accomplishment of another kind was the successful imaging of water distribution in an operating polymer electrolyte fuel cell (PEFC), by means of high-resolution neutron radiography. Extremely tight collimation (0.9 mrad) was required to image the cell’s membrane edge-on. This PEFC experiment was performed in collaboration with Dr. Richard Bellows of Exxon Research and Engineering. A report of these results has been accepted for publication in Electrochimica Acta. (M. Arif, D. Jacobson, A. Thompson, T. Gentile, and A. Ioffe)

• Asymmetries of the Weak Interaction and the Neutron Lifetime. During CY97, one major experiment was completed and a second one has begun at the NIST cold neutron user facility for fundamental neutron physics.

  The time-reversal asymmetry experiment ran for five reactor cycles during CY97, accumulating more than 15 million electron-proton coincidence events on a polarized neutron beam at the end station of the NG6 cold neutron guide. This large collaboration called emiT involves physicists from the University of Washington, University of California-Berkeley, University of Michigan, University of Notre Dame, Los Alamos National Laboratory, and NIST. Despite serious problems with proton energy loss in the ion-implanted silicon proton detectors, it is anticipated that the results of these runs will permit an improvement of about a factor of two in the current best limit on the time-reversal asymmetry (triple-product correlation) coefficient in neutron beta decay. Detailed data analysis is still going on at Berkeley, Washington, and Michigan. A proposal has been submitted to DoE that would support replacement of the ion-implanted detectors with surface-barrier detectors in future runs planned for FY99.

  The Harvard-NIST experiment to measure the neutron lifetime by creating and trapping ultra cold neutrons in a very cold liquid helium cell (0.1 K) has just begun operating at the NG6 station. All systems seem to be working as expected, and various sources of background counts are being evaluated. Various methods of observing scintillations from neutron decay in the superfluid helium cell are being tested, and several alternative methods for reducing background are being tried.

  A new monochromatic beam line has been set up near the end of the NG6 guide, with the new beam going into the laser-polarization hut. In this hut at NG6 much less space is now required for the laser setup, because of conversion to compact laser-diode arrays; and the remaining room in the hut will accommodate other experiments on the monochromatic beam line. The laser-polarization work will coexist with the cryogenic neutron calorimeter or the alpha-gamma black detector or the boron total cross section experiment in the hut during the coming year.

  Two NIST physicists joined Russians from the Petersburg Nuclear Physics Institute (PNPI) at the Institut Laue-Langevin in Grenoble to work on a measurement of the antineutrino asymmetry coefficient for neutron beta decay. This provided a good opportunity for the NIST staff to learn the subtleties of operation of the 2A2F (two analyzer, two flipper) neutron polarimeter, which NIST purchased from the PNPI. A preliminary report of this experiment has been accepted for publication in a Russian journal (in Russian). A more complete report (in English) is nearly ready for submission to Phys. Rev. C. (J. Nico, F. Wietfeldt, A. Thompson, M.S. Dewey, G. Jones, J. Adams, and D. Gilliam)
• Laser Polarization of ³He for Neutron Spin Filters and MRI Applications. The five-year competence program in Laser Polarization of ³He for Neutron Spin Filters has grown into a fully integrated part of our research and services in neutron interactions and dosimetry. This work is receiving both continued internal support and DoE funding through a collaboration with Indiana University.

  A closed-loop ³He recirculation system has been built and tested for preparing polarized ³He at atmospheric pressure by the metastable optical-pumping method. The system consists of an optical-pumping cell, cold traps, a modified (now non-magnetic) commercial diaphragm pump, storage cell, getter, and capillary return line. In non-recirculation mode, polarization in the storage cell of up to 20 % was obtained at a pressure of about one atmosphere. Pressures up to 2.5 atmospheres have been obtained in the storage cell by collecting the polarized gas at liquid-nitrogen temperature and then letting it warm up. This current level of polarization is already high enough for medical imaging applications, and the low cost of the present system may be important in these applications. Much higher levels are required for most neutron spin-polarization experiments, and several modifications of the present system are in progress to see what may be achievable. In addition, our collaborators at Indiana University (IU) are constructing a much more sophisticated (and expensive) compressor system, similar to a very high-performance system developed at the University of Mainz.

