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Technical Highlights

• Modifications to the Mammography and 100 kVp to 300 kVp X-Ray Calibration Ranges. A replacement Mo-anode x-ray tube was purchased and has been characterized for the NIST mammography proficiency testing and for routine calibrations. All NIST reference class chambers have been re-calibrated to the new, slightly different mammography beam qualities. The previous, repaired Mo-anode tube will be also be characterized and integrated into the calibration system. The data acquisition software upgrade is in the final testing stage and is being used to calibrate NIST reference chambers for proficiency testing. The tungsten x-ray calibration facility which houses the primary standard for x-rays between 100 kV and 300 kV has been completely redesigned. The x-ray facility is used to perform calibrations of ionization chambers to both NIST and ISO reference bremsstrahlung photon techniques. The recent upgrades allowed NIST to host an international primary-standard comparison. The modifications now permit quality-assurance comparisons between the NIST primary x-ray standards to be more easily conducted with improved reproducibility. The new design permits identical calibration techniques for all types of chambers, resulting in a lower uncertainty and maintaining high quality assurance. Moreover, the modifications will allow NIST to offer low-rate reference-beam-quality conditions that can be used to better support low-level (environmental) dosimetry. The x-ray scatter conditions are much improved, and more ideally meet the requirements for the production of ISO beam qualities. The modifications included the rotation of the x-ray tube 90 degrees to use the longer and wider portion of the available space in the 300 kV calibration range. The range is now 2.5 times longer (over 5.7 m) than the previous range, and is now equipped with precision chamber-alignment mounts and support stand for the primary standard. (C.M. O’Brien, J.H. Sparrow, P.J. Lamperti, and M.R. McClelland)

• X-Ray Comparison Between NIST, NPL and NRCC for Low- and Medium-Energy Tungsten and Mammography Qualities. NIST hosted an international comparison of x-ray standards, which included a direct comparison between three US and two UK primary x-ray standards and an indirect comparison with two Canadian reference-class chambers. The standards were compared at energies ranging from 10 kVp to 300 kVp produced by tungsten and molybdenum x-ray anodes. The beam qualities used for the comparison included six narrow-spectra ISO beam qualities, two mammography qualities and 16 NIST beam qualities, including recently developed diagnostic beam qualities. The comparison between primary standards was conducted at NIST because the large size of the NPL 300 keV standard prohibits its use at the BIPM. The mammography standards were compared at the NIST because the BIPM does not maintain traceability to mammography beam qualities. The preliminary agreement for all energies shows agreement to better that 0.5 %. (C.M. O’Brien, P.J Lamperti, and J.H. Sparrow)

• Gamma-Ray Ranges for Radiation-Protection Calibrations. Five gamma-ray sources are used for calibration of instruments and passive dosimeters, in terms of air-kerma and exposure, to support protection-level measurements in the U.S. The calibrations are directly traceable to measurements with the national primary standard for gamma-ray exposure, graphite-cavity ionization chambers. The ranges provide a wide range of air-kerma rates: two ¹³⁷Cs sources provide air-kerma rates from 4.5 mGy/h to 110 mGy/h; a third ¹³⁷Cs source provides air-kerma rates of 2.3 Gy/h and 3.6 Gy/h; and two ⁶⁰Co sources provide air-kerma rates from 0.25 mGy/h to 5.4 mGy/h. In addition, two ⁶⁰Co teletherapy-level sources are available. Programs of regular, calibrated exposures of thermoluminescent dosimeters provide direct support for the worker-protection measurement programs of a number of agencies, including the U.S. Navy. NIST standards are also disseminated through a number of secondary instrument calibration laboratories to provide traceability of protection-level measurements. Plans are underway to develop a calibration capability at much lower air-kerma rates, approaching those involved in environmental-level measurements. (R. Minniti, P.J. Lamperti, and J. Shobe)

• Air-Kerma and Absorbed-Dose Calibrations of ¹⁹²Ir Brachytherapy Seeds. NIST calibration of ¹⁹²Ir seed sources was first introduced in 1980. Because of a half-life of only 74 days, it is impractical for NIST to maintain calibrated sources for subsequent calibration of instruments. Rather, measurement results are transferred to a reentrant ionization chamber that then serves as a secondary standard. In recent years, ¹⁹²Ir sources have become increasingly popular for use in the treatment of certain cancers, and new seed designs are being introduced. Manufacturers send such seeds to NIST for calibration, and those seeds are used to calibrate transfer chambers used by the manufacturer to characterize seeds for shipment and used by secondary calibration laboratories to further calibrate instruments used in therapy clinics. New ¹⁹²Ir seed sources are currently being calibrated in terms of both air-kerma and absorbed-dose-to-water. A comparison between the absolute calibration method of 1980 and newer methods have been performed. The calibration has been transferred to a NIST well-ionization chamber. Work is continuing to determine the reference absorbed dose rate from the new seeds. (J. Shobe, C.G. Soares, and M.G. Mitch)

