Ionizing Radiation Division

Division Overview   |   Program Directions   |   Major Technical Highlights

Major Technical Highlights

• Calibration of Low-Energy Photon Brachytherapy Sources. Small radioactive "seed" sources used in prostate brachytherapy, containing either the radionuclide ¹⁰³Pd or ¹²⁵I, are calibrated in terms of air-kerma strength using the NIST Wide-Angle Free-Air Chamber (WAFAC). The WAFAC is an automated, free-air ionization chamber with a variable volume that directly realizes air kerma (or exposure). Seeds of twenty different designs from fourteen manufacturers have been calibrated using the WAFAC. On-site characterization at seed manufacturing plants for quality control, as well as at therapy clinics for treatment planning, relies on well-ionization-chamber measurements. Following the primary-standard measurement of air-kerma strength, the responses of several well-ionization-chambers to the various seed sources are determined. Such response factors enable well-ionization-chambers to be employed at therapy clinics for verification of seed air-kerma strength, a quantity used to calculate dose rates to ensure effective treatment planning.

  To verify that seeds of a given design calibrated at NIST are representative of the majority of those calibrated in the past, several additional tests have been implemented. Mapping the distribution of radioactive material within a seed using radiochromic-film contact exposures as well as angular x-ray emission measurements enable characterization of the degree of anisotropy present in seed emissions. The relative response of calibration instruments has been observed to depend on such anisotropy. Data from two Accredited Dosimetry Calibration Laboratories (ADCLs) and the seed manufacturer, in addition to the results of NIST measurements, are compiled and checked as a function of time to ensure the continuous validity of the calibration traceability chain from NIST to ADCLs and manufacturers and to the clinic. (M.G. Mitch, P.J. Lamperti, S.M. Seltzer, and B.M. Coursey)

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

• 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, also used to characterize sources in the cylindrical geometry for transaxial uniformity. In addition, irradiations of planar sheets of film at various depths in water-equivalent plastic are 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. The equipment used for these studies is augmented with two micro-scintillator detection systems, two automated three-dimensional water-tank scanning systems, various well-ionization-chambers, and two small fixed-volume ionization chambers.

  Collaborations were continued between NIST and Guidant, Inc., for dosimetry of their ³²P wires, and over 350 of their well-ionization-chambers were calibrated. Collaborations also continued with Novoste, Inc., for dosimetry of ⁹⁰Sr/Y seed trains, and a measurement-assurance proficiency test was performed with two AAPM secondary laboratories for these sources. Collaborations were inaugurated with Radiovascular for the calibration of their ³²P shell source, and with Medtronic and with Xoft for the calibration of their miniaturized x-ray sources. Coupled with the continuing collaboration with Photoelectron Corporation for the calibration of their x-ray probes, it is anticipated that these collaborations will lead to a new standard for low-energy photon absorbed dose measurement.

  A new collecting electrode with a much better defined collecting area was designed for the NIST extrapolation chamber. Successful construction of this electrode is expected to reduce the current large uncertainty in the NIST calibration of beta-particle brachytherapy sources. (C.G. Soares and M.G. Mitch)

• Beta-Particle-Emitting Ophthalmic Applicator Calibration Service. With the advent of the calibration service for ophthalmic applicators at the University of Wisconsin Accredited Dosimetry Calibration Laboratory, the role of NIST in these routine calibrations has diminished considerably. NIST's role in this field will be more geared towards providing transfer standard sources and fields both to secondary laboratories and source manufacturers. The results of the major international intercomparison of ophthalmic applicator dosimetry, which included the dosimetry of both curved and flat sources of ⁹⁰Sr/Y and ¹⁰⁶Ru/Rh was published in Medical Physics and are included in the ICRU report on beta particles for medical applications. A new high-sensitivity electrometer has been purchased to allow measurement of lower activity sources with this system. An intercomparison on high-dose-rate beta-particle dosimetry between NIST and PTB was begun using a PTB secondary standard. (C.G. Soares and M.G. Mitch)

• Development of a New X-ray kV Calibration Service. The NIST Ionizing Radiation Division is in the process of developing a new x-ray beam kV calibration service. The plan is to offer calibrations of devices used to measure the accelerating potential applied to x-ray generators. A precision voltage divider, calibrated by EEEL and traceable to the NIST primary standard for voltage, will be installed on the x-ray generators used in the NIST x-ray beam calibration range, covering constant accelerating potentials up to 300 kV. Customers can submit devices to be irradiated in NIST beams of their choice. Such devices usually are based on some indirect measurement, such as the differences in transmission among x-ray filters of different compositions and/or thicknesses. The results from the device will be reported along with the measured accelerating potential, with an accuracy anticipated to be typically 1 kV or better at the 95 % confidence level. (C.M. O'Brien)

• 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 for attenuation coefficients for x rays and gamma rays, including cross sections for Compton and Rayleigh scattering, atomic photo-effect, and electron-positron pair production, as well as on energy-transfer, energy-absorption and related coefficients needed in radiation dosimetry. Work on charged-particle cross sections and radiation transport data includes 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. A new evaluation of cross sections for the elastic scattering of electrons and positrons by neutral atoms has recently been completed. Current efforts are focused on the development of a suite of computer programs to evaluate the differential, total, and energy-loss cross sections for Compton scattering of photons from electrons bound in atoms.