  Several flat-windowed, ¹⁰B-free cells were produced for use on NIST cold neutron beams. For two of these cells, the initial polarization lifetime was more than 130 hours, although these slowly degraded to less than 50 hours. As part of our DoE-supported collaboration with IU, we made high-pressure cells for epithermal neutron experiments at Los Alamos National Laboratory (LANL), as well as both polarizer and analyzer cells for a LANL experiment on neutron spin rotation in ¹²⁹La.

  We have assisted researchers at the University of Pennsylvania in making high-quality lung images and a sinus cavity image by magnetic resonance imaging (MRI) with polarized ³He inhalation. Additional collaborations in MRI applications are being explored with researchers at Johns Hopkins University, the University of Nottingham, and the company Intermagnetics General. (T. Gentile, A. Thompson, and G. Jones)
• Neutron Dosimetry for Reactor Safety Assessment. Through a cooperative agreement with the Office of Nuclear Regulatory Research, NIST provides measurement-assurance services and consultation related to neutron dosimetry and nuclear reactor safety.

  Round-robin tests of ²³⁷Np and ²³⁸U fissionable dosimeters involving four U.S. participants and one international participant are currently under way. These fissionable dosimeters were irradiated to NIST-certified fluences in the Materials Dosimetry Reference Facility at the Phoenix Laboratory of the University of Michigan. The fission product of interest in these tests is ¹³⁷Cs, which has a very suitable half-life but a rather low-energy gamma-ray requiring careful handling of interferences from higher-energy fission-product gamma rays.

  Progress continues on assistance to the Nuclear Regulatory Commission in preparation of Draft Guide 1053, Calculational and Dosimetry Methods for Determining Pressure Vessel Neutron Fluence. A resolution-of-comments document is currently being prepared in response to formal comments from power reactor operators and suppliers regarding the first public release of the draft guide.

  NIST has organized a ⁹³Nb activation round-robin and is expecting four participants. This dosimeter is of particular interest because the energy dependence of its cross section closely matches the energy dependence of the iron damage function. Several major technical difficulties are encountered in x-ray spectrometry for ⁹³Nb, and inaccurate results are often obtained unless there is careful standardization of the sample purity and spectrometry technique. (J. Adams and D. Gilliam)
• Calibrations, Irradiation Services, and Special Tests. The numbers of calibrations of radiation protection survey instruments increased somewhat this year despite our encouragement of many potential customers to use secondary calibration laboratories. Continuing efforts to achieve better consistency between calibrations made at NIST and calibrations at the Pacific Northwest Laboratories have resulted in good agreement for measurements made with unmoderated californium fission neutrons, but some difficulty remains in resolving discrepancies in measurements with the moderated californium fission neutron source.
  
  Special tests of industrial neutron detectors were carried out at the reactor thermal column for two industrial customers, Westinghouse Science and Technology Center and the Imaging and Sensing Technology Corporation; and two irradiations were made at the Californium Neutron Irradiation Facility to test a fast neutron imaging system being developed by Industrial Quality, Inc. and the University of Virginia. (J. Adams, A. Thompson, and J. Nico)
• Neutron Cross Section Standards. The NIST role in the advancement of neutron cross section standards is now performed primarily through evaluation, compilation, and dissemination of standards in collaboration with the U.S. Cross Section Evaluation Working Group and two international committees, the International Nuclear Data Committee (INDC) and the Nuclear Energy Agency Nuclear Science Committee (NEANSC). The NEANSC has planned a new measurement of the ¹⁰B(n,α) branching ratio, to be carried out at the Institute for Reference Materials and Measurements in Geel, Belgium. The INDC has published a new report "Update to Nuclear Data Standards for Nuclear Measurements."
  