• X-Ray Spectrometry of Prostate Brachytherapy Sources. To understand the relationship between well-ionization chamber response and WAFAC-based air-kerma strength for prostate brachytherapy seeds, x-ray emission spectra are measured with an HPGe detector. Pulse-height distributions from the spectrometer are unfolded to obtain the true photon spectra emitted from the seeds in the transaxial direction. ¹⁰³Pd seeds from all five manufacturers emit very similar photon spectra, while there are five distinct spectra emitted by ¹²⁵I seeds from eleven manufacturers. These differences in ¹²⁵I seed emission spectra are a result of fluorescence x-rays emitted by the radionuclide support material, either silver or palladium. The effect of these fluorescence x-rays is to lower the average energy of the emitted spectrum, resulting in a lower well-ionization chamber current relative to air-kerma strength because of the greater energy sensitivity of the well-chambers compared to that of the WAFAC. Knowledge of seed emission spectra allows separation of well-ionization chamber response effects due to spectral differences from those due to seed internal structure and self-absorption, and also allow the refinement of correction factors that must be applied to the WAFAC measurements. (M.G. Mitch, S.M. Seltzer, and P.J. Lamperti)

• Alanine-EPR Film Dosimeter. A new polymer-based film dosimeter containing alanine has been tested. The films tested were 10 µm, 100 µm, and 300 µm in thickness, and were irradiated over a range of 50 Gy to 200 kGy. The average relative standard deviation of the response over this range was ~1 %. The EPR signal for a 100 µm film at 50 µGy was measurable but required enhancement to obtain a reasonable signal-to-noise level. Comparative measurements of alanine films irradiated to the same dose but of different thickness revealed that the signal was roughly proportional to the thickness. A more comprehensive study of the alanine system is planned. Extensive tests of the alanine films are underway for application to low-energy electron beams frequently used in radiation processing. (M.F. Desrosiers and J.M. Puhl)

• 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. 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 that 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 published in the journal Medical Physics. The equipment used for these studies was augmented with the addition of an automated micro-scintillator detection system and various well-ionization chambers. Use of all this equipment has been centralized in a newly refurbished laboratory. Collaborations were continued with Guidant for dosimetry of a ³²P wire, with Novoste for dosimetry of ⁹⁰Sr/Y seed trains, with Washington Hospital Center for dosimetry of various sources, with Cordis for the dosimetry of ¹²⁵I wires, and with Best Industries for ¹⁸⁸W/Re wire and ¹⁹²Ir seed sources. A refinement was made to the NIST calibration procedure using the extrapolation chamber that resulted in a +15 % change to the Novoste seed-train calibrations. This value now more closely agrees with the reference dose rates predicted by theoretical models and measured source activities. (C.G. Soares and M.G. Mitch)

• Beta-Particle Emitting Ophthalmic Applicator Calibration Service. With the advent of the calibration service for ophthalmic applicators at the University of Wisconsin Accredited Dosimetry Calibration Laboratory, the routine role of NIST in these calibrations has diminished considerably. NIST's role in this field in the future will be directed towards providing transfer standard sources and fields both to secondary laboratories and source manufacturers. The results of the major international comparison of ophthalmic applicator dosimetry, which included the dosimetry of both curved and flat sources of ⁹⁰Sr+⁹⁰Y and ¹⁰⁶Ru+¹⁰⁶Rh, were submitted to Medical Physics for publication in 2001 and are included in the ICRU report on beta particles for medical applications. LabVIEW code has been written and is now being used for the automation of the measurements within the framework of the NAMT automated source calibration facility. A new high-sensitivity electrometer has been purchased to allow measurement of lower activity sources with this system. (C.G. Soares and M.G. Mitch)

• Compton Scattering of Photons. The availability of high-speed computing devices on the desktop has enhanced the applicability of Monte Carlo transport methods. Many of these new applications such as radiation treatment planning, detector modeling and radiography require modeling at lower and lower energies for accuracy. At these low energies, there is an increased need for differential cross section data for Compton scattering of photons from bound electrons. The Photon and Charged Particle Data Center is developing an inelastic-photon-scattering database that will provide such differential data, as well as a consistent set of integrated cross section data. The approach we are taking is based on the impulse approximation, the most widely used method of accounting for the main contributions to the Compton scattering cross section. However most simulations, including Monte Carlo transport codes, rely on the accuracy of the underlying physical databases of basic interaction cross sections. We have utilized the impulse approximation to develop a number of sets of differential scattering data (Compton profiles) that enable the calculation of the cross section, doubly differential in scattered photon energy and angle, for all elements up to Xenon. We are in the process of utilizing these databases to understand the sensitivity of the Compton profile to the various atomic models employed in their derivation. Further, we are quantifying the applicability of the Compton profile approach to the impulse approximation. Once these analyses are complete, a tabulated set of Compton scattering data will be available with an enhanced understanding of the theoretical uncertainties. (P.M. Bergstrom, D.V. Rao, J.H. Hubbell, and S.M. Seltzer)

• Accelerator Radiation Sources. Our electron accelerator facilities continue to support active research in radiation interaction studies. These electron sources include the 32 MV linear accelerator of the Medical and Industrial Radiation Facility (MIRF), the 4 MV Van de Graaff accelerator, and the 500 kV cascaded rectifier electrostatic accelerator. These machines allow us to provide research beams ranging in electron energy from 10 keV up to 32 MeV. Our present efforts include characterizing and refining beam delivery on the 500 kV accelerator to support studies in electron beam dosimetry for radiation-processing applications, in materials research and in detector characterization. Future plans include a program to measure the low-energy response of radiochromic and alanine routine dosimeters using the 500 kV accelerator. Emphasis will be placed on developing standards for low-energy electron-beam dosimetry based on calorimetric methods. This will help us to provide guidelines and standards for the radiation processing industry. (F.B. Bateman and M.R. McClelland)

• Photon and Charged-Particle Data Center. The Data Center compiles, evaluates, and disseminates data on the interaction of ionizing radiation with matter. The data on photons and charged particles, with energies above about 1 keV, include fundamental information on interaction cross sections as well as transport data pertaining to the penetration of radiation through bulk material. Databases are developed and maintained on attenuation coefficients for x rays and gamma rays, including cross sections for Compton and Rayleigh scattering, atomic photo-effect, and electron-positron pair production, as well as on energy-transfer, energy-absorption and related coefficients relevant to radiation dosimetry. Work on charged-particle cross sections and of radiation transport data has entailed significant effort on the evaluation of the stopping powers and ranges of electrons, positrons, protons, and alpha particles, the elastic scattering of electrons and positrons, and the cross section for the production of bremsstrahlung by electrons. The compilations of the Data Center rely heavily on the synthesis of available theory to extend the data and provide for comprehensive coverage over broad ranges of energy and materials. Thus we have long been involved in complex computational analyses and in the development of highly sophisticated transport-theoretic methods. Our Monte Carlo transport calculations also are incorporated into some of the most widely used general-purpose radiation transport codes. With the help of the NIST Physics Laboratory's Office of Electronic Commerce of Scientific and Engineering Data, a number of the Center's databases can be accessed on the worldwide web. (S.M. Seltzer, P.M. Bergstrom, J.H. Hubbell, M.J. Berger, and D.V. Rao)

• Retrospective Dosimetry for the Techa River Study. The DoE is collaborating with NIST to conduct a focused EPR dosimetry study coupled with photo-stimulable phosphor imaging-plate characterization and ⁹⁰Sr analysis, on selected teeth from the Techa River population and others in the Southern Urals (for background data). The results of this study will significantly contribute to efforts to validate the dose estimates of Project 1.1 of the US-Russia Joint Coordinating Committee on Radiation Effects Research (JCCRER). The main areas of improvement will be focused on the development of non-destructive methods, lowering the cost of the dose assessment, and enhancing the quantitative aspects of the procedures. The main focus will be the application of the non-destructive image-plate method; employment of this method will lower cost and increase the throughput of the dose assessment while maintaining the quantitative aspects of the procedures. Dose validation through EPR measurement is considered to be critical to the success of the project. However, in the case of internal emitters, the applicability of the measured dose has been somewhat ambiguous. Our work will provide the bridge to make the EPR dose much more biologically relevant. Quantitative image-plate data coupled with EPR doses will enable the differentiation of internal and external dose contributions. This approach is the first step in the convergence of retrospective methods with real-time measurement methods (e.g., whole-body counting). This path will provide a credible basis for the derivation of radiogenic risk factors. (A.A. Romanyukha, V. Nagy, and M.F. Desrosiers)

Strontium-90 Imaging of Dental Tissues. Tooth-enamel EPR dosimetry provides a tool to reconstruct the total life-accumulated radiation dose to individuals. A unique test case is provided by residents in the Urals region of Russia near the Mayak nuclear weapons plant, who received radiation exposures due to radioactive pollution of the Techa River. For those local residents who had permanent teeth during the exposure period, the total dose consists of two main components; a dose due to internal exposure from 90Sr (accumulated mainly in the dentin) and a dose due to external gamma-ray exposure. Thus, in order to determine the external dose component, it is necessary to subtract the 90Sr dose contribution from the total dose measured by EPR. The 90Sr component of the tooth enamel dose can be determined from 90Sr mapping of dental tissues with an image plate (Fig. 1). To map 90Sr in dental tissues the tooth will be vertically cross-sectioned into two approximately equal halves, and then the flat side of each tooth half is placed on the imaging plate for 3 h to 40 h, depending on the level of 90Sr deposition. In order to quantify the 90Sr activity producing the dose distribution in the resulting image, a calibrated 90Sr source will be constructed and prepared. The non-destructive assessment of activity can be used to estimate the absorbed dose from the internally deposited 90Sr, and the tooth (both halves) can then be used to prepare the tooth-enamel sample for EPR dose-reconstruction measurements. (A.A. Romanyukha, V. Nagy, M.F. Desrosiers and M.G. Mitch)

Figure 1. Images of radiation exposure to photostimulable-phosphor array (Fuji plate) for a tooth from a resident of the Techa River region. For teeth from a number of individuals, a strong correlation is found between Fuji plate intensities and the body burdens of strontium-90 as measured by whole body counting.