  The quality of the work of the Data Center is reflected 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. Our data and Monte Carlo transport algorithms are incorporated into the most widely used general-purpose radiation transport codes. We have used two Monte Carlo codes for calculations of the wall-correction factors for the NIST graphite-wall cavity-ionization chambers that serve as the U.S. standard for gamma-ray air kerma and exposure. The results will be used to modify these U.S. standards in the near future, in concert with many other national primary standards laboratories around the world. (S.M. Seltzer, P.M. Bergstrom, J.H. Hubbell, and M.J. Berger)

• Internet-Based Calibration Services for the Radiation-Processing Industry. An Internet-based calibration service has been built for fast remote calibration of high-dose radiation sources against the U.S. national standard gamma-radiation source. The new service will electronically deliver calibration results to the industry customer, on-demand and at a lower cost. The industrial site will have a dosimeter reader and an Internet link to the NIST server. When a calibration is requested, the industrial site will connect to the NIST calibration web site and initiate the process. In order to ensure a reliable dose assessment, the NIST server will lockout the local user and fully control the measurements remotely. The remote control process will include the spectrometer calibration, setting of readout parameters, real-time spectral acquisition and validation. The post-measurement evaluation process involves a rigorous verification of the quality of the acquired spectra, and dose calculations from calibration curves maintained at NIST. The system has been demonstrated at national and international dosimetry symposia. Current efforts include further refinements in software/hardware compatibility and development work with future industrial partners. (M.F. Desrosiers, V. Nagy, and J.M. Puhl)

• Validation of the EPR Method for Tooth-Enamel Dosimetry. Knowledge is required on dose-effect relationships for radiation-induced stochastic and deterministic effects. Therefore, the acquisition of dosimetric effects from populations with chronic exposure is of special interest (Chernobyl, Techa River, etc.). 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 is essential before these data can be used reliably in epidemiological studies from which recommendations are made for occupational exposures. Significant effort has gone into developing sound protocols for the preparation of tooth-issue sample for EPR analysis, and into the analysis and interpretation of the EPR results. Results have been obtained for members of the Techa riverside population from 1945-1949, exposed to radioactive waste from the Mayak nuclear weapon plant near the Techa River, Urals, Russia. (A.A. Romanukha, M.F. Desrosiers, and V. Nagy)

• 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. The second-generation beta-particle secondary standard system (BSS2), which includes the isotope ⁸⁵Kr, is now utilized routinely for calibrations and research into standard extrapolation-chamber data-handling techniques. The sources were calibrated both at the Physikalisch Technische Bundesanstalt (PTB) and at NIST, allowing a direct intercomparison of calibrations. The systems are also being used for the dosimetry characterization of a photo-stimulatable-luminescence-phosphor imaging system. The standardized techniques developed at PTB and NIST are now included in an International Organization for Standardization (ISO) draft standard and are being implemented in the NIST calibration service. A new high-sensitivity electrometer has been purchased to replace the 15-year old high-sensitivity electrometer currently being used for these measurements. (C.G. Soares and M.G. Mitch)

• Gamma-Ray Sources for Radiation-Protection Calibrations. Five gamma-ray sources are used for calibration of instruments and passive dosimeters, in terms of air-kerma and exposure, to support protection-level measurements in the U.S. The calibrations are directly traceable to measurements with the national primary standard for gamma-ray exposure, graphite-cavity ionization chambers. The ranges provide a wide range of air-kerma rates. Two ¹³⁷Cs sources provide air-kerma rates from 4.5 mGy/h to 110 mGy/h; a third ¹³⁷Cs source provides air-kerma rates of 2.3 Gy/h and 3.6 Gy/h; and two ⁶⁰Co sources provide air-kerma rates from 0.25 mGy/h to 5.4 mGy/h. Programs of regular, calibrated exposures of thermoluminescent dosimeters provide direct support for the worker-protection measurement programs of a number of agencies, including the U.S. Navy. NIST standards are also disseminated through a number of secondary instrument-calibration laboratories to provide traceability of protection-level measurements. Work is underway to develop a calibration capability at much lower air-kerma rates, down to 5 μGy/hr. (R. Minniti, P.J. Lamperti, and J. Shobe)

• Fundamental Neutron Physics –User Facility and In-house Program. The Neutron Interactions and Dosimetry (NID)Group supports a national user facility for research in fundamental neutron physics. Our collaborators and customers represent institutions in at least 11 different states and 7 other countries. (J.S. Nico, P.R. Huffman, M.S. Dewey, T.R. Gentile, A.K. Komives, and A.K. Thompson)

  Figure 1. The tiled assembly of intercalated graphite is seen as it is held by gloved fingers within an inert atmosphere chamber.