  In addition, a limited NIST experimental role continues, with plans for a new measurement of the ¹⁰B total cross section at the new NG6M monochromatic neutron beam and with NIST participation in a collaborative re-measurement of the H(n,n) angular distribution measurement at Ohio University (OU) at 10 MeV. This work at OU is particularly timely, because European standards groups have recently expressed considerable skepticism about the latest U.S. evaluation of the H(n,n) angular distribution and because the H(n,n) cross section is one of the most widely used cross section standards. (A. Carlson)
• Dissemination of National Standards of Radioactivity. The Radioactivity Group disseminated the National Standards of Radioactivity mainly through the following three activities: (1) over 500 Radioactivity Standard Reference Materials (SRMs) were provided; (2) over 200 comparative measurements and Reports of Traceability were provided 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 four research associates in cooperative testing programs; and (3) over 60 calibrations of customer sources were provided. (L.L. Lucas, J.T. Cessna, and J.M.R. Hutchinson)
• 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. A continuous-wave Titanium-sapphire (Ti-Saph) laser was incorporated into the system to perform initial highly selective Z discrimination before isotopic selection in the mass spectrometer. Preliminary tests of this system have been performed. (J.M.R. Hutchinson)
• Environmental Radioactivity SRM for Radiochemical Metrology. Over the past decades, on the order of 10¹⁵ Bq of waste have been stored in the oceans by the nuclear countries. Potential contamination by leaking nuclear waste, including the 17 sunken Russian nuclear submarines in the Arctic Ocean, has caused worldwide concern for the exosphere and commercial food chain. The determination of mBq · g^-1 activity concentrations in marine sediments is a difficult analytical task. The new NIST Ocean Sediment Standard Reference Material (SRM 4357) provides a benchmark for quality assurance, methods verification, and data comparability for this type of natural-matrix material. The SRM was developed using a composite of 1 % contaminated Irish Sea sediment and 99 % Chesapeake Bay sediment by weight. Ten radionuclides including ⁴⁰K, ⁹⁰Sr, ¹³⁷Cs, ²²⁶Ra, ²²⁸Ra, ²²⁸Th, ²³⁰Th, ²³²Th, ²³⁸Pu, and ^239-240Pu were certified through a comparison study among twenty international laboratories. The mean values were reported for an additional 11 uncertified radionuclides: ¹²⁹I, ¹⁵⁵Eu, ²¹⁰Po, ²¹⁰Pb, ²¹²Pb, ²¹⁴Bi, ²³⁴U, ²³⁵U, ²³⁸U, ²³⁷Np, and ²⁴¹Am. Interestingly, the measurement results for the primordial radionuclides were distributed normally, while those for the anthropogenic radionuclides were best described by Weibull distributions.
• NIST Radiochemistry Intercomparison Program (NRIP). The NIST Radiochemistry Intercomparison Program NRIP marks the successful completion of the first year of its measurement traceability program for low-level environmental radioactivity measurements. Three rounds of testing (water, air filters, synthetic urine) were completed. The laboratories underwriting the program in the first year of testing were Auburn University (Auburn, AL), Carlsbad Environmental Monitoring and Research Center (Carlsbad, NM), Environmental Evaluation Group (Carlsbad, NM), National Air and Radioactivity Environmental Laboratory (Montgomery, AL), Sandia National Laboratory (Albuquerque, NM), Westinghouse Savannah River Company (Aiken, SC), and Westinghouse Waste Isolation Division (Carlsbad, NM). All participants agreed that the program is vital for relating low-level radioanalytical measurements to the National Standards. Test materials include both environmental (water, soils, sediments, vegetation, etc.) and bioassay (urine, fecal material) sample matrices. Laboratories were issued Certificates of Traceability for the matrices and nuclides of the tests.