• Tests of the Weak Interaction and the Neutron Lifetime. The cold neutron guide hall at the NIST Center for Neutron Research (NCNR) provides a unique opportunity for the U.S. to compete for a leading role in research on the physics of fundamental particles. High precision measurements at very low neutron energies complement high-energy research at national and international particle accelerator laboratories. Measurement techniques developed for this research improve NIST's ability to provide measurement services and calibrations. The Neutron Interactions and Dosimetry Group pursues a research program of its own, as well as supporting a national user facility for industrial and university researchers.

  Two different neutron lifetime experiments, one a Harvard-led ultra cold neutron (UCN) experiment, the other a NIST-led cold neutron beam experiment, are currently able to take data on our polychromatic neutron beam at the NCNR.

  Complete three-dimensional magnetic trapping of ultracold neutrons was demonstrated by the Harvard/NIST collaboration for the first time, as reported in the journal Nature in January 2000. Trapping of neutral and charged particles is an invaluable tool for the study of both composite and elementary particles. The main advantages of trapping are long interaction times and isolation from perturbing environments. In the present work, inelastic scattering in superfluid ⁴He is used to load neutrons into the trap, and the helium also acts as a scintillator for detection of neutron decay. The work described in the Nature article verified the theoretical predictions of the loading process and the technique of magnetic trapping of neutrons. During CY00 major upgrades to the apparatus were installed including a larger trapping magnet and a monochromator to prefilter the incident beam. These improvements should result in a precise measurement of the beta-decay lifetime of the neutron sometime in CY01CY01.

  The NIST-led lifetime experiment utilizes a Penning trap for decay protons and a thin ⁶Li neutron fluence monitor to measure the lifetime of free neutrons in a 30 K thermal neutron beam. During CY00 final data taking runs of the experiment were completed. A major source of uncertainty in this experiment comes from measurement of the neutron fluence. In the past the neutron monitor calibration factor was calculated from a cold neutron capture cross section, the areal density of the ⁶Li deposit through which the neutrons pass, and the measured solid angle of the detector. Now in collaboration with Indiana University, an independent calibration of the neutron fluence monitor is being achieved by use of a totally absorbing cryogenic calorimeter. Several absorbing targets are being intercompared: LiPb, LiMg, and liquid ³He. The LiMg target results have verified the thin ⁶Li monitor results within about 0.4 %, as reported in a recent Ph.D. thesis. When completed in early CY01, this work is expected to reduce the uncertainty in the neutron fluence by at least a factor of two.

  Preliminary work has begun on a NIST-led experiment to measure the electron-antineutrino correlation in neutron beta decay using a novel asymmetry technique that does not rely on precise proton spectroscopy. This correlation coefficient was last measured in 1978 and has a 5 % uncertainty (by comparison the other better known correlation coefficients are known to slightly better than 1 %). This experiment requires an electron spectrometer with a high degree of backscatter suppression to minimize the low-energy tail in the response function. During CY00 a prototype electron spectrometer consisting of a conical array of plastic scintillator detectors coupled to photomultipliers was designed and constructed, and testing was begun on our 4 MeV electron Van de Graaf.

  In CY00 the large collaborations responsible for two previous experiments on our polychromatic beam continued major drives to upgrade those experiments in preparation for further running on our beam. Both projects, a search for time-reversal symmetry violating correlations in neutron decay and a search for parity violating spin rotation of neutrons in bulk media, are expecting to be ready for beam time sometime in late CY01. These projects have produced three recent Ph.D. theses and have already made modest improvements on the best preceding results. The results of the time-reversal asymmetry experiment were recently published in Phys. Rev. C.

  Throughout the year our group explored promising new ideas for experiments that probe the weak interactions with scientists from Los Alamos National Laboratory (LANL). The most promising ideas would lead to participation in a LANL-led measurement of the parity-violating gamma-ray asymmetry in the "npd" reaction (polarized neutron plus proton goes to deuteron plus gamma-ray) at the spallation neutron source at LANL, and to participation in a LANL-led measurement of the electric dipole moment of the neutron using UCN. (M.S. Dewey, J. Nico, P. Huffman, F. Wietfeldt, J. Adams, A. Thompson, with M. Arif, T. Gentile, F. Bateman, D. Gilliam, D. Jacobson)

• Polarized ³He for Neutron Spin Filters. The primary focus of the polarized ³He program is the development of neutron spin filters and application of these devices to both materials science and fundamental physics. We are developing two optical pumping methods, spin-exchange and metastability-exchange, for producing the polarized ³He gas. In collaboration with materials scientists, we are conducting experiments at the NCNR to demonstrate the advantages of ³He spin filters for certain materials science applications. In addition, we are contributing to an experiment in weak interaction physics at the Los Alamos National Laboratory, and we are collaborating with Indiana University in applications at the Intense Pulsed Neutron Source (IPNS) at Argonne National Laboratory.
  

  Figure 2. Neutron Polarization Analyzer for Small Angle Neutron Scattering (SANS) Experiments. Conventional neutron polarization analyzers would destroy the delicate SANS signal, but the He-3 spin filter does not. This permits differentiation of magnetic scattering effects and nuclear scattering effects.

  In May 2000, we employed polarized ³He-based neutron polarization analysis for an experiment on the NCNR NG7 small angle neutron scattering (SANS) instrument (Fig. 2). The goal was to separate the magnetic and nuclear scattering obtained from a sample of the colossal magnetoresistive (CMR) material La_0.85Sr_0.15Mn0₃. Materials in this class are of interest for application of polarized ³He because no other method exists for isolation of the magnetic scattering. Data was acquired using cells polarized by both optical pumping methods. The ferromagnetic transition of the material at 235 K was clearly identified by depolarization of the neutron beam by the sample. However, analysis of the scattering data has proved to be problematic, which may due to the overwhelming structural scattering in this sample. Hence we are planning experiments on other samples so that the method can be demonstrated clearly in a simple case, before tackling such difficult samples again.

  Besides production of the polarized ³He gas required for spin filters, an important issue in the application of spin filters is the interaction with the sample and instrument environments. Relaxation of the ³He polarization in different holding field scenarios is being studied with a diode-laser-based apparatus for metastability-exchange optical pumping.

  The ³He polarization achievable with the diaphragm compressor apparatus has been improved using a more efficient optical pumping scheme. For the SANS experiment the polarizations obtained from each of the optical pumping methods were both just under 50 %, but the spin-exchange cell was the better performer because of the extremely long relaxation time of the spin-exchange cell (130 hours). Hence the focus of attention for the metastable work will shift from improving the polarization to improving the cell relaxation times by using alkali coatings.

  We are also investigating spin-exchange optical pumping with spectrally narrowed high power diode lasers. One of the reasons we typically use high ³He pressure (3.5 bar) for spin-exchange cells is to increase the pressure-broadened absorption width to better match the broad spectrum of high power diode lasers. Recently spectrally narrowed diode lasers have been developed by other researchers, which might allow reduced pressures for spin-exchange cells. In addition to relaxing the mechanical constraints on spin-exchange cells, lower pressure cells would allow for the possibility of extremely long cell relaxation times because of the reduced dipole-dipole relaxation. We have recently produced a neutron-compatible cell at 1 bar with a relaxation time of at least 400 hours, a world record to our knowledge. Such cells would allow for quite stable polarization even in the absence of continuous optical pumping.

  NIST is collaborating with LANL in the application of ³He polarizers to the proposed npd LANSCE experiment. A recent test run at LANSCE (November 2000) utilized a NIST "double" spin-exchange cell (i.e., separate volumes for optical pumping and spin filtering). Indiana University recently employed a metastability-exchange cell prepared and tested at NIST for an experiment to demonstrate ³He-based neutron polarization analysis in polarized neutron reflectometry. (T. Gentile, A. Thompson, and D. Rich)

• Neutron Interferometry and Optics Facility (NIOF). During the past year at the NIOF a number of fundamental and applied physics experiments have been started and/or completed with collaborators from the University of Melbourne, Australia, the University of Missouri, the University of Indiana, and the University of North Carolina: Phase Contrast Radiography, Neutron-Deuteron Scattering Length in Gaseous Deuterium, Dynamical Diffraction Measurement of the Neutron-Electron Scattering Length, and Visualization of Lithium Ion Migration in Ion Conductors.

  The results of an experiment to produce quantitative Phase Contrast Radiographs of small objects were reported in the journal Nature (November 2000). This experiment was the result of collaborative efforts between NIOF, the University of Melbourne, and the University of Missouri. This Phase Contrast Radiography technique is novel since it provides a way of extracting phase information in an image without the use of an interferometer. Images taken have shown that this technique makes small and delicate features much more prominent. Eventually this technique will be applied to neutron phase contrast tomography as well.

  Major progress has been made in the design and construction of a highly sensitive experimental assembly for the precision measurement of the scattering lengths of gases in a neutron interferometer. This will lead to an upcoming experiment that will measure the scattering length of deuterium gas. This achievement is important since the neutron interferometer represents the only neutron optical device capable of precisely measuring the scattering lengths of gaseous samples. Accurate knowledge of the scattering length is important in many theoretical models of the neutron interaction in few body problems. By using gaseous samples, one can avoid some of the difficulties encountered in interpreting the results of similar scattering length measurements in solid state systems, e.g., Bragg reflection interferences.

  The results of an ongoing experiment to measure the neutron-electron interaction amplitude via the scattering length b_ne have been extremely promising. This very fundamental quantity b_ne is critical to the understanding of the charge structure of the neutron and of the deuteron charge structure in atomic physics. After four decades of sustained effort discrepancies between different experimental values and between experiment and theory persist. Using a novel dynamical diffraction effect for a perfect single crystal inside a neutron interferometer, we are attempting to measure b_ne directly and accurately. This measurement does not suffer from many of the systematic errors common to most of the previous experiments.

  Ongoing ATP supported experiments to image lithium ion conduction in batteries have shown that neutron imaging is a very effect technique to reveal the isotopic lithium concentration in ion conductors. This research effort aims at providing new methods for evaluating the effectiveness of future battery technologies.

  Finally, after a highly successful effort to design, construct, and operate a mobile 3-D neutron imaging station, new funding has been secured to aid in the construction of a more permanent neutron tomography facility. The DoE Office of Transportation Technologies Fuel Cell Program is supporting the establishment of a fuel cell test and evaluation facility here at NIST. This station will afford industrial researchers the tools of neutron tomography to aid the future development of advanced fuel cell related technologies. (M. Arif, D. Jacobson, S.A. Werner, P. Huffman, T. Gentile, and A. Thompson)

• Neutron Dosimetry. For the first time we provided a neutron survey instrument calibration in an Am-Be neutron field. We still have some development work to do on this capability, before we can provide these calibrations with uncertainties as low as those done with ²⁵²Cf sources. Nevertheless, the customer was very pleased to get a calibration that relates directly to his own Am-Be transfer source.

  Another important development was the initiation of accelerated testing for neutron-induced soft failures in SRAM and DRAM chips. This accelerated testing was done both with thermal neutrons and ²⁵²Cf neutrons. In this work, it was shown conclusively that thermal neutrons at normal environmental levels (from cosmic rays) have begun to be a serious problem for some batches of SRAM and DRAM chips that incorporate borosilicate glass films in their manufacture.

  The neutron source calibrations this year included one calibration for a secondary standards laboratory and two calibrations for industrial customers. The radiation protection dosimetry calibration customers included one DoE fuel processing facility and four industrial customers. Special tests for two neutron detector manufacturers were performed with thermal neutron beams at the reactor thermal column. (J. Adams, A. Thompson, J. Nico, and D. Gilliam)

• Neutron Cross Section Standards. The NIST Neutron Cross Section Standards Project has played an important role in the improvement of the neutron cross section standards through both evaluation and experimental work. We are leading an effort that will result in a new international evaluation of the neutron cross section standards. This has involved motivating and coordinating new standards measurements, examining the standards database, and pursuing the extension of the standards over a larger energy range. This work is taking place through participation in the U.S. Cross Section Evaluation Working Group and two international committees, the International Atomic Energy Agency (IAEA) and the Nuclear Energy Agency Nuclear Science Committee. An IAEA Coordinated Research Project has been formed to provide resources to allow meetings of the contributors to this evaluation process. An objective is to complete the evaluation in time for the major international cross section evaluation projects to use the improved standards in forming new versions of their libraries.

  This NIST project has continued to maintain a limited experimental role in the measurements of the standards. This role has led to a new, NIST-LANL-Ohio University collaborative measurement of the H(n,n) angular distribution at Ohio University at 10 MeV neutron energy. The final results of the data indicate differences with the most recent U.S. evaluation of this angular distribution. As a result of these differences measurements are planned at about 15 MeV neutron energy to determine the energy dependence. This work was initiated as a result of concerns about that evaluation expressed by European standards groups. The H(n,n) angular distribution is one of the most important neutron cross section standards. Also, plans are being made for a new measurement, which will lead to an improvements in the ¹⁰B(n,) standard at low neutron energies. This work will be done at the new NIST monochromatic neutron beam facility on NG6. (A. Carlson and D. Gilliam)

• Dissemination of National Standards of Radioactivity. The Radioactivity Group disseminated the National Standards of Radioactivity mainly through the following three activities: (1) over 650 Radioactivity Standard Reference Materials (SRMs) were sold in 2000. (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. (3) over 50 calibrations of customer sources were provided. (L.L. Lucas, J.C. Cessna and L.R. Karam)

• Calibration of ⁶⁸Ge/⁶⁸Ga Source for NIST Time and Frequency Division. The radionuclide ⁶⁹Ge and its positron-emitting daughter, ⁶⁸Ga, are often used as long-lived (270 d) sources to calibrate detector systems for positrons as well as the 511 keV radiation that results from their annihilation. A NIST Physics Laboratory Researcher in Boulder was using such a commercially available source to calibrate his instrumentation and observed a factor of three discrepancy between expected yields based on the activity certificate from the manufacturer and experimental data. We performed an activity measurement on the source using high-purity germanium gamma-ray spectrometry. The expanded uncertainty was 7.6 % and was mainly due to uncertainties in fitting the 511 keV annihilation peak and the uncertainty in the decay data. Despite the relatively large uncertainty, we were able to determine that the manufacturer's certified value was not the source of discrepancy, although the manufacturer had understated their uncertainties by a factor of about five. (B.E. Zimmerman, L. Pibida, and L.R. Karam)

• Calibration of Pure-Beta-Emitting Intravascular Brachytherapy Sources. Considerable work has continued on providing NIST-based activity calibrations for manufacturers of intravascular brachytherapy sources. These sources are intended for use in the prophylactic inhibition of restenosis following balloon angioplasty in heart-disease patients. Their use has an ultimate potential of possibly serving over 600,000 patients per year in the US alone, with an economic value in excess of a billion dollars. In the past year, three sets of nondestructive ionization-chamber-based calibrations were performed for ³²P "hot-wall" angioplasty-balloon catheter sources that are manufactured by Radiance Medical Systems Inc. (Irvine, CA). These calibration measurements utilized previously established ionization-chamber calibration factors that were in turn derived from destructive assays. The results of the calibrations were used by Radiance for internal quality control and to transfer the NIST calibrations to two other calibration laboratories, viz., the Radiation Calibration Laboratory at the University of Wisconsin-Madison and the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Germany. Independent destructive assays of the ³²P content of balloon catheters by PTB were in agreement with the NIST to within less than 1 %. Ionization-chamber-based calibrations were also performed the Novoste Corp. (Norcross, GA) sources. These are stainless-steel-encapsulated, ceramic-based ⁹⁰Sr-⁹⁰Y sources whose calibration factors were also derived from earlier destructive radionuclidic assays by NIST. (R. Collé)

• Development of Radioactivity Standards for New Brachytherapy Device for Brain Cancer Therapy. Proxima Therapeutics is developing a new radiation treatment technique to prevent the recurrence of aggressive brain tumors after surgical resection. This new treatment modality involves the implantation of a small balloon device, named the "GliaSite," in the cavity that remains after the tumor is removed. The balloon is subsequently filled with a solution containing radioactive ¹²⁵I, marketed under the trade name "Iotrex". The Radioactivity Group has experimentally analyzed the response of Capintec CRC-12, CRC-35R, and Vinten 671 dose calibrators to NIST-calibrated solutions of Iotrex. The use of new NIST data for dial settings and correction factors significantly improves the accuracy of Iotrex assays. Furthermore, these data constitute an important part of the documentation that must be submitted by Proxima in order to receive FDA approval for the device. (B.E. Zimmerman and J.T. Cessna)

• Development of Radioactivity Standard for ¹⁸⁸W/¹⁸⁸Re. The availability of the ¹⁸⁸W/¹⁸⁸Re generator has increased interest in using ¹⁸⁸Re for a wide variety of nuclear medicine applications. The daughter, ¹⁸⁸Re, is milked from the generator and is currently under investigation for radioimmunotherapy, radiation synovectomy, intravascular brachytherapy, and as a bone palliation agent. Wires containing ¹⁸⁸W are currently being studied for possible applications in intravascular brachytherapy. Because of its short (t_1/2 = 17 h) half-life, it is impractical to produce a Standard Reference Material (SRM) of ¹⁸⁸Re to distribute to the large number of users worldwide. Rather, by producing a standard of ¹⁸⁸W in secular equilibrium with ¹⁸⁸Re, users can calibrate their instrumentation for ¹⁸⁸Re with a source that has a relatively long (t_1/2 = 69 d) half-life. Using liquid scintillation counting, we have succeeded in calibrating solutions of ¹⁸⁸W in equilibrium with ¹⁸⁸Re with an expanded uncertainty (k=2) of 0.79 %. This calibration was transferred to the NIST "4" ionization chamber to facilitate subsequent calibrations. Calibration factors were also derived for the Capintec CRC-12 and CRC-35R dose calibrators maintained at NIST in the standard 5 mL ampoule geometry. (B.E. Zimmerman and J.T. Cessna)

• Resonance Ionization Mass Spectrometry. RIMS has been evaluated for measuring ¹³⁵Cs/¹³⁷Cs isotopic ratios. Spectroscopic measurements of 6s ²S_1/2(F = 4)  6p ²P_3/2(F = 5) transition frequency shifts for ¹³⁵Cs and ¹³⁷Cs confirmed existing values and demonstrated that it is possible to perform such measurements on sub-picogram samples. Optical isotopic selectivity of ~10³ for both ¹³⁵Cs and ¹³⁷Cs against stable ¹³³Cs was observed and, when combined with a quadrupole mass spectrometer, overall selectivity of greater than 10⁹ was demonstrated. This selectivity appears to be limited by neutral particles generated during the atomization of the samples and could possibly be improved by using a non-axial geometry that prevents direct line-of-sight transport between the atomization source and the ion detector region. Because of the inherent elemental selectivity of the resonance ionization process, no interference could be observed from barium isobars in the RIMS measurements. Overall detection efficiency for the RIMS process was found to be 1 - 2x10^-6, limited in approximately equal parts by the efficiency of converting Cs salts in the atomization source to neutral gaseous Cs atoms and the efficiency of ionizing the Cs atoms entering the laser beams. Test measurements on samples containing as much as 4x10⁸ excess of ¹³³Cs were performed and demonstrated detection limits of 1 - 2x10⁸ atoms. While this only corresponds to ~100 mBq for ¹³⁷Cs, the activity equivalent for ¹³⁵Cs is ~10^-3 mBq, which could not be detected by normal decay counting methods. Further, both radioisotopes can be measured simultaneously by the same method, and this method is independent of the nuclear decay properties. Isotope ratios for ¹³⁵Cs/¹³⁷Cs were compared using both RIMS and conventional TIMS and found to be in excellent agreement. The isotope ratio measurements were able to precisely date a standard sample whose isotopic composition had been accurately measured two decades previously; however, measurements on another standard with unknown ¹³⁵Cs content yielded an anomalous ratio that illustrates problems that may arise because of the neutron-flux dependent fission yield of ¹³⁵Cs. However the neutron-flux dependence for the fission yield of ¹³⁵Cs can provide information about the origin of the samples studied. (L. Pibida, L.R. Karam, and J.M.R. Hutchinson)

• Status of Low-level Radiochemical Analysis in the U.S. The NIST Radiochemistry Intercomparison Program has four years of performance evaluation data from fifteen commercial, university, advocacy, National, and Federal Agency laboratories across the US for ⁹⁰Sr, ²³⁸U, ²³⁸Pu, ²⁴⁰Pu, ²⁴¹Am, at 0.03 Bq to 0.3 Bq per sample in water, air-filters, soil, synthetic urine, and synthetic feces. A systematic evaluation of the data provides a glimpse at the status of low-level radiochemical analyses in the U.S. The relative mean difference from the NIST massic activity values across all matrices and radionuclides ranged from –1 % to –6 % with standard deviation of the means from 2 % to 7 %. Although there is an improvement in laboratory performance since the inception of NRIP, analysis of variance indicated that the analytical methods used was a factor in the difference of measured ²⁴¹Am and ⁹⁰Sr values from the NIST massic activity values, whereas the test matrix was a determining factor for ²³⁸U analyses. Furthermore, about 90 % of traceability evaluations were acceptable when compared to the ANSI N42.22 criteria for performance evaluations. The majority of the failures to pass the ANSI N42.22 criteria were due to unrealistically small estimates of measurement uncertainties. The Radioactivity Group has provided workshops to assist the NRIP participants with estimating their measurement uncertainties. (Z.Y. Wu, C. McMahon, Z. Lin, and K.G.W. Inn)

• NIST Radiochemistry Intercomparison Program (NRIP). The NIST Radiochemistry Intercomparison Program NRIP marks the successful completion of the fourth year of its measurement traceability program for low-level environmental radioactivity measurements. Four rounds of testing (five total matrices were offered: water, air-filters, soil, synthetic urine, and synthetic feces) were completed. The analyte list includes ⁹⁰Sr, ²³⁴U, ²³⁸U, ²³⁸Pu, ²⁴⁰Pu, ²⁴¹Am, at 0.03 Bq to 0.3 Bq per sample. Participants in the program include: NM State Carlsbad Environmental Monitoring and Research Center, Evaluation Group, Martin Idaho Technologies Company, Institute of Nuclear Energy Research (Taiwan), Los Alamos National Laboratory, EPA Air and Radiation Environmental Laboratory, Oak Ridge National Laboratory's Bioassay Laboratory, Oak Ridge National Laboratory's Intercomparison Studies Program, Oak Ridge Institute for Science and Education, STL Richland Laboratory, Sandia National Laboratory, and Westinghouse Waste Isolation Pilot Plant. The program is vital for relating low-level radioanalytical measurements to the National Standards. The test matrices and analytes for each fiscal year are to be determined at the upcoming Annual Conference on Bioassay, Analytical and Environmental Radiochemistry. Continued growth in the number of participating laboratories is anticipated from the commercial national and international communities. (K.G.W. Inn, C. McMahon, Z.Y. Wu, and Z. Lin)

• International Equivalence. As part of NIST's efforts in the international arena of measurement activities, the Radioactivity Group has prepared a listing of calibration capabilities for Appendix C (list of calibration and measurement capabilities, or CMC's) of the Mutual Recognition Arrangement and submitted it to our RMO. The final version of the Group's CMC's had nearly 400 entries covering a range of dozens of nuclides, geometries and measurement techniques and will be part of a much larger database which will include the CMC's from National Metrology Institutes around the world. (L.R. Karam and L.L. Lucas)