Ionizing Radiation Division

Technical Highlights

• International Comparison of Mammographic X-ray Exposure Standards. In June of 1998 NIST completed a direct comparison of primary standards for mammographic x-rays at the National Physical Laboratory (NPL) in Teddington, UK. The Ritz chamber, a NIST free-air ionization primary standard used between 20 and 100 kV was compared to the NPL primary standard used for calibrations of mammography x-ray qualities. Good agreement, between 0.4 % and 0.6 %, was found between the two primary standards. In 1997 an indirect comparison with the German primary standard for mammography showed similar results to the recent NPL-NIST comparison. This indirect comparison was conducted through the use of NIST reference class ionization chambers, which were calibrated at NIST and at the German standards laboratory. The results from the calibrations at both national laboratories agreed to within 0.5 % for the six different mammographic beam qualities used in this comparison. The NIST primary standard for mammography x-rays, the Attix chamber, was also directly compared with another NIST national standard for low energy x-rays, the Lamperti chamber. The results from the comparison of the two primary standard chambers agreed to within 0.4 %, for the four molybdenum entrance x-ray beam qualities. (C.M. O'Brien and P.J. Lamperti)
• Implementation of ISO Bremsstrahlung and NIST Techniques. NIST has continued the efforts to implement the photon techniques specified by the International Organization for Standardization (ISO). In addition to the implementation of the 41 new ISO beam qualities, NIST has responded to the requests of the medical community through the development of several new diagnostic x-ray beam qualities. The development project of the diagnostic M80 and M120 beam qualities was identified by CIRMS as a high-priority need and will soon be successfully completed. Calibrations to the ISO beam qualities and the diagnostic beam qualities will be available upon the completion of an evaluation of the correction factors for the primary standards used to measure these new beam qualities. A spectral determination of all new beam qualities will be used to verify the half-value layers of the newly established beam parameters. (C.M. O'Brien, P.J. Lamperti, and S.M. Seltzer)
• International Comparisons of Low-Energy X-ray Primary Standards. Low-energy x-ray comparisons of two NIST primary standards were conducted at both the National Physical Laboratory (NPL) in Teddington, UK, and at the Bureau International des Poids et Mesures (BIPM) in Sevres, France, in the summer of 1998. The NPL-NIST 1998 low-energy portion of the comparison involved a series of air-kerma measurements with both the NIST Lamperti and Ritz free-air chambers and comparisons with results from the NPL 1 cm³ standard chamber. The NPL x-ray beam qualities used for the comparison were produced between 10 kV and 50 kV. Agreement was found to be within 0.5 %. The diagnostic-energy portion of the NPL-NIST 1998 comparison involved a series of air-kerma measurements with the NIST Ritz chamber and the new NPL high-energy standard chamber at beam qualities produced at 50 kV and 80 kV. Preliminary results indicate agreement to within 0.6 %. The BIPM-NIST 1998 comparison included a repeat of air-kerma measurements made in 1966 with the NIST Lamperti chamber. The goal was to replicate the measurement conditions of the 1966 comparison without compromising the experiment. Although charge-measurement techniques have changed since 1966, the beam-quality parameters, chamber apertures and alignment techniques have remained unchanged at BIPM. The five BIPM beam qualities for the Lamperti chamber comparison were produced by a tungsten anode at 10, 25, 30, and 50 kVp. The NIST Lamperti chamber reproduced the results of 32 years earlier to within 0.1 %, and agreement with the BIPM standard in this range was found to be within 0.6 %. Comparisons of results from the NIST Ritz chamber and the BIPM standard were made at six BIPM tungsten beam qualities produced at 25, 30, 50, 80 and 100 kVp. Preliminary results indicate agreement that varies from 0.4 % to 1.0 %. (C.M. O'Brien and P.J. Lamperti)
• 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 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, J.H. Hubbell, and M.J. Berger).
• Theoretical Dosimetry for Brachytherapy Sources. Brachytherapy sources, encapsulated radioactive material emitting photon and/or beta particles, are used in a variety of therapeutic applications. Such sources include ophthalmic applicators for the treatment of ocular lesions, low-energy photon "seed" implants for the treatment of prostate cancer, and photon- and beta-emitting intravascular sources of various design intended for the irradiation of artery walls to prevent re-closing (restenosis) after balloon angioplasty. Numerous calculations have been done for a wide variety of source designs to determine the spatial distribution of absorbed dose per unit activity in the source to aid in source optimization, to provide basic dosimetric data, and to determine correction factors for measured results. These calculations are done with state-of-the-art Monte Carlo codes, such as ITS, MCNP4b, and EGS4, and with more rapid point-kernel-based methods. (S.M. Seltzer, C.G. Soares, and F. Mourtada)
• ¹⁰³Pd Brachytherapy Source Characterization. There has recently been a significant increase in the demand for low-energy photon brachytherapy sources used in the treatment of prostate cancer. We are working with source manufacturers and the medical physics community in order to develop a new standard for the dosimetry of brachytherapy seeds containing the radionuclide ¹⁰³Pd. Measurement of the air kerma strength of ¹⁰³Pd seeds is accomplished using the Wide-Angle-Free-Air Chamber (WAFAC). Ionization current measurements using well-ion chambers and K_α photon fluence measurements are also carried out on each seed. Various ratios of these quantities are computed and evaluated for suitability as calibration factors for source manufacturer quality control measurements and source user treatment planning. (M.G. Mitch, P.J. Lamperti, B.E. Zimmerman, F.J. Schima, C.G. Soares, S.M. Seltzer, and B.M. Coursey)
• Brachytherapy Source Dosimetry Using the Imaging Plate. The continuing development of new source geometries with lower required activities to achieve desired doses has prompted an investigation of the possible utilization of Fuji plate phosphoimaging to characterize weak brachytherapy sources. The Imaging Plate contains a photo-stimulable phosphor which "stores" energy from radiation sources. Following exposure, the plate is placed into a reader, which scans the plate surface with a laser, releasing photo-stimulated luminescence, which is converted into a two-dimensional digital image. A preliminary depth-dose study using a ³²P intravascular brachytherapy wire source in an A150 plastic photon showed the ability of the imaging plate to successfully measure dose rates in the µGy/s range, below that accessible with radiochromic film. (M.G. Mitch and C.G. Soares)
• 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 which can be used to predict the dose rate at arbitrary locations around the sources using a modified form of the AAPM Task Group 43 Protocol. A publication describing this work was 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 was centralized in a newly refurbished laboratory. Collaborations were continued between NIST and NeoCardia for dosimetry of a ³²P wire, with Washington Hospital Center for dosimetry of various sources, and with Radiance Medical Systems for dosimetry of a radioactive balloon source. Collaborations were also begun with Best Industries for ¹⁸⁸W/Re wire sources. Funding was received from the National Advanced Manufacturing Testbed (NAMT) Program at NIST for this work, with the emphasis on the automation of the source handling for the measurements. An international workshop on brachytherapy source dosimetry was organized and hosted by NIST in April 1998. (C.G. Soares and M.G. Mitch, with F.A. Mourtada, T. Wheatley, and R. Densock, Div. 823)
• International Gamma Ray High-Dose Comparison. The national metrology institutes of the United States (NIST), France (BIPM), Great Britain (NPL), Germany (PTB), Italy (INMRI-ENEA), and the International Atomic Energy Agency, have undertaken a high-dose comparison. The BIPM coordinated the comparison and will analyze and publish the results. The protocol was created through consultation with the two issuing laboratories, NIST and NPL. The dosimeters have been processed by the irradiating laboratories and are currently being measured by the issuing laboratories. (M.F. Desrosiers and V. Nagy)
• Internet-Based Calibration Services for the Radiation-Processing Industry. In cooperation with industry, academia and other government agencies, an Internet-based system will be built for fast remote calibration of high-dose radiation sources against the U.S. national standard gamma-radiation source. The new project will be facilitated by the National Advanced Manufacturing Testbed (NAMT) based in NIST's Manufacturing Engineering Laboratory. The new service will deliver immediate calibration results to the industrial customer on-demand at a lower cost. All prerequisites are now present for automating NIST's calibration services on the basis of modern technologies and commercially available products. Once built, the Internet-based transfer calibration process will be the most modern calibration service available and offer many options for expansion into other areas of dosimetry and metrology in general. The new service will entirely redesign the source calibration process to make it faster, less laborious, much cheaper, and, consequently, much more accessible for present and potential customers. (M.F. Desrosiers, J.M. Puhl, S.M. Seltzer, T. Vorburger, and T.B. Renegar)
• Validation of the EPR Method for Tooth Enamel Dosimetry. The main objective of radiation protection is the development of systems of protection and decision making that help minimize unfavorable consequences of occupational and non-occupational human exposures. To meet this objective, knowledge is required on dose-effect relationships for radiation-induced, stochastic and deterministic effects. Therefore, the acquisition of dosimetric data from populations with chronic exposure is of special interest. Several new sources (Chernobyl, Techa River) for these studies have arisen and are presently underway. Electron Paramagnetic Resonance (EPR) is the only physical method available to retrospective biological dosimetry studies. Validation of the method and rigorous analysis of critical steps in the method have not been demonstrated in the scientific literature. Validation is essential before these data can be used reliably in epidemiological studies from which recommendations are made for occupational exposures. The objective of this work is to validate the EPR-tooth dose assessment method and produce a standardized procedure with well-defined uncertainties. (M.F. Desrosiers, O.F. Sleptchonok, V. Nagy)
• Alanine-EPR Film Dosimeter. Prototypes of a new polymer-based film dosimeter containing alanine have been manufactured and tested. This development program, under a CRADA with the W.R. Grace Co., is testing a variety of formulations for the film dosimeter. These tests were followed by comparative measurements on films of the same polymer/alanine ratio manufactured by different methods. The selected film formulation was mass-produced and is undergoing extensive tests of its radiation response characteristics. Using NIST accelerators and with the cooperation of Risø National Laboratory and the Hungarian Academy of Sciences, a study of the energy response of the films confirmed previously published data on alanine. (M.F. Desrosiers, V. Nagy, W.L. McLaughlin, C.E. Dick, and J.M. Puhl)
• Production and Characterization of Radioendofullerenes. An investigation of novel, fullerene-based, delivery methods for radionuclides in medical/industrial imaging and tracing applications is in progress. Radioendofullerenes have radionuclides trapped inside their all-carbon, cage-like structures, isolating chemically reactive isotopes from the environment. The Kratschmer-Huffman method of fullerene synthesis was carried out in a Fullerene Production Chamber (FPC) using high-current arc burning of graphite rods doped with the isotope to be encapsulated. High-pressure liquid chromatography (HPLC) and thin layer chromatography (TLC) prior to identification of various, purified, fullerene species by optical absorption spectroscopy and mass spectrometry. Current studies have focused on the encapsulation of iodine, as it has several medically useful isotopes. Following development and execution of an encapsulation protocol for "cold"" ¹²⁷I, the radionuclide ¹²⁶I was produced by the reaction ¹²⁷I(γ,n) ¹²⁶I upon irradiation of the sample with bremsstrahlung from our MIRF (Medical Industrial Radiation Facility) linac. The radiolabel was identified by MultiPhoton Detection (MPD) and Fuji plate phosphoimaging. Direct encapsulation of the radionuclide ¹²⁵I has recently been accomplished, and characterization of this new radioendofullerene species is currently in progress. (M.G. Mitch, L.R. Karam, B.M. Coursey, and K.L. Rowley)
• Neutron Interferometry and Optics Facility (NIOF). During the past year a diverse selection of experiments in fundamental and applied physics have been successfully carried out at the NIOF. These experiments have so far resulted in the writing of six articles for publication and have partially satisfied requirements for two separate Ph.D. dissertations. This work has involved collaborations with the U. Missouri-Columbia, the Hahn-Meitner Institute, the U. Innsbruck, the Exxon Research and Engineering Corporation, and the NIST Polymers Division.

  One of these experiments was aimed at searching for the recent predictions of quantum entanglement of the nuclear states in a mixture of fluids. The existence of such entanglement would suggest that the neutron refractive index, n, of a mixture cannot be calculated from the knowledge of refractive indices for the constituent elements alone. The experiment measured n for various mixtures of H₂O and D₂O. Some theorists have predicted a 5 % to 10 % deviation from the traditional theory due to quantum entanglement of H and D at room temperature. However, the NIOF experimental data agreed with the traditional theory to within the statistical precision of the data, which was of order 0.4 %.

  The first, successful, neutron interferometric measurement of the mass density of a thin polymer film (thickness <1 µm) was made at the NIOF in collaboration with the NIST Polymers Division. This new technique to measure thin film density is self-calibrating and does not require complex, mathematical modeling of the makeup or surface profile of the film.

  The capabilities of the NIOF have been augmented by the addition of a transmission neutron polarizer and a RF gradient spin flipper. This polarizer provides the NIOF with neutron polarization in excess of 98 %. This new capability was exploited in an experiment that directly demonstrated the 4π spin rotation symmetry of the neutron wave function under space rotation. This experiment was unique in that the neutron guide field was gently rotated by 180°, allowing the neutron spin direction to adiabatically follow the field. The measured phase shift is strictly due to the rotation of the spin direction since it is independent of both the strength of the magnetic field and the magnitude of the neutron magnetic moment.

  A new, neutron diffraction, radiographic and tomographic, imaging setup has been constructed, installed, and successfully tested. Along with this new addition to the NIOF there exists a 2-dimensional, high resolution, CCD, neutron detector. In addition to radiography, many other types of diffraction imaging experiments were performed with this setup. Researchers from Exxon exploited this facility to study hydrogen distribution in operating fuel cells. Researchers from the U. Innsbruck, Austria, studied the diffraction of neutrons (λ=0.235 nm) from macroscopic objects ~0.1 mm in size. Finally, the Neutron Phase Contrast Imaging technique was demonstrated by a high school student, resulting in his recognition as a semi-finalist in the 57^th Westinghouse Science Talent Search. (M. Arif, D. Jacobson, A. Ioffe, P. Huffman, T. Gentile, and A. Thompson)
• Polarized ³He for Neutron Spin Filters and MRI Applications. The primary focus of the polarized ³He program is the development and application of neutron polarizers for both condensed matter and fundamental physics. As a spin-off of this technology, the program also includes collaboration with medical researchers in polarized gas magnetic resonance imaging (MRI). A unique feature of the ³He effort is the concurrent development of the spin-exchange and metastability-exchange optical pumping methods. These two methods have different strengths and weaknesses, and pursuit of both allows for a versatile approach to the needs of different applications.

  Neutron polarizer tests have been performed with a compact neutron polarizer test facility located on the new NG6 monochromatic beamline. This apparatus has a laser system for on-line, spin exchange, optical pumping of cells, whereas cells polarized by the metastable method are presently transported from another lab to the test facility. For the metastable method, polarized gas is compressed using a modified diaphragm pump.

  The first tests of polarizing neutrons have been performed on this apparatus. A spin-exchange cell produced 88 % neutron polarization of the NG6M beam with 8.1 % transmission. For a metastable-exchange cell, the results were 67 % neutron polarization with 18.5 % transmission. For both methods, the ³He polarization was 45 %; the difference in the results for neutron polarization and transmission are related to the "thickness" of gas.

  In collaboration with C. Glinka, J. Barker, B. Hammouda, and J. Lynn of the NIST Center for Neutron Research, we have performed the first application at a U.S. neutron facility of polarized ³He to condensed matter research. The purpose of this test was to evaluate the use of a ³He-based polarization analyzer in a small angle neutron scattering (SANS) apparatus. Conventional neutron polarizers are not practical for this goal because of the relatively high divergence of the scattered beam. The experiment successfully demonstrated the separation of coherent from incoherent scattering using polarization analysis, which had not previously been performed in SANS experiments.

  We have performed collaborative experiments in polarized gas MRI with the U. Pennsylvania and the U. Virginia. Until our recent work, there had been no application of the metastable-exchange method to polarized gas MRI. We have obtained four lung images, three of patients with lung disease, by using the compression apparatus to polarize ³He gas at NIST, and then transporting the gas to U. Pennsylvania and U. Virginia. (T. Gentile, A. Thompson, G. Jones)

• Asymmetries of the Weak Interaction and the Neutron Lifetime. During FY98, two different neutron lifetime experiments, one a NIST-led, cold neutron beam experiment, the other a Harvard-led, ultra cold neutron (UCN) experiment, were conducted on our polychromatic neutron beam at the NIST Cold Neutron Research Facility. On our adjacent monochromatic neutron beam, work was begun to calibrate, using a lithium calorimeter, the efficiency of the neutron fluence monitor belonging to the neutron lifetime experiment.

  The NIST lifetime experiment utilizes a continuous, cold, neutron beam, a Penning trap to catch decay protons, and a thin foil, neutron fluence monitor to measure the lifetime. During FY98, 11 series of runs were completed. The statistical uncertainty achieved in those runs was an impressive 1.9 seconds. Nevertheless, our greatest success came in a better understanding of the systematic errors that have influenced results in the past. Major problems affecting stability and reliability of voltages in the Penning trap were identified and eliminated. Proton backscattering from the solid state proton detector is another major concern in this experiment. A correction is possible only when the dead layer of the detector has been characterized.

  The UCN lifetime experiment exploits three-dimensional, magnetic trapping of UCN produced by inelastic scattering of cold neutrons in a reservoir of superfluid ⁴He. As the trapped neutrons decay by beta emission, the energetic electrons generate scintillations in the liquid He that can be detected with nearly 100 % efficiency. A measurement of the scintillation rate as a function of time is used to determine the neutron beta-decay lifetime. The apparatus was moved onto the beamline late in 1997. Since then two surprisingly strong sources of background have been studied: a constant background rate and a time dependent rate coming from a trapped species other than neutrons. Work is presently underway to remove these signals. The constant background will be reduced by using additional neutron shielding, muon veto counting, and coincidence between two photomultiplier tubes. Indications are that the coincidence method of running will also reduce the trapped signal to acceptable levels. (M.S. Dewey, J. Adams, M. Arif, T. Gentile, D. Gilliam, P. Huffman, D. Jacobson, G. Jones, J. Nico, A. Thompson, and F. Wietfeldt).
• Neutron Dosimetry for Reactor Safety Assessment. The Neutron Interactions and Dosimetry Group provides metrology services and research studies on neutron metrology to the USNRC Office of Regulatory Research in support of their program in nuclear reactor safety assessment. We are presently completing a multinational, round-robin intercomparison of fissionable, neutron fluence monitors of a design currently employed by the nuclear industry for measuring the RPV fast neutron fluence. In particular, the fission reaction in ²³⁷Np fissionable dosimeters is especially useful because it monitors a significantly larger portion of the neutron damage spectrum than do other dosimetry materials. However, several complicating factors (e.g., the presence of a substantial ²³³Pa equilibrium activity) contribute to considerable differences observed in the results for the fast-neutron fluence obtained from neptunium dosimeters, as compared to other dosimeters. From these tests we will be able to specify details for the accurate use of ²³⁷Np as a neutron damage monitor. As a complementary study, a round robin test of ⁹³Nb dosimeters is planned for 1999. (J. Adams and D. Gilliam)
• Neutron Cross Section Standards. NIST 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 has taken place through participation in the U.S. Cross Section Evaluation Working Group and two international committees, the International Nuclear Data Committee and the Nuclear Energy Agency Nuclear Science Committee. 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.

  NIST 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. Preliminary results indicate differences with the most recent U.S. evaluation of this angular distribution. This work was initiated as a result of concern about that evaluation expressed by European standards groups. The H(n,n) angular distribution is one of the most widely used cross section standards. Also, plans are being made for a new measurement of the ¹⁰B total cross section at our new, monochromatic, neutron beam facility on NG6. This work will support efforts to improve the ¹⁰B(n,αγ) standard at low neutron energies. (A. Carlson)

• Dissemination of National Standards of Radioactivity. The Radioactivity Group disseminated the national standards of radioactivity as follows: 1) over 500 radioactivity Standard Reference Materials (SRMs) were provided to customers; 2) over 200 comparative measurements and reports of traceability were provided to federal regulatory agencies, radiopharmaceutical manufacturers, commercial suppliers of calibration sources and services, and the nuclear-power industry. Industrial steering committees guided the work of four research associates in cooperative testing programs; and 3) over 60 calibrations of customer sources were provided. (L.L. Lucas, J.C. Cessna, and L.R. Karam)
• Glow-Discharge Resonance Ionization Mass Spectrometry (RIMS). Work continued on the development of a glow-discharge initiated, mass spectrometer system that would permit the direct, compositional analysis of soils and sediments for radioactive and non-radioactive elements. The mass spectrometer was upgraded with improvements in the vacuum systems, improvements in the ion collection systems, renovation of the Faraday Cup insertion and control system, purchase and installation of several needed electronic components, and development of software programs for the automatic manipulation of various physical parameters. Tests were performed with the glow-discharge source aimed at maximizing the neutrals-to-ions ratio for cesium. The response of the output to changes in the glow-discharge current, the internal pressure of the argon gas in the source, and source construction and configuration were examined and compared with theoretical expectations. Much work was performed to obtain the optimal setup for resonance ionization with the TiSapph coupled with an Ar ion laser for the ionization stage. The purchase of appropriate optical parts enabled optimal focusing and alignment of the lasers. After these system improvements were made, the effort was turned to developing a RIS ion beam. Cesium was chosen as a first source element partly because of its low ionization potential, which could lead to less demanding proof of principle and, hence, the exploration of optimal conditions on critical parameters. Initially, a RIS response using a thermal ionization source was obtained using a double filament that allowed vaporization of the source material at low enough temperatures to remove mainly the ion component and, at the same time, allow large amounts of neutrals to be produced. Subsequently, a RIMS beam was produced with the glow-discharge source. Work is now underway to optimize the sensitivity and selectivity of this process. (L.R. Karam, J.M.R. Hutchinson, and L. Pibida)
• Environmental Radioactivity SRMs for Radiochemical Metrology. Over the past decades, on the order of 10¹⁵ Bq of waste have been stored in the oceans by the nuclear countries. Potential contamination by leaking nuclear waste, including the 17 sunken Russian nuclear submarines in the Arctic Ocean, has caused worldwide concern for the exosphere and commercial food chain. The determination of mBq/g levels of radioactivity in ocean sediments is a difficult task. The NIST Ocean Sediment Standard Reference Material (SRM 4357) now serves as the basis for all measurement assurance programs, methods verification procedures, and data comparisons. This SRM was developed using a composite of 0.5 % contaminated Irish Sea sediment and 99.5 % Chesapeake Bay sediment by weight. Ten radionuclides, including ⁴⁰K, ⁹⁰Sr, ¹³⁷Cs, ²²⁶Ra, ²²⁸Ra, ²²⁸Th, ²³⁰Th, ²³²Th, ²³⁸Pu, and ^239-240Pu, were certified through a comparison study among 20 international laboratories. The mean values were reported for an additional 11 uncertified radionuclides: ¹²⁹I, ¹⁵⁵Eu, ²¹⁰Po, ²¹⁰Pb, ²¹²Pb, ²¹⁴Bi, ²³⁴U, ²³⁵U, ²³⁸U, ²³⁷Np, and ²⁴¹Am. Interestingly, the measurement results for the primordial radionuclides were distributed normally, while those for the anthropogenic radionuclides were best described by Weibull distributions. SRM 4357 will be followed by additional low-level radioactivity natural matrix SRMs. The Ashed Bone matrix was certified in 1998 and will be followed by Rocky Flats soil II (1999) and the Japan, White, and Irish Seas composite Shellfish. (Z.C. Lin and K.G.W. Inn)
• Radionuclide Speciation in Soils and Sediments. Potentially, billions of dollars of remediation costs could be saved and social concerns could be addressed with reliable and interpretable information on the chemical forms (speciation) of the nuclides and their distribution in the environment. The Ionizing Radiation Division will be developing a new series of environmental SRMs for radionuclide speciation. As a first step towards this goal, the Division, in collaboration with Florida State University, is developing a phase-selective, standard, extraction protocol. The future, environmental, radionuclide SRMs will be characterized for radio-plutonium, uranium, thorium, strontium, and cesium species and will be used by those making decisions for risk assessment, selection of cost-effective, mitigation strategies and technologies, and developing and executing long-term monitoring. The effects of critical variables (time, temperature and reagent concentration) of the leaching protocol are being evaluated for selective dissolution of the exchangeable, carbonate, reducible metal oxide, organic, and acid leachable phases. Once the protocol is documented and pilot tested, a network of collaborations will be developed for the future round-robin testing of the protocol and interlaboratory comparisons for the certification of the future SRMs. (K.G.W. Inn).
• ²³⁹Pu Intercomparison of ICP-MS, TIMS and FTA at µBq Levels.. The goal of this project was to evaluate the state-of-the-art accuracy (precision, and minimum detection amount) for ²³⁹Pu in artificial urine measurements by inductively-coupled plasma, thermal ionization, mass spectrometry and fission track analysis in the range of 18 to 278 µBq/sample (approximately 3 million atoms of plutonium per 200 g sample). The plan for diluting NIST ²³⁹Pu SRM from kBq/kg to µBq/kg in artificial urine was developed by NIST and the Yankee Atomic Environmental Lab. With careful selection of reagents, labware, and high precision metrological techniques, the test materials were made, their concentrations confirmed, and samples sent to participating laboratories. Analysis of the data indicated that ICP-MS currently is capable of making quantitative measurements of ²³⁹Pu from urine down to the 18 µBq/sample level within an uncertainty of 20 %. Some potential, technical difficulties by some of the participating laboratories include: 1) data transcription; 2) sample tracking; 3) estimation of limits of detection; 4) statistical control over analytical processes; and 5) control of analytical background. A second study is being planned to include interferences in order to assess the technologies under more realistic conditions. (K.G.W. Inn)
• Experimental Determination of Dose Calibrator Settings for ¹⁸⁸Re. Rhenium-188 is proving to be one of the most versatile of the radionuclides currently being investigated for use in nuclear medicine. Among the applications being studied for ¹⁸⁸Re are bone pain palliation therapy, radioimmunotherapy, radiation synovectomy, and, more recently, intravascular brachytherapy (IVBT) for the prevention of coronary restenosis following balloon angioplasty. Radioassays are normally performed in the clinic using re-entrant well ionization chambers ("dose calibrators") using a dial setting, or "calibration factor," recommended by the manufacturer. Accurate measurements of radionuclides in the clinic or radiopharmacy therefore demand that the correct calibration factor be applied. The dose calibrator settings for the medical radionuclide ¹⁸⁸Re (as ¹⁸⁸ReO₄^-) were experimentally determined using solution sources prepared and calibrated by the Radioactivity Group. These new settings result in activity readings 28 % to 30 % lower than those obtained using the manufacturer's recommended setting. This discrepancy most likely results from an underestimation of the total radiation yield from ¹⁸⁸Re decay by the manufacturer when calculating the dose calibrator response to derive the dial setting. This study emphasizes the need for experimental determinations of dose calibrator settings in the geometry in which the measurements will be performed. (B.E. Zimmerman and J.T. Cessna)
• A New Technique for Measuring Contained Activity in Balloon Catheters for Use in Intravascular Brachytherapy: Radioassays and Film Dosimetry of ¹³³Xe. Accurate dosimetry of liquid- or gas-filled balloon catheters for use in intravascular brachytherapy requires accurate knowledge of the amount of contained activity. However, assumptions made in estimating this quantity often lead to serious errors. The Radioactivity Group has developed a technique to make direct measurements of contained activity in balloon catheters using a commercially-available dose calibrator with a modified shielding liner. Xenon-133 was used as a first test, since a gas is expected to provide the most homogeneous distribution within the balloon, without concern for air bubbles that might be present in liquid sources. A special shielding insert was constructed that would leave the balloon exposed, while shielding the dose calibrator chamber from radiation in the catheter. The dose calibrator was calibrated for ¹³³Xe in the appropriate geometries at NIST. The entire assembly was then tested by assaying balloon catheters (3 mm diameter × 20 mm length or 3.5 mm diameter × 20 mm length) for the amount of contained ¹³³Xe. GAFChromic^TM film measurements were performed using balloon catheters that had just been assayed for the amount of contained activity of ¹³³Xe and the results compared to Monte Carlo calculations. Because of the low energy of the ¹³³Xe radiation, many source inhomogeneities were evident, leading to large (~50 %) uncertainties. Despite this, agreement with Monte Carlo calculations is good and should improve with higher-energy beta-emitters. (B.E. Zimmerman, M.P. Unterweger, C.G. Soares, and S.M. Seltzer, and R.C. Chan [WHC])
• Approaches in Intravascular Brachytherapy. We developed a system of entrapping ^99mTc within liposomes of various charged (positive, negative or neutral) membranes to be used as a possible, technetium-packaging unit for intravascular irradiation (IVBT) after balloon angioplasty (using the Infiltrator^TM catheter system). After preliminary labeling of liposomes showed the feasibility of this approach, labeling of liposomes with ^99mTc at the Washington Hospital Center (WHC) was performed. Labeled liposomes were injected into pig models and a time course was followed over three hours for persistence of liposomes at artery sites (both with and without injury). Fuji imaging of excised arteries (with free ^99mTc and ^99mTc-labeled liposomes) showed that ^99mTc-labeled liposomes persisted longer at the application site than free ^99mTc. We have demonstrated that liposome encapsulation of ^99mTc is highly efficient at volumes up to 1 mL and that the liposomes are stable (do not leak) under in vivo types of conditions (in presence of serum) although stannous chloride is necessary for efficient encapsulation. Work has begun to determine the feasibility of using other ^99mTc-labelled compounds (Sestamibi^TM, Ceretec^TM) in the same system. (L.R. Karam, B.E. Zimmerman, with R. Waksman and R. Chan [WHC])
• Determination of Calibration Factors for the Nondestructive Assay of Pure Beta-emitting Brachytherapy Sources. Ionization-chamber calibration factors have been determined for two, commercially-manufactured, intravascular, brachytherapy sources: a TiNi-encapsulated ³²P source developed by Guidant Intravascular Intervention (Houston, TX), and a stainless-steel encapsulated ⁹⁰Sr-⁹⁰Y developed by Bebig Isopentechnic und Umweltdiagnostik GmbH (Berlin, Germany) in collaboration with the Novoste Corp. (Norcross, GA). The calibration factor for the former was derived from ionization current measurements with a Capintec CRC-12 ("dose calibrator") which is the nuclear-medicine community's de facto standard instrument, followed by very quantitative, destructive assays of the ³²P content in the sources. Similarly, for the former sources, NIST "4πγ chamber A" calibration factors were determined for both bare ceramic and stainless-steel encapsulated source configurations. (R. Collé)
• Development of Procedures for the Chemical Digestion and Radionuclidic Assay of Encapsulated, Pure Beta-Emitting, Intravascular Brachytherapy Sources. These methods have been applied to two very different types of sources: a polymer-based, ³²P-containing source with TiNi encapsulation; and a ceramic-based, ⁹⁰Sr-⁹⁰Y source with stainless-steel encapsulation. The internal compositions for both types of sources had constituents that were chemically impervious. The assays involved partial dissolutions (or extractions) followed by 4πβ liquid scintillation (LS) spectrometry (with ³H-standard efficiency tracing) of the resulting solutions. The procedures for the former included provisions for accounting for all possible losses of ³²P in the digestion procedure (based on radiochemical tracing experiments), for any unrecovered activity in the remaining source material, and for any residual activity in the solution- and source-handling tools. The procedures for the latter source consisted of extracting a fraction of the ⁹⁰Sr activity from the ceramic-like material for LS assay, and determining the fraction of unextracted activity by before and after ionization current measurements on the extracted source material. The uncertainties in the assays were typically 2 % to 4 % for two standard uncertainty intervals. These destructive assays were required for relating radiochromic-film measurements of the absorbed dose spatial distributions for the sources to theoretic dose modeling, and for establishing calibration factors for subsequent, non-destructive, radionuclidic measurements on the sources. (R. Collé)
• New "Primary" Standardizations of ²²⁶Ra and ²²²Rn. All extant activity standards and calibrations for ²²⁶Ra and ²²²Rn are based on comparative measurements against an artifact radium mass standard (namely the international 1935 Hönigschmid standards and its derivatives). Efforts are currently underway to remove this dependence and to develop and perform primary standardizations for both ²²⁶Ra and the ²²²Rn subseries based on 4παβ LS spectrometry with ³H-standard efficiency tracing. We have successfully employed this new standardization technique to ²²²Rn, such as for the calibration of the NIST radon-in-water standard generator and for measurements of the polyethylene-encapsulated ²²⁶Ra-solution emanation standards (SRM 4968). The techniques are now being extended to the entire ²²⁶Ra decay series. Preliminary findings have demonstrated that the method will be able to adequately resolve the ²¹⁰Pb subseries (i.e., from ²²⁶Ra and the ²²²Rn subseries) in even aged radium solutions. The ability to perform such a resolution is significant in that it will allow direct intercomparisons between the various ²²⁶Ra standards issued by NIST over the past 50 years, irrespective of their present degree of radioactive equilibrium between ²²⁶Ra and ²¹⁰Pb. (R. Collé)
• Standardization of New Tritiated-Water Standards. The calibration of the new, tritiated-water standard, SRM 4927F, by internal-gas counting and liquid scintillation counting is in progress. This standard and the lower activity SRM 4962F will be used extensively worldwide for ground-water hydrology. The standards and their predecessors are and will be used as the basis for a uniform measurement scale for environmental tritium studies. The NIST tritiated-water standards are also used extensively in liquid scintillation counting. A redetermination of the half-life of tritium is also being done. An accurate value for the half-life is very important in extending the useful lifetime of the tritium standards, i.e., the decay of the certified value is not significantly effected by the uncertainty in the half-life value. (L.L. Lucas and M.P. Unterweger)
• Titanium-44 Half-life. The 1157 keV gamma ray resulting from the decay of ⁴⁴Ti (and its 3.93 hour daughter ⁴⁴Sc) has been observed from the supernova remnant, Cassiopeia A. The flux of this gamma ray and the ⁴⁴Ti half-life would yield a production abundance of the isotope at the time of the explosion. This is an important "data point" for the test of theoretical models of supernova events. Historically, the measured ⁴⁴Ti half-life values have ranged from 39 years to 66 years, much too uncertain to be useful.

  The Radioactivity Group has had ⁴⁴Ti sources since 1978. The emission rate of the 1157 keV gamma ray from these sources has been measured from time to time with the Group's carefully calibrated, gamma-ray detector systems. These emission rate measurements were carefully assembled and documented. Over the last year, additional emission rate measurements have been made and a half-life (60.7 years, with an estimated standard error of 1.2 years) determined. (F.J. Schima, F.E. Wietfeldt, and B.M. Coursey)

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