  The program has been implemented to meet the demand for traceability as defined under ANSI N42.23. The voluntary standard defines a hierarchy of traceability with an unbroken linkage from service laboratories to NIST. Under the standard, the testing materials are (1) of appropriate matrices (i.e., matrix categories commonly analyzed by the laboratory) and (2) of appropriate (ambient) activity concentration ranges for these matrices. Currently NRIP is the only traceability program providing performance evaluation materials at environmental levels. Consequently, it has been necessary for radioactivity sample measurements to participate in NRIP.

  In the coming year aqueous solutions, glass-fiber, artificial urine and fecal material, and soil will be the test matrices. The analyzed list has been expanded to include ⁹⁰Sr, ²³⁴U, ²³⁸U, ²³⁸Pu, ²⁴¹Am, at 0.03 Bq to 0.3 Bq per sample. Continued growth in the number of participating laboratories is anticipated from the commercial national and international communities. (K.G.W. Inn and M. Schultz)
• Radionuclide Speciation in Soils and Sediments. Potentially, billions of dollars of remediation costs could be saved and social concerns could be addressed from reliable and interpretable information on the chemical forms (speciation) of the nuclides and their distribution in the environment. The Ionizing Radiation Division will be developing a new series of environmental SRMs for radionuclide speciation. As a first step towards this goal, the Division, in collaboration with Florida State University, is developing a phase-selective standard extraction protocol. The future environmental radionuclide SRMs will be characterized for radio-plutonium, uranium, thorium, strontium, and cesium species and will be used by those making decisions for risk assessment, selection of cost-effective mitigation strategies and technologies, and developing and executing long-term monitoring. The effects of critical variables (time, temperature and reagent concentration) of the leaching protocol are being evaluated for selective dissolution of the exchangeable, carbonate, reducible metal oxide, and organic antacid leachable phases. Once the protocol is documented and pilot tested, a network of collaborations will be developed for the future round-robin testing of the protocol and interlaboratory comparisons for the certification of the future SRMs. (K.G.W. Inn)
• ²³⁹Pu Intercomparison of ICP-MS, TIMS and FTA at mBq Levels. The Department of Energy, Office of International Health Programs (EH-63), is in the process of assisting Marshall Islanders to resettle their islands after five decades. The DoE and the resettled residents require assurances that the radiation dose to residents will not exceed recognized international standards or recommendations. One of the remaining radionuclides that could contribute to internal radiation dose from inhalation and ingestion intake pathways is ²³⁹U. The uptake of ²³⁹Pu is estimated from the excretion of ²³⁹Pu in the urine of an individual. The analytical technique must have sufficient sensitivity to quantify ²³⁹Pu at or below a level of 20 mBq/kg.

  The goal of this phase of the project was to evaluate the state-of-the-art (accuracy, precision and minimum detection amount) for ²³⁹Pu in artificial urine measurements by inductively coupled plasma, thermal ionization mass spectrometry and fission track analysis in the range of 18 mB/sample to 278 mB/sample (approximately 1.5 million atoms of plutonium per 200 g sample). A portion of the tasks was performed by the Yankee Atomic Environmental Laboratory (YAEL), in terms of establishing the stability of ^99mTc in the artificial urine, executing the dilutions, conformational measurements and distributing the samples to participating laboratories.

  Preliminary analysis of the data indicates that ICP-MS currently has the capabilities of making quantitative measurements of ²³⁹Pu from urine down to the 18 mBq/sample level within 20 % bias. Preliminary analysis also indicate, however, some potential technical difficulties by some of the participating laboratories include: a) data transcription; b) sample tracking; c) estimation of limits of detection; d) statistical control over analytical processes; and e) control of analytical background. A second study is being planned to include interferences to be able to assess the technologies under more realistic conditions. (K.G.W. Inn)
• Calibration by Gamma-ray Counting. Two different mixtures of radioactive solutions were calibrated, in part, by gamma-ray counting. An efficiency function was determined by measurement of well characterized and calibrated solution standards in nominal-5 ml glass ampoules. In turn, 5 ml ampoules of ⁷Be, ¹²⁴Sb, and ¹²⁵Sb were measured and the gamma-ray emission rate of representative transitions were determined. The corresponding activities were obtained with these measurement and the best available decay data. For this work, geometric corrections had to be made for the solution column height of each ampoule measured, both the standards and the samples. The two solution mixtures consisted of: (A) ¹²⁴Sb, ¹²⁵Sb, and ¹³⁴Cs; and (B) ⁷Be, ⁵⁸Co, ⁶⁰Co, and ⁶⁵Zn. (F.J. Schima and D.B. Golas)
• Photo-Activation by MIRF. Activities of ¹¹C, ¹⁸F, ^34mCl and ¹²⁶I were produced from a single MIRF irradiation and subsequently gamma-ray counted in a sequence determined by the half-life. The target holder for the irradiation consisted in a rotating holder that held the individual target samples in front of the photon converter. Activities of ¹¹C and ¹⁸F were determined from the annihilation radiation peak (511 keV) counting rate that had the appropriate half-life. The 511 keV peak counting rate typically exhibited a compound decay. This compound decay was fitted with the known half-lives of the components, which yielded the corresponding activity at the end of the irradiation. A second MIRF irradiation of lead nitrate yielded activities of ^204mPb and ²⁰³Pb. The lead target had the natural isotopic distribution and the yield of the two activities was found to be 1 to 2.2, respectively, at the end of the irradiation. (F.J. Schima and B.M. Coursey)
• Standardization of ⁶²CuPTSM. The availability of generator-produced radiopharmaceuticals for use in Positron Emission Tomography (PET) will greatly increase the number of PET procedures that can be performed by eliminating the requirement of a cyclotron at the PET center. One such generator currently undergoing FDA review is ⁶²Zn/⁶²Cu, which elates ⁶²Cu as a pyruvaldehyde bis(N⁴-methyl) thiosemicarbazone (PTSM) complex for use in cardiac perfusion imaging. The Radioactivity Group has been requested to provide a standard of this radiopharmaceutical for Proportional Technologies, Inc. (Houston, TX), the manufacturer of the generator. Because the half-life is very short (9.7 min), the standard will consist of a dose calibrator setting for a 35 mL polyethylene syringe containing nominally 33 ml of ⁶²CuPTSM solution. This setting is determined by preparing liquid scintillation sources and syringe sources from the same eluent each time the generator is milked. (B.E. Zimmerman and J.T. Cessna)
• The Half-life of ⁶²Cu. The half-life of the positron-emitter ⁶²Cu was measured using 4πβ liquid scintillation (LS) spectrometry and counting in the NIST 4πgamma; re-entrant ionization chamber (IC). A total of twelve independent LS measurements and two independent IC measurements were made, all of which spanned 2 to 4 half-lives. Weighted nonlinear fits of the data resulted in half-lives of 9.68 min ± 0.04 min and 9.673 min ± 0.026 min (uncertainties are combined standard uncertainties) for the LS and IC data, respectively (a difference of only 0.07 %). The weighted mean of these two values is 9.67 min ± 0.03 min, which is shorter than the ENSDF-recommended value of 9.74 min ± 0.02 min by 0.7 %. (B.E. Zimmerman, M.P. Unterweger, and J.T. Cessna)
• Technetium-99. ⁹⁹Tc is a very long-lived (the half-life is 211 thousand years) fission product that is present in nuclear waste. ⁹⁹Tc decays by pure beta-particle emission and hence its calibration is more difficult than for most radionuclides for which standards are issued. The massic activity was measured in the NIST 4πβ(LS)-γ-anticoincidence counting system using ⁶⁰Co as the efficiency-tracing radionuclide. Measurements were also made in commercial 4πβ liquid-scintillation counters using theoretically calculated detection efficiencies. The calculated detection efficiencies depend very strongly upon the beta-spectrum shape assumed for ⁹⁹Tc. The experimentally measured beta-spectrum shape gives the most accurate calculated efficiencies. The latest reissue of this standard is now available as SRM 4288A. (L.L. Lucas)

• Holmium-166m. ^166mHo is a long-lived (the half-life is 1200 years) radionuclide that emits a large number of gamma rays with energies from 80 keV to more than 1400 keV. The large number of gamma rays, the wide energy range, and the long half-life make ^166mHo a very desirable gamma-ray source for determining the detection efficiency of germanium detectors and for monitoring their long-term stability. High-purity stable ¹⁶⁵Ho was neutron-irradiated to produce ^166mHo, which is now being calibrated in terms of activity. The gamma-ray emission probabilities and their uncertainties are also being evaluated. SRM 4274, ^166mHo solution, will be issued in FY 1998. (L.L. Lucas, F.J. Schima, and B.E. Zimmerman)
• Chemical Digestion and Radionuclidic Assay of TiNi-Encapsulated ³²P Intravascular Brachytherapy Sources. Work is nearing completion on a project that was intended to relate the ³²P activity content of one type of interstitial brachytherapy source to measurements and Monte Carlo calculations of the source's absorbed dose spatial distribution. The sources (of proprietary design) are thin cladded cylinders of TiNi (having a diameter of less than 0.5 mm and lengths up to a few cm), called "seeds," that are used to treat the wall of blood vessels during angioplasty to prevent restenosis (reclosing due to scar tissue formation in the healing process). For this work, a radiochemical procedure was developed to chemically digest the seed's encapsulating jacket for the release of ³²P into a solution which could subsequently be assayed for ³²P (and ³³P impurity) by 4πβ liquid scintillation (LS) spectrometry. The procedure had to be devised to ensure both a quantitative release of the ³²P from the seed and the quantitative retention of the ³²P in the solution. The radiochemical assays will also allow us to develop calibration factors for non-destructive measurements of the seed’s external radiations (e.g., with combination NaI(Tl) and re-entrant ionization chamber "dose calibrators"). (R. Collé)
• Liquid Scintillation Cocktail Composition and Mismatch Effects. Liquid scintillation (LS) spectrometry is one of the principal measurement tools used by the Radioactivity Group. The effects of LS cocktail composition on radionuclidic standardizations is very poorly known and understood. Our very broad, systematic evaluations of such LS cocktail composition effects in the assay and standardization of low-energy β-emitters and low Z (atomic number) nuclides that decay by electron capture (EC) have been continuing. These studies have re-confirmed the previously-observed, critical importance of the aqueous fraction of cocktails prepared with commercial scintillants, and extended the findings to another class of scintillants. (R. Collé)
• Calibrations of Large-Area Beta Sources. Studies are continuing on the effects of beta-backscattering to develop a systematic method for determining the effective source thickness needed for relating the measured rate to activity. This will be incorporated into an ANSI standard for the use of large-area beta sources. (M.P. Unterweger, J.M.R. Hutchinson, and P. Hodge)
• International Intercomparison of ¹³³Xe and Tritiated Water. These intercomparisons were sponsored by EUROMET and the results were reported at the Conference on Radionuclide Metrology and Its Applications - ICRM '97. The ¹³³Xe intercomparison by internal gas counting was undertaken to try and resolve differences between results of ionization chamber measurements carried out under the auspicious of BIPM. The results indicate a need to recalibrate our iodization chamber for ¹³³Xe. The tritiated water intercomparison using both internal gas counting and liquid scintillation counting indicates the need for further examination of possible systematic errors arising between the two methods before the tritiated water samples prepared at NPL can be used as an international standard. (M.P. Unterweger)

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