  Two separate measurements of the beta-decay lifetime of the neutron have been our primary focus during the past year. In one experiment, the decay rate of magnetically trapped ultracold neutrons (UCN) will be measured to extract the neutron lifetime. This experiment is led by Harvard University and recently the techniques were demonstrated in a proof-of-principle measurement. The apparatus has been upgraded in preparation for a lifetime measurement. The major improvement this year was the development, fabrication and testing of a neutron monochromator made of potassium-intercalated graphite (KC24). This monochromator allows neutrons with wavelengths near 0.9 nm to be Bragg reflected from the primary beam with greater than 85% reflectivity, allowing for a significant suppression of background events from neutrons with other wavelengths. The monochromator assembly consists of nine pieces of KC24 that must be encapsulated in an inert atmosphere to minimize the chemical reactivity of the potassium with air (see fig. 1). The assembled monochromator has been completely characterized and is performing well above initial expectations. Also note that the 0.9 nm wavelength is about a factor of two longer than that of any other monochromatic beam in the NCNR facility, and the NCNR management and material scientists are in the process of replicating this technology for instruments in the facility. (P.R. Huffman)

  The other lifetime experiment, which is led by NIST staff, measures the neutron decay rate in a beam passing through a well-defined volume. The key components are a Penning-trap to confine the decay protons and a high-precision neutron monitor to measure the average number of neutrons within the trap volume. The methods and associated systematic uncertainties in this experiment are completely different from that of the UCN lifetime experiment, so that consistent results from the two different types of experiments will constitute a very reliable determination of this important quantity. The result to be submitted for publication from this Penning-trap experiment is approximately n = 885 s ± 4 s. The four-second uncertainty should be reduced to ± 2 s once cross-calibrations can be performed among the high-precision neutron monitor and two other "black" neutron counters. The refinements in neutron counting from these calibrations are also of interest in verifying the neutron emission rate from the standard neutron source NBS-I, which is the basis for two of our calibration services and much of our neutron dosimetry work. (M.S. Dewey and J.S. Nico)
  		Figure 2. The diagram shows stages of intercalation within the hexagonal close-packed structure of crystalline graphite. The KC24 compound is denoted as a "2nd stage" compound because it has potassium in every other layer. The large spacing of the potassium layers produces a long wavelength monochromator.

• Neutron Interferometry and Optics Facility (NIOF) - User Facility and In-house Program. The NIOF is the other national user facility operated by the NID Group. Radiography and tomography services and research involve both academic and industrial customers and collaborators.

  Initial measurements were completed this year on the coherent neutron scattering lengths for hydrogen, deuterium, and ³He. Preparation of publications is still in progress, but the data analysis is essentially complete. The estimated uncertainties from these new measurements are about a factor of ten lower than those of the best preceding measurements. These data are of interest for theoretical studies of "few-body" interactions (6 quark, 9 quark, and 12 quark), and also for Los Alamos stockpile-stewardship calculations. All of these were measurements on gaseous samples at pressures of several atmospheres.

  A complete set of measurements was also made this year in an innovative approach to the measurement of the neutron-electron scattering length, by observing the large phase shifts as a thin silicon crystal was rotated through the Bragg angle within a neutron interferometer. The neutron-electron scattering length is related to the distribution of electric charge within the neutron and the mean square charge radius of the neutron (which has a net charge of zero). A precise value of this rather fundamental quantity is also of interest in the theory of few body nuclear systems and related precision measurements.

  Development of neutron radiography and tomography facilities is continuing, with work now concentrating on the BT-6 beam position, which has just been granted for this purpose by the NCNR management. Accomplishments this year included near real-time imaging of water formation in an operating fuel cell, a tomographic image of a fuel cell, and a radiograph showing lithium ion transport in a commercial battery. (M. Arif and D.L. Jacobson)

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

  The most important development this year was the production of large diameter cells with very long polarization lifetimes for both optical pumping methods. Spin-exchange optical pumping is most efficient in small volume, high pressure (3 bar to 10 bar) cells, primarily because of the reduced number of rubidium atoms and to a lesser degree because of better spectral overlap with broadband diode lasers. However, construction of the large-diameter glass cells needed for typical neutron beams, is difficult (and hazardous) at such pressures.

  Recently, we have tested spin-exchange cells at near-atmospheric pressure and found that with sufficient laser power, polarization comparable to that obtained in higher pressure cells can be obtained in these cells. Operation at such low pressure has an additional advantage: because of the reduced dipole-dipole relaxation at lower pressure, we have been able to achieve polarization lifetimes as long as one month, about two to three times longer than had ever been achieved for spin-exchange cells previously. In addition, the long lifetime permits higher polarization at a given laser power.

  Our low-pressure cells are expected to be employed as the polarizer for the n+p  d+ experiment, as well as having important utility for material science and other fundamental physics experiments. The success of our apparatus for the preparation of sealed spin-exchange cells has also led to the production of two refillable cells for the metastability-exchange method with 200 h lifetimes. Our development of lower pressure cells was an indirect outgrowth of tests of spectrally-narrowed high power diode lasers, developed by collaborators at the University of Wisconsin. (T.R. Gentile and A.K. Thompson)

• Neutron Dosimetry. Section III (neutron measurements) of the Consultative Committee on Ionizing Radiation (CCRI) is currently carrying out three international comparison exercises, and we are participating in all of them. NIST is leading a comparison of thermal neutron fluence measurements; the PTB is leading a comparison of fast neutron fluence measurements; and the National Physical Laboratory (NPL, Teddington) is leading a neutron source emission rate comparison.

  For the thermal neutron comparison, we prepared a transfer instrument system based on an active ¹⁰B ionization chamber. This system was used successfully for the comparison measurements at the NPL, but one problem with an electrical contact had to be resolved. The system was returned to NIST for a check on the integrity of the ¹⁰B deposits and will be sent on to China and Japan next year, if international shipping restrictions permit.

  Measurements of neutron inelastic scattering in steel have been made in collaboration with Ohio University and Penn. State. These measurements are needed to better understand data for nuclear reactor pressure vessel damage estimation. Initial measurements with a spherical shell of 4 cm thickness were recently reported at the Nuclear Data 2001 meeting in Tsukuba.

  Calibration services for radiation protection dosimetry and neutron source emission rate measurement continued at a fairly busy pace. (J.M. Adams, A.K. Thompson, J.S. Nico, and D.M. Gilliam)

• Nuclear Cross Section Standards. The neutron cross section standards are important since almost all cross sections are measured relative to them. Any improvement in a cross section standard leads to improvement in all measurements that have been or will be made relative to that standard. The NIST neutron cross section standards project has played a significant 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, detailed examination of the standards database, and pursuing the extension of the standards over a larger energy range.

  This work is taking place through participation in the U.S. Cross Section Evaluation Working Group (CSEWG) and two international committees, the International Atomic Energy Agency (IAEA) and the Nuclear Energy Agency Nuclear Science Committee (NEANSC). The NIST representative heads a standards Task Force of the CSEWG, coordinates a Subgroup on standards of the Working Party on International Evaluation Cooperation of the NEANSC, and was chairman of an IAEA Consultants’ Meeting on Improvement of the Standard Cross Sections. Final approval was recently obtained for an IAEA Coordinated Research Project which will provide resources to allow meetings of the contributors to the standards evaluation process. An objective is to complete the evaluation in time for the major international cross section evaluation projects to use the improved standards in forming new versions of their libraries.

  This NIST project has continued to maintain a limited experimental role in the measurements of the standards. This role has led to a new, NIST-LANL-Ohio University collaborative measurement of the H(n,n) angular distribution at Ohio University at 10 MeV neutron energy. The final results of the data indicate differences with the most recent U.S. evaluation of this angular distribution. The work was accepted for publication in the Physical Review. (A.D. Carlson and P.R. Huffman)

• Development of a Radiotherapy Nuclide Standard. In an effort to solve the problem of distributing a standard of the short-lived (t_1/2 = 17 h) nuclide, ¹⁸⁸Re, which is of interest in many areas of nuclear medicine, we have standardized solutions of ¹⁸⁸Re in equilibrium with its parent nuclide, ¹⁸⁸W. This will provide researchers the ability to calibrate their instruments for both of these radionuclides with a relatively long (t_1/2 = 64 d) source of activity. A series of ampoules was prepared from a stock solution and measured in several different types of ionization chambers in order to determine calibration factors for these instruments. Primary activity determinations were made using liquid scintillation (LS) counting with the CIEMAT/NIST ³H-standard efficiency tracing method. Using the data from the sources deemed to be stable, the activity of the stock solution was determined with an expanded (k = 2) uncertainty of 0.74 %. Calibration factors were determined for all of the ionization chambers in the standard NIST geometry of 5 mL of liquid in a flame-sealed ampoule. (B.E. Zimmerman, J.T. Cessna, and M.A. Millican)

• Calibration of Intravascular Brachytherapy Sources. Considerable work has continued on providing NIST-based activity calibrations for manufacturers of intravascular brachytherapy sources. In the past year, two sets of nondestructive ionization-chamber-based calibrations were performed for ³²P "hot-wall" angioplasty-balloon catheter sources that