Ionizing Radiation Division

Technical Highlights

Radiation Interactions and Dosimetry Group

A. Theoretical Dosimetry

• 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 photoeffect, 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 quality of the work of the Data Center is reflected in the many requests for our data from other laboratories and in the use of our data in engineering and scientific compendia, books and review articles, and in the reports and protocols of national and international standards organizations. 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 are incorporated into some of the most widely used general-purpose radiation transport codes. (S.M. Seltzer, J.H. Hubbell, and M.J. Berger)
• Beta-Ray and Hot-Particle Dosimetry Calculations. Highly refined theoretical methods have been applied to beta-particle radiation protection practice. Participating in the work of an ICRU/ICRP Joint Task Group in updating fluence-to-dose conversion factors used in radiation protection, extensive electron Monte Carlo transport calculations have been done of the depth-dose distribution by electrons incident on phantoms of water, PMMA, and tissue. Tables have been prepared of the personal dose equivalent at depths of 7, 40, 300 and 1000 mg/cm², for electrons incident at 18 energies from 50 keV to 10 MeV and 7 angles from 0° to 89°. 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 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. (S.M. Seltzer)
• Energy Deposition and Radiation Quality of Radon and Radon Daughters. We have developed a quantitative description at the micrometer and nanometer level of the physical interactions of the alpha particles from radon and its daughters with the cells at carcinogenic risk in the lung and bronchial epithelium. This information is basic to a detailed understanding of the mechanisms of the biological effectiveness of radon and provides input information to biophysical and biochemical models of radiation interaction (as, for example, in a collaboration with Dr. Werner Hofmann, University of Salzburg, Austria). The results of these calculations should improve our understanding of the interaction of radon and radon-daughter alpha particles with the relevant tissues through comparison with experimental biophysical, biochemical and biological information. (L.R. Karam and R.S. Caswell)
• Space-Shielding Radiation Dose Calibrations. With support from NASA's Life Sciences Biomedical Research Program, a computerized database and code package was developed for the routine prediction of the absorbed dose from incident electrons and their secondary bremsstrahlung, and from incident protons, as functions of the thickness of aluminum shielding of structures in space. This work represents a thorough updating, extension, and refinement of our earlier SHIELDOSE package, which has found wide use in the space-radiation-effects community. The database is based on extensive Monte Carlo calculations of the penetration, scattering and energy loss of electrons in aluminum slabs, the production of secondary bremsstrahlung, and the penetration and scattering of these photons to greater depths. The proton dose distributions were evaluated in a straight-ahead approximation; a partial study of the effects of nonelastic nuclear interactions of the protons with aluminum nuclei and the transport of nuclear secondaries was included. The user code performs the necessary interpolation over the database and the integration for any specified spectra of incident electrons and protons, giving the distribution in a variety of simple geometries of dose in small detector volumes of Al, graphite, Si, air, bone, CaF₂, LiF, GaAs, SiO₂, tissue, or H₂O. Numerous copies of this software have been distributed to the international space-radiation-effects community, and a collaboration is underway to incorporate this new work into an existing commercial package for estimating space radiation effects in any Earth orbit. (S.M. Seltzer)

B. Industrial Dosimetry

• Electron-Beam Radiation Sources. The electron accelerators operated by the Radiation Interactions and Dosimetry Group continue to be used for a large variety of radiation interaction studies. The principal sources that are currently being employed are the 4 MeV electron Van de Graaff and the 32 MeV MIRF (Medical-Industrial Radiation Facility). The activities carried out on these sources are specified in greater detail in other areas of this report but will be mentioned here. The Van de Graaff has been used principally to check the radiation response of new and existing types of diodes and solar cells. These include silicon on silicon, gallium arsenide on silicon and gallium arsenide, and indium phosphide on germanium and indium phosphide types. The MIRF has been used for a large variety of programs including: production of radioactive isotopes of carbon, chlorine, fluorine, iodine and molybdenum; precipitation of heavy metals (lead and cadmium) from solution; degradation of polychlorinated biphenyls (PCBs); investigation of remote laser telemetering dosimetry; radiation curing of epoxies; production of radioactive fullerenes; investigation of radiochromic dye-gels; and the investigation of scatter from medical treatment beams. In addition to these ongoing programs, the beam line for the production of monoenergetic photon beams from the interaction of electrons in single crystals is nearing completion. (C.E. Dick and M.R. McClelland)
• Waste Treatment by Electron Beam. Electron beam studies, centered at NIST's MIRF facility, have proceeded in several areas. In a collaboration under a CRADA with the Department of Materials and Nuclear Engineering of the University of Maryland, electron-beam-induced degradation of toxic metal salts in aqueous solutions has been demonstrated, with efficiencies of precipitation of mercury as high as 99.9 % at typical radiation processing absorbed doses (~40 kGy). The model radiation chemical reactions in such systems has been substantiated, and the roles of oxygen and hydroxyl radical scavengers (e.g., ethanol) have been established. We also have been investigating applications of radiation (gamma-ray and electron-beam) technology for the reduction and elimination of polychlorinated biphenyls in waste water. We are performing mass spectral analyses of 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. In collaboration with the University of Miami, an aqueous dye dosimeter is being developed for quality control in the electron beam dehalogenation (e.g., PCBs) and purification of toxic ground water supplies. NIST is also helping the DoE in evaluating the progress of legislated contracts on hospital waste treatment by electron beams as a viable alternative to incineration. Electron-beam treatments for remediation of several environmentally hazardous wastes are being developed as emerging U.S. technologies for small business entrepreneurs. (W.L. McLaughlin, L.R. Karam, M. Al-Sheikhly, M. Chayehian, A. Sadeghi, and B. Wise)
• Electron and Gamma-Radiation Process-Control Dose Intercomparisons. During 1995, NIST participated in two dosimetry intercomparisons involving eight international standards laboratories engaged in standards and measurement services for industrial radiation processing quality control. The laboratories include NIST, the U.K. National Physical Laboratory (NPL), the Atomic Energy Laboratory of Canada, Ltd. (AECL), the French Laboratoire de Mesure des Rayonnements Ionisants (LMRI), the Institute of Isotopes of the Hungarian Academy of Sciences, the Russian National Scientific and Research Institute for Physical, Technical and Radiotechnical Measurements (VNIIFTRI), the Ris National Laboratory of Denmark, and the Japan Atomic Energy Research Institute (JAERI). The "double-blind" intercomparison used the Dosimetry Laboratory of the International Atomic Energy Agency (IAEA) as the issuing and analytical laboratory for the alanine transfer dosimeters and the Bureau International des Poids et Mesures (BIPM) as the coordinating laboratory for interpreting the intercomparison results and their statistical treatment. The two intercomparisons involved the irradiation by ⁶⁰Co gamma radiation and by 7 to 10 MeV electron beams to absorbed doses unknown by the issuing and analyzing laboratory. The ability of NIST to provide accurate irradiations in the radiation processing dose range (10 kGy to 50 kGy), using both kinds of radiation, was confirmed by these intercomparisons. The results of the intercomparisons will be published in an IAEA Technical Report in 1996. (W.L. McLaughlin, J.C. Humphreys, M.F. Desrosiers, J.M. Puhl, and M.L. Walker)
• High-Dose Irradiations and Dosimetry at Semiconductor Detector Interfaces. In collaborations with the NIST Semiconductor Electronics Division, Computer Devices, Inc., Sacramento, CA, and Analogue Devices, Inc., Wilmington, MA, high-intensity electron beam and gamma-ray sources have been used to deliver surface doses as well as controlled lateral and depth doses in ceramic layers and adjacent germanium, nickel, and silicon layers. In one experiment, thin radiochromic films (8 µm thick) were calibrated with the NIST Van de Graaff 1 MeV electron beam and were read by a He-Ne scanning laser micro densitometer. The films were used to measure doses at the interfaces between the adjacent ceramic-metal-semiconductor layers. These studies were done to evaluate enhanced radiation effects near metal components of radiation detectors on DoD and NASA space vehicle missions. In another study, high-dose gamma-ray irradiations of semiconductors were done, using an accurate temperature-control device built for the NIST high-intensity Gamma-cell, in order to generate dose-temperature dependence data for the evaluation of detector performance at temperatures up to 200 °C. (W.L. McLaughlin, M.L. Walker, C.E. Dick, J.C. Humphreys, J.M. Puhl, and S. Witczak)
• Alanine Studies in the High-Dose Region. The alanine dose-response curve generated by the peak-to-peak measurement method begins to saturate at dose levels greater than 500 kGy. However, the linearity of the response can be extended to higher doses by using the integral of the alanine EPR spectrum. Furthermore, the resolution of hyperfine lines in the alanine EPR spectrum decreases with increasing dose. In a search for a basis to these changes, we investigated the possibility of a second radical at high doses with an EPR technique called spin trapping. For alanine irradiated to doses >500 kGy, additional EPR resonances are detected in the spin-trap experiment. These resonances are the first evidence of a new radical present incrystalline alanine irradiated to high doses. Elucidation of the chemical structure of this radical was attempted using several different isotopically labeled alanine compounds. Although new EPR signals were evident in the high-dose spectra, the data did not reveal a clear or simple solution to the structure of the second radical. The future efforts will employ selective photo-bleaching experiments to enhance the view of the spectrum for the second radical. (M.F. Desrosiers and S.B. Taylor)
• EPR Spectrometer Response Reference System. The accuracy and precession 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 long-term testing of many different organic (various pitch samples, sucrose char, tempol, fusinite) and inorganic materials (copper and manganese salts), it was decided that the material that best corrected for random spectrometer fluctuations is synthetic ruby crystal. In extensive tests the 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. Efforts are under way to modify spectrometer hardware to permanently include the ruby reference material 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. After selection of the optimum polymer/alanine ratios the dosimeter will be mass produced and undergo further testing. The dosimeter will be primarily useful for electron beam processing but, due to the relative ease of handling for films over pellets (the most common alanine dosimeter shape), the film dosimeter could become a popular form for high-dose gamma-ray processing. (M.F. Desrosiers)

C. Medical Dosimetry

• Mammographic X-Ray Exposure Standards. A free-air ionization chamber has been established as the NIST primary air-kerma standard for mammographic x-rays. The Attix chamber, named for its original designer Herb Attix, is a variable volume chamber designed for absolute in-air exposure measurements for x-ray energies up to 50 kVp. The Attix chamber has been extensively compared to a well-established standard, the Ritz chamber. These comparisons required the determination of the correction factors for both free-air chambers for the tungsten (W), molybdenum (Mo), and rhodium (Rh) beam qualities. The Attix chamber, which will be used routinely to establish calibration factors for customers' mammographic ionization chambers, resides in the recently completed NIST Mammography Calibration Range. The range features a constant potential, highly stabilized high-voltage generator and two stationary anode x-ray tubes. Seventeen beam qualities for x-rays from the Mo and Rh anodes have been established for the calibration of mammographic ionization chambers. The beam qualities available include anode/filter combinations of Mo/Mo, Mo/Rh, and Rh/Rh. The mammography range was developed in collaboration with the U.S. Food and Drug Administration's (FDA) Center for Devices and Radiological Health who require traceability to NIST for exposure measurements made in the inspection/certification program of the Mammography Quality Standards Act (MQSA) of 1992. (C.M. Johnson, P.J. Lamperti, S.M. Seltzer, and J.H. Sparrow)
• International Comparison of Gamma-ray Standards. NIST is in the process of comparing its gamma-ray standards with those of the BIPM (Bureau International des Poids et Mésures) in Paris, France using ⁶⁰Co gamma radiation. The transfer standards being used for this purpose were previously used with the Italian National Laboratory in Rome (Ente Per Le Nuove Tecnologie, L'Energia E L'Ambiente - ENEA). These two comparisons will provide NIST with additional corroboration of standards used for therapy, and further contribute to the harmonization of international radiation dosimetry. (P.J. Lamperti)
• Wide-Angle Free-Air Chamber for ¹²⁵I. The wide-angle free-air chamber (WAFAC) will serve as the NIST standard air-kerma-strength measurements for ¹²⁵I brachytherapy seeds and perhaps other low-energy photon emitters. The chamber and associated fixtures are in the process of being moved to another laboratory room. The mechanical support mechanism, source holder, and filter holder are being redesigned at this time. Redesign of the WAFAC support mechanism will allow for greater flexibility in determining the WAFAC parameters and it is anticipated that redesign of the source support mechanism will reduce uncertainties due to source holder scatter and attenuation. In addition, work is in progress to automate the measurement and analysis procedure, including a motorized filter/shutter wheel. Locating the WAFAC in a separate facility will provide an opportunity for a dedicated measurement facility. (P.J. Lamperti, J.T. Weaver, and J. Shobe)
• Use of Radiochromic Film Dosimetry for Brachytherapy Source Characterization. The high resolution capability coupled with the relative insensitivity of the radiochromic dye film system suggested its use for characterizing the small, high-dose rate sources used in brachytherapy. Both ¹²⁵I and ¹⁹²Ir sources are being studied. Films were irradiated in various geometries and read with a high-resolution scanning densitometer. The most promising geometry, involving wrapping a single layer of film around a 6 mm diameter plastic cylinder in which a seed is placed, was studied in 1995. Other cylinders, made with solid water with radii of 2, 4, and 5 mm were used as well. This geometry yields information on seed axial and transaxial uniformity, as well as dose rates at depths of 2, 3, 4, and 5 mm in tissue. Results of these measurements will be reported at the annual meeting of the American Association of Physicist in Medicine (AAPM). (C.G. Soares)
• Intravascular Brachytherapy Source Dosimetry. An exciting development in 1995 was the beginning of the use of beta-particle emitting brachytherapy sources for the prevention of restenosis (re-closing) of coronary blood vessels after angioplasty. Angioplasty is performed more than 300,000 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 used also 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 to predict the dose rate at arbitrary locations around the sources using a modified form of the AAPM Task Group  3 Protocol. A publication describing this work is in progress, and collaborations with other workers in this field are in the planning stages. (C.G. Soares)
• Development of Dose-Mapping Instrumentation for Radiation Therapy Treatment Planning. In a CRADA with Photoelectron Corp. of Waltham, MA and in collaboration with ISP Technology Inc. of Wayne, NJ, Harvard-Massachusetts General Hospital, the Mayo Clinic, and Georgetown University, radiation detectors and CCD microspectrodensitometers are being developed for two- and three-dimensional dose mapping for the treatment of cancer by clinical photon, electron, and proton beams. A paper on novel radiochromic films for clinical dosimetry presented at the 11th Solid State Dosimetry Conference in Budapest, July 1995, was honored as Best Presentation at the Conference. This work also involves the development of 3D gels which can be analyzed in terms of isodose data after beam irradiation. Such data will provide a new, inexpensive and more accurate method of radiotherapy treatment planning and viewing of radiographic images. (W.L. McLaughlin, C.G. Soares, J.M. Puhl, C.A. Christou, and L. Karam)
• Scattered Fraction Measurements of High-Energy X Rays. High-energy photon beams from medical electron linear accelerators, used for cancer therapy, require radiation shielding of not only the direct beam but also the radiation scattered from the patient. A program for the determination of scattered dose fractions for 6, 10, 18, and 25 MeV photon beams has been undertaken, with some of the measurements taken using NIST's MIRF medical linac. The "patient" is replaced by a cylindrical water phantom (with radius 12.81 cm) placed at the treatment position in the radiotherapy x-ray beams. Measurements were made 200 cm from the isocenter at angles ranging from 0° to 160° with respect to the central axis of the incident beam. The ionization current measured at each angle is reported as a fraction of the ionization current measured at the center of the phantom. The results of these measurements are being compared to Monte Carlo calculations which are often used to determine shielding requirements in medical radiotherapy facilities. Secondary neutron measurements have been made at the same positions for 18 MeV and 25 MeV beams utilizing thermoluminescent dosimeters (TLDs) placed in a 12.7 cm polyethylene moderating sphere. The results of this work are being used by an AAPM task group assigned to reappraise the design and evaluation of structural shielding for medical facilities that use high-energy x rays. (J. Shobe)
• Alanine Dosimeter Response in Proton Therapy Beams. The use of proton beams for radiation therapy of various malignancies is being studied at a number of institutions worldwide. Because of the wide geographic distribution of these facilities and the variety of dosimetry methods in use, there is a need for a reliable mailable dosimetry system that has a uniform response over the energy range used in proton therapy. A comparison was made of the time-dependent response of alanine dosimeters irradiated in the radiation-therapy proton beam at the Harvard Cyclotron in Cambridge, MA. Dosimeters were placed near the entrance point of the beam in the phantom and at the Bragg peak. Time-dependent changes differed slightly between these two positions. A decrease in intensity (<0.5 %) over several hours was measured in the Bragg peak region after which the intensity remains constant over the next several days; dosimeters irradiated at the entrance point continued to decrease from the earliest time of measurement continuing over the course of a few days (<1 %) before reaching a constant level. (M.F. Desrosiers, V. Nagy, and K. Gall)
• Development of Radiosensitive Dosimetric Gels. We have prepared and done preliminary studies of radiochromic gels (gelatin containing the radiochromic dye 2,3,5-triphenyl-2H-tetrazolium chloride), showing the dose-coloration response to gamma irradiation. Based on those results, we have subjected similarly prepared gels to electron beams from our MIRF accelerator and have observed a comparable response. Gels were also provided to other laboratories for comparison in radiation therapy beams. (L.R. Karam, W.L. McLaughlin, and B.M. Coursey)
• Slot Camera Imaging of High Energy Electron and Photon Beams. Preliminary experiments have been undertaken to image various materials in a side-scatter geometry using high-energy (10 to 25 MeV) electron and photon beams. Images of plastic and metal phantoms have been recorded using a Fuji plate as the scattered-particle detector at 90° to the incident beam direction. The images were recorded using both pinhole and slot geometries to define the images. Measurements were made of the resolution of the system for various electron energies and slot sizes. The results of these studies will be used to implement the slot camera under development for one-sided computed tomography. The main goal of this work is the development of an imaging camera with the potential of simultaneously measuring in real time the profiles of tissue density and of radiation dose in a patient undergoing radiation therapy. (W. Deye, J.W. Motz, C.E. Dick, and M.R. McClelland)
• Radionuclides for Nuclear Medicine. One of the most widely used isotopes for nuclear medicine procedures is ^99mTc (T_1/2 = 6 h, γ = 140 keV) fed by the decay of ⁹⁹Mo (T_1/2 = 60 h) which usually is produced in a nuclear reactor. In view of the trends toward nuclear reactor shut downs, due to questions of nuclear waste hazards and economic efficiency, alternate methods of production using high-energy electrons are being investigated in an effort to maintain the availability of this important isotope. Using a custom-designed molybdenum target, the production efficiency with a 30 MeV electron beam from the Medical-Industrial Radiation Facility (MIRF) has been measured. Electron beam energy is first converted to bremsstrahlung photons, and the isotope is then produced via the (γ,n) reaction in the naturally abundant Mo target. The measured activity showed rather good agreement with theoretical estimates derived from Monte Carlo calculations based in part on electron transport algorithms developed in our Group. (L. Lidsky, C.E. Dick, and B.M. Coursey)
• MultiPhoton Detector Development for Biomedical Applications. This project focuses on the continuing development of the MultiPhoton Detector (MPD) for the measurement of radiolabelled compounds of very low specific activities. Because of 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. This Division is providing electron-capture (EC) nuclide standards, performing high pressure liquid chromatographic (HPLC) analyses of ¹²⁵I-labelled steroids and peptides, consulting on other potential biomedical applications of the technique, and performing background measurements. In addition, we organized a Workshop in July 1995, addressing issues involved in ultrasensitive detection in biological systems. (L.R. Karam, B.M. Coursey, L. Schorrs, I. Sagdeev, and A. Drukier)
• 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. Because fullerene cages are capable of physically and chemically isolating radioisotopes from their associated pharmaceutical, a flexibility in choosing radioisotopes for specific tracing or therapeutic applications, not possible with currently available radiopharmaceuticals, would be possible. The first objective of this project is the successful development of radioactive fullerenes ("radiofullerenes") suitable for use in medical imaging, such as technetium and radioactive carbon (¹¹C). We have used the MIRF electron beam to convert non-radioactive ¹²C to radioactive ¹¹C in C₆₀ fullerenes obtained commercially as well as C₆₀ and larger species purified in our facility. Yield and purity of ¹¹C heterofullerenes have been determined by high pressure liquid chromatography (HPLC), multiphoton detection (MPD), liquid scintillation counting (LSC), thin layer chromatography (TLC), and autoradiography using storage phosphor imaging. In addition, we have produced, extracted and purified by HPLC fullerenes produced in the presence and absence of radioactive ^99mTc and have quantified the amount of radioisotope encapsulated by MPD, by LSC and by storage phosphor imaging after separation on TLC plates. (L.R. Karam, M.G. Mitch, and B.M. Coursey)

D. Worker Protection and Accident Dosimetry

• 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 1995 the system was used to calibrate 9 protection level beta-particle point sources, and 4 extrapolation chambers. In addition, the system was used to irradiate customers' extremity dosimeters. A high activity ²⁰⁴Tl source was obtained and characterized and is now also available for calibrations. In 1995, NIST participated in an international round-robin intercomparison of the calibration of a ²⁰⁴Tl source organized under the auspices of the European Radiation Dosimetry Group (EURADOS). Preliminary results indicate agreement to better than 2 % with the Physikalisch-Technische Bundesanstalt (PTB) of Germany. Other measurements performed in 1995 included detailed depth-dose characterizations of the NIST sources. The 2.6 mg/cm² entrance window of the NIST standard extrapolation chamber was also replaced with one of 0.67 mg/cm², allowing depth dose studies to be extended to this shallower depth. (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 supported in the implementation of the bremsstrahlung photon techniques specified by the International Organization for Standardization (ISO). In 1995 the interchangeable filter wheel assemblies with center-driven rotary tables for the filter wheels were mounted on our two x-ray calibration ranges, and the NIST beams were re-characterized in this new configuration. This will allow a transition from the current NIST techniques to the internationally recommended techniques. (C.G. Soares, P.J. Lamperti, and J.T. Weaver)

Neutron Interactions and Dosimetry Group

A. Fundamental Neutron Physics

• Neutron Interferometry. The neutron interferometry station at the NIST cold neutron facility became operational in 1994. During the last year, two new large interferometers have been fabricated, one of the Laue-Laue-Laue type and the other of the Skew-Symmetric type. The normal operating energy of these interferometers is 4 meV, and they will use the higher flux available at this energy resulting from the cold source upgrade. Due to the combined effect of the reactor upgrade and the new interferometer geometry, an increase of more than a factor of five is expected in the count rate for the outgoing interferometer beam. The new interferometers are capable of operating in the energy range of 4 meV to 15 meV, and have much larger dimensions to accommodate larger samples (2.5 cm × 2.5 cm). These are very important factors in the utilization of a neutron interferometer in solid state and material science studies.

  A CCD type neutron detector capable of 40 µm spatial resolution has been acquired. This detector will be used to image structural and magnetic properties of samples by the neutron "phase contrast" imaging technique and will allow real time visual investigation of these properties. In some cases, the sensitivity of this technique will be three orders of magnitude higher than standard neutron radiography. A new variable length, helium-filled beam path has been built to allow the interferometer to operate within the whole energy range mentioned earlier. Numerous other hardware and software upgrades have been carried out to enhance the data acquisition, servo control mechanisms, and interferometer motion controls. Previously, vibration isolation of 1 × 10^-7 g (g as in the earth's gravitational acceleration), positional stability of 2 µm, and rotational stability of 1 mrad had been attained for the interferometer setup. Interferometer fringe visibility of >70 % and phase stability as good as 5 mrad/day were also observed. It is expected all these numbers will improve with the changes and enhancements carried out to the setup.

  A number of experiments in the areas of materials science and fundamental physics have been scheduled in collaboration with research teams in the U.S. and Europe. These experiments will begin as soon as the testing of the new interferometers and the facility is complete.

  In addition, preliminary steps have been taken to build a neutron optics station for 2D and 3D neutron imaging, suitable for both materials science and basic physics research. (M. Arif and A. Thompson)

• Development of Neutron Spin Filters by Laser Polarization of ³He. The developmental program to produce polarized neutron beams using a ³He spin filter at NIST has seen major advances this year with optimization of ³He polarization using the Ti:Sapphire laser, and beginning the conversion to using inexpensive diode laser arrays for optical pumping of Rb. The setup used to polarize the ³He could filter a beam of cold neutrons with sufficient ³He polarization.

  The spin filter is based on the spin-dependent absorption of neutrons by polarized ³He in the reaction ³He(n,p)³H. The polarized ³He is produced either by spin-exchange with laser optically pumped rubidium vapor or by direct optical pumping of metastable ³He. The polarization of the ³He is measured either using NMR, or by measuring the circular polarization of light emitted from an excited state of ³He (the second method only works for the metastable polarization technique).

  A ³He polarization of 25 % was measured in the spin-exchange setup early in the year using the Ti:Sapphire laser. We have begun conversion to a diode laser array, and have already achieved 15 % polarization using this laser. The diode laser requires higher density ³He for optimal performance, so we have started construction of our cell filling system and a new oven to better match the properties of the diode laser.

  The metastable apparatus has also made major advances this year. The optical polarimeter was completed, and a 40 Pa test cell was polarized to almost 80 % polarization. We worked with our collaborator W. Mike Snow of Indiana University to design a compressor system so this low-density gas can be compressed to the high density required for a spin filter.

  Immediate plans are to complete the apparatus to produce and use high density spin-exchange cells, and then maximize the ³He polarization produced in the spin-exchange setup. We are improving the data acquisition and control system for the metastable system, and starting to build an NMR polarization monitor in order to test compressor designs. We hope to test the spin-exchange based spin filter with neutrons in spring or summer of 1996. (A. Thompson and T. Gentile)
• Weak Interaction Studies. During the reactor shutdown, the main thrust of the program to determine the neutron lifetime was directed toward reducing the data acquired from October 1993 through May 1994. In addition, more beam filter material was installed and tighter collimation was employed in anticipation of the increased flux resulting from the installation of the new liquid hydrogen cold source. These improvements will permit a much lower background rate while maintaining the same signal rate. The neutron detector will be operated with a lithium rather than the boron foil used in the initial running. This change reduces the statistical uncertainty on the mass of the material on the foil and serves as an important check on unanticipated systematic effects. Data acquisition began again during November of 1995, and the upgrades to the neutron lifetime experiment are working as anticipated.

  EMIT, a collaborative experiment to search for time-reversal violating effects in the decay of polarized neutrons, will soon be ready to begin data acquisition. Progress on the beam line for EMIT is proceeding well. The polarizing super mirrors are in-hand, and the construction of the requisite mounting hardware and shielding is complete. The design and fabrication of the neutron beam line is also complete, and vacuum testing of the components is currently in progress. The detector, which is composed of an octagonal array of alternating proton and electron detector segments, is nearing completion. Construction of the proton detectors is complete, and all of the electron detectors are built and tested. Emphasis is now being placed on the completion of the electronic and data acquisition systems. EMIT is on track to begin the physics run on NG6 in 1996 when EMIT is currently scheduled for beam time.

  As part of our program in weak interaction studies with cold neutrons, we are collaborating with the University of Washington on an experiment to measure the parity non-conserving neutron spin rotation in a liquid helium target. The Seattle group has designed and constructed the detector apparatus for the experiment. They are in the process of performing the cryogenic tests of the system. Our group at NIST is working on the supermirror polarizer (and analyzer) and developing the polarized neutron beam along with some beam line apparatus. Many of the polarized beam requirements for this experiment overlap with those of the EMIT experiment. We have also assisted in testing the helium neutron counter on the NG7 monochromatic beam. The spin rotation collaboration intends to commence data acquisition on NG6 during the first part of 1996. (M.S. Dewey, J. Nico, F. Wietfeldt, and J. Adams)

B. Standard Neutron Fields and Applications

• Engineering Benchmark Neutron Field for Electric Power Reactor Dosimetry. The Materials Dosimetry Reference Facility (MDRF) is an engineering-benchmark neutron field operated in the pool adjacent to the FORD reactor at the University of Michigan's Phoenix Memorial Laboratory. The MDRF has as a 5 cm thick iron cylindrical shell as its principal moderator of core-leakage neutrons. This produces a slowing-down spectrum similar to that at the inside surface of a PWR reactor pressure vessel and the fluence rate (E > 1 MeV) at the center of a dry-tube specimen-irradiation position is 2.7 × 10¹¹ n cm^-2 s^-1.

  The spectrum and fluence level have been characterized by collaborative measurement and calculational efforts at NIST and the University. The fluence level is uniquely established with reference to the standard ²³⁵U fission neutron field routinely produced within the thermal column of the NIST research reactor.

  The MDRF spectrum characterization work includes a doctoral thesis that develops a method for determining photofission contributions in ²³⁷Np and ²³⁸U without the need for detailed knowledge of gamma spectrum or photofission cross sections.

  The MDRF is presently being used to expose the mentioned fissionable materials for round robin intercomparison of the power industry's dosimetry methods. Irradiations for nominally three days, or 6.5 × 10¹⁶ n/cm², are sufficient to produce accurately measurable quantities of 30 yr ¹³⁷Cs. However, evidence of different counting procedures, involving different geometries, activity loss compensation, and (in particular) methods to handle effects of the large fraction of activity from ²³³Pa in equilibrium with ²³⁷Np are making this intercomparison a tougher job than anticipated. A separate study that tracks fission product gamma spectra and derived dosimetry predictions over a 30 month period is being conducted at NIST.

  The 30 month study also compares newer fissionable oxide dosimeters, fabricated by the Geel (Belgium) Establishment, Central Bureau for Nuclear Measurements, with fission dosimeters from Oak Ridge National Laboratory. The Geel dosimeters are composed of 0.5 mm diameter fission oxide spheres in thin vanadium tubing, while the Oak Ridge dosimeters are sintered oxide wires in vanadium tubing. Results, to date, suggest better than 2 % agreement but on a very limited sample of specimens (i.e., two). (E.D. McGarry and J. Adams)



• Iron Sphere Benchmark Experiment. Integral and differential spectrum measurements inside and outside of an iron sphere benchmark at the CNIF Facility are in progress to check the latest ENDF/B-VI iron inelastic cross section. Based on scoping calculations, a 25 cm diameter sphere was designed with a central plug for locating the Cf fission source at the center of the sphere. A radial penetration is included for activation detector measurements. External measurements are being made with NIST double fission chambers for three fissionable isotopes: ²³⁷Np, ²³⁸U, and ²³⁵U. (J. Grundl, J. Nico, and C. Eisenhauer)

C. Neutron Cross Section Standards

• International Coordination of Measurements and Evaluation. The collaboration to improve the¹⁰B(n,α) standard cross sections has produced a series of important measurements. These include the ¹⁰B(n,α₁γ) and ¹⁰B total neutron cross section measurements at the ORNL-ORELA and WNR-LANL facilities in the U.S. and at the GELINA and Van de Graaff facilities at the IRMM in Belgium. Measurements are also planned at the IRMM using gridded ionization chambers to obtain angular distributions and branching ratio data. These measurements will significantly improve the data base for use in an improved R-matrix analysis of the ¹¹B system. Efforts are now underway to obtain the most recent measurements and their uncertainties for an R-matrix analysis by G. Hale at LANL. This analysis will produce a new evaluation which should extend the energy range over which this cross section can be used as a standard so that a smooth and easy transition to higher energy standards such as H(n,n) can be obtained. (A. Carlson)



• The H(n,p) Angular Distribution at 10 MeV Neutron Energy. Measurements of this important standard were initiated in order to resolve the approximately 3 % differences between recent evaluations of this cross section in the 10 MeV energy region. The experiment was performed at the Ohio University Tandem Accelerator facility. Measurements were made of the angular distribution from 60 degrees to 180 degrees in the center of mass by detecting recoil protons. The analysis of this data indicates that the shape of the angular distribution is somewhat more consistent with the SM95 analysis of Arndt than with the ENDF/B-VI evaluation. A paper on this work was presented at the Del Mar '95 Workshop on Nuclear Data for Fusion Applications. Further analysis is underway and new measurements are under consideration. (A. Carlson and O. Wasson)

Radioactivity Group

• Dissemination of National Standards of Radioactivity. The Radioactivity Group disseminated the National Standards of Radioactivity mainly through the following three activities: (1) Over 700 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. (3) Over 60 calibrations of customer sources were provided. (L.L. Lucas, J.C. 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. (X. Xiong and J.M.R. Hutchinson)
• Natural Matrix Standards. In October 1977, the International Committee for Radionuclide Metrology sponsored a symposium at Woods Hole, Massachusetts to define needs and initiate programs to develop natural matrix radioactivity standards. The term "natural matrix standard" (NMS) refers to a standard of radioactivity which is homogeneously contained in a matrix, such as soil or vegetation, in the same chemical forms as are found in the environment. Since that time six NMS have been issued, River Sediment, Human Lung, Human Liver, RockY Flats Soil #1, Freshwater Lake Sediment, and Peruvian Soil. Two are under development as follows:

  Over the past decades, on the order of 10¹⁵ Bq nuclear waste have been stored in the oceans. Potential contamination of the oceans from leaking nuclear waste has caused world wide concern. Because the determination of low-level radioactivity in ocean sediment is a difficult technical task, a basis for measurement quality assurance, methods verification, and data comparability is needed. The NIST ocean sediment SRM-4355 is being developed using a composite of 1 % contaminated Irish Sea sediment and 99 % Chesapeake Bay sediment by weight. Twelve radionuclides including ⁴⁰K, ⁹⁰Sr, ¹³⁷Cs, ²²⁶Ra, ²²⁸Th, ²³⁰Th, ²³²Th, ²³⁴U, ²³⁵U, ²³⁸Pu, and ^239,240Pu were certified. The mean values were reported for an additional 10 uncertified radionuclides: ¹²⁹I, ¹⁵⁵Eu, ²¹⁰Po, ²¹⁰Pb, ²¹²Pb, ²¹⁴Pb, ²¹⁴Bi, ²²⁸Ra, ²³⁷Np, and ²⁴¹Am. The standard reference material in unit quantities of about 100 g each will be available by the end of 1996.

  Bone as a sink of a number of long-lived radionuclides is a key organ for biokinetics model development and dosimetry studies. Development of a bone standard for the bone-seeking radionuclides is one of the most important tasks in ensuring quality control in bone sample analysis and for providing a common basis for data comparison and evaluation. The development of a bone ash standard for low-level radioactivity measurements provides a great analytical challenge in radiochemistry because of its high calcium and phosphate content. The standard is being characterized for ⁹⁰Sr, ²¹⁰Pb, U, Th, and Pu. (K.G.W. Inn, Z. Lin)
• Chemical Speciation of Environmental Radioactivity. The primary concerns associated with radionuclides in the environment are: (1) migration through natural systems; and (2) bioavailability via the food chain. The problem is that ionizing radiation in sufficient doses can affect a variety of processes in higher organisms. Since the mobility and bio-availability of radioactive elements in the environment must be dependent upon the element's chemical speciation, the characterization of the element's physico-chemical forms in soils and sediments is a key factor in understanding and predicting the migrational behavior of trace metals and radionuclides. One widely used empirical approach for describing speciation is the application of sequential-chemical extractions.
  
  

  NIST is now exploring the possibility of certifying SRM's for soils and sediments by "fraction" as well as for "total" concentration as indicators of the bioavailability of radionuclides in the environment. To test the robustness and expectations in such an approach, a sequential leaching, radiochemical separating, and low-level beta-particle counting procedure was designed and carried out to study the speciation of radionuclides in natural matrix standard reference materials, SRM 4350B (Columbia River Sediment) and SRM 4353 (Rocky Flats Soil-1). The nine-step procedure extracted ⁹⁰Sr from the following fractions: water solubles, exchangeable, carbonates, reducibles, organics, iron and manganese oxides, acid leachables, micas, and silicates. The majority of ⁹⁰Sr in the soil samples was found in the exchangeable fraction. By contrast the ⁹⁰Sr in the sediment is more evenly distributed among the various leached fractions.

  A workshop was also held to discuss the problems and status of available fractionation techniques. All participants agreed that an empirically-based sequential extraction approach would be most likely to result in meaningful information at a reasonable cost. The group felt that a defensible sequential-extraction method would require that optimum conditions for each sequential fraction be experimentally determined. Four experimental conditions (reagent concentration, reagent-to-sample ratio, duration of extraction period, and temperature of reaction) were identified as potentially significant parameters. An experimental plan was designed to test three of these four parameters for each extraction. (K.G.W. Inn and J. Thomas)
• Photo-Nuclear Produced Radioactivities. The MIRF facility has been used to produce positron emitting radioactivities. Techniques have been developed to characterize those sources. Efficiencies for positrons which have been stopped and annihilated, have been developed and compared, when appropriate, with associated gamma-ray emission from the decay. Radioactivities characterized this last year included ¹⁸F, ^34mCl, and ¹²⁶I. The last radionuclide is of particular interest as a high gamma-ray energy emission rate standard for Ge detector calibration. ^34mCl has a 3303.6 keV gamma-ray with a probability per decay of 0.123. This abundant emission and readily prepared radioactivity allows the extension of the current efficiency curve to that energy. (F.J. Schima, B.M. Coursey, and C.E. Dick)
• ⁶³Ni Standardization and Decay Studies. Standard solutions of ⁶³Ni have recently been prepared and will be disseminated as NIST SRM 4226C. The solutions were calibrated by 4πβ liquid scintillation (LS) spectrometry with ³H-standard efficiency tracing. This radionuclide is of interest to the nuclear reactor community because it is often found in reactor environments as the neutron activation product of nickel present in the steel used in construction of those facilities. Certain physical properties, namely a low β^- energy (66.945 ± 0.004 keV) and a relatively long half-life (101.1 ± 1.4 a) also make it attractive for radionuclidic metrology studies, as it tends to be a more sensitive indicator of effects in measurement technique and procedures than would a radionuclide with a higher β^- energy.

  In order to identify and quantify as many of these effects as possible, over 975 measurements involving 55 different LS samples were performed. Among the variables studied were aqueous fraction of LS sample, LS sample mass (volume) dependence, LS sample age (time between sample preparation and measurement) dependence, and effects due to different scintillation fluors. In addition, an exhaustive uncertainty analysis was performed.

  This SRM is gravimetrically related to two others previously prepared by NIST/NBS (SRM 4226, prepared in 1968 and calibrated by microcalorimetry; and SRM 4226B, prepared in 1984 and calibrated with 4πβ LS spectrometry with ³H-standard efficiency tracing), allowing a comparison to be made between the three sources as a check of both measurement consistency and solution stability. After adjusting the data from the 1968 and 1984 measurements to include the latest available nuclear data for both ⁶³Ni and ³H, the three measurements were found to be in agreement to within 0.3 %. Because of this remarkable consistency over a 27 yr span, the three data sets provided enough data to make the first-ever determination by radioactive decay of the ⁶³Ni half-life. Using these data, the half-life was determined to be 101.06 ± 1.97 a. A critical review of measurements of the ⁶³Ni half-life was performed, resulting in a new recommended value of 101.1 ± 1.4 a. A series of articles addressing all of these topics is being prepared for publication. (B.E. Zimmerman and R. Collé)
• Development of Standard for the Palliative Therapy Radionuclide. The American Cancer Society estimates that about 180,000 new cases of breast cancer and an equal number of cases of prostate cancer are diagnosed each year in the United States. New radiopharmaceuticals are being developed which have been shown to relieve pain that is unresponsive to narcotic treatment in many of these cases. NIST in collaboration with the Nuclear Energy Institute (NEI) has distributed standard reference materials (SRMs) for over 20 yr which is an important first step in ensuring the accuracy of injected radiopharmaceuticals in the U.S.

  Over the past two years NIST scientists in the Radioactivity Group have worked to standardize two new nuclides for bone palliation: ⁸⁹Sr (half-life 50.5 days) and ^117mSn (half-life 14 days).

  The ⁸⁹Sr used in SRM 4426A was produced in a fast-flux nuclear reactor in Obinsk, Russia and purified at NIST. The material was standardized by high-accuracy liquid-scintillation counting, and decay-scheme data were established by Ge(Li) gamma-ray spectrometry. The high-activity SRM 4426H-A was then distributed to the FDA's calibration laboratory and to North American radiopharmaceutical manufacturers, and a lower-activity SRM 4426L-A was distributed mainly to instrument companies and medical centers. (B.M. Coursey, D.B. Golas, and F.J. Schima)

  As part of an increasingly active program to develop national standards for radionuclides of interest to the nuclear medicine community, this group has recently performed a calibration of ^117mSn, which is currently under study for use in palliative therapy for pain associated with metastatic bone cancer. The calibration was performed using three techniques: γ-ray spectrometry with HPGe detectors, γ-ray spectrometry with a 4π 30 cm NaI(Tl) system, and πβ liquid scintillation (LS) spectrometry. While the data are in the final stages of analysis, initial results indicate good agreement between the results obtained with each of the three methods. Data were also obtained for the re-determination of the half-life and probabilities per decay of the major emissions. A procedure for the direct standardization of this isomeric radioactivity based on sum coincidence peaks in a Ge spectrum of the ^117mSn is under investigation. This appears to be one of those rare cases in which a ^117mSn source can be calibrated by this method when using a Ge detector with an adequate resolution. This radionuclide is particularly exciting because of the greater uptake of ^117mSn(4+) DTPA in bone tissue relative to the marrow. Compared to other commonly-used bone palliation radionuclides such as ³²P, ⁸⁹Sr, and ¹⁸⁶Re, there is as much as a 4-fold increase in the ratio of bone surface dose to bone marrow dose with the use of ^117mSn(4+)DTPA. This suggests that a much higher dose can be given to the patient before marrow toxicity levels are reached, possibly leading to the ability to treat the metastases themselves. An additional advantage in using ^117mSn is the presence of a 159 keV γ ray, which allows the uptake and distribution of the radionuclide to be studied with conventional imaging devices. (B.E. Zimmerman, J.T. Cessna, B.M. Coursey, and F.J. Schima)
• A Mock "Soil" Calibration Standard for γ-ray Spectrometery. At the request of the Nuclear Energy Institute (NEI), a spiked, mixed-radionuclide, mock "soil" calibration standard (in a 1-L Marinelli beaker configuration) was gravimetrically prepared, and calibrated by high-resolution photonic-emission spectrometry. The matrix consisted of a well-characterized (in terms of densities, particle sizes, etc.) and blended standard Ottawa sand. The matrix was spiked with ²⁴¹Am, ¹⁰⁹Cd, ⁵⁷Co, ¹³⁹Ce, ²⁰³Hg, ¹¹³Sn, ⁸⁵Sr, ¹³⁷Cs, ⁸⁸Y, and ⁶⁰Co. Source homogeneity and possible spike losses were evaluated for every spiking radionuclide. Measurements on aliquants (30 g samples out of 1850 g of spiked matrix) taken as a function of blending times (40 min to 400 min) indicated source homogeneity to within several percent. The only observed spike loss was for ²⁰³Hg which is due to the chemical instability and volatility of deposited mercury compounds. A quantitative, verifiable, and efficacious spiking protocol was devised and developed as part of this work, and used to prepare the Marinelli-beaker calibration standard. The calibration obtained in this work will allow NIST to provide calibration services for other similar Marinelli beaker sources. Such sources are of vital importance for measurements made by the nuclear energy industry. In addition, the developed protocol will be of great benefit to other laboratories for use in preparing their own Marinelli beaker calibration sources. A publication describing the protocol is in preparation. (R. Collé and F.J. Schima)
• Imaging Plate Technology (IPT) and Large-Area Alpha Sources. Criteria are being established for the use of an imaging plate (which stores a radiation image as a distribution of F-centers in a photostimulable phosphor) in the determination of the homogeneity and 2πα emission rate of large area α sources. Since sources using dot matrix distributions are increasingly being used, the shape of the sampling area (circular or square) becomes important in purchasing specifications. This information will be incorporated into an ANSI standard specifying requirements for large area sources.
  
  Other work using IPT includes examining the response of radiation from soils for purposes associated with site remediation measurements and measurement of very low levels of radioactivity (a few disintegrations per day) for the examination of microchip construction material. (M.P. Unterweger, P. Hodge, and T. Cheng)
• Iodine-129. Iodine-129 is a very long-lived (the half-life is 15 million years) fission product that can be significantly concentrated by some organisms. Hence, there is interest in monitoring this radionuclide in food and in the environment. Isotopically enriched ¹²⁹I was obtained from the Oak Ridge National Laboratory. This material contains approximately 96 atom-percent ¹²⁹I. The activity was calibrated by 4πβ (LS)-γ-anticoincidence counting and the material is now available as SRM 4949C. (L.L. Lucas)
• Iron-55. Iron-55 is a radionuclide with a half-life of 2.9 yr that is produced with great efficiency whenever iron is irradiated with neutrons. It is a very common by-product of reactor operation. ⁵⁵Fe decays by electron capture and emits only low-energy (5 keV) x rays. Hence, its calibration is more difficult than most of the radionuclides for which standards are issued. New measurements have been made and intercompared with the ⁵⁵Fe calibration at the National Physical Laboratory in the United Kingdom. The agreement is excellent. The half-life data are being re-evaluated and SRM 4929E will be issued shortly. (L.L. Lucas)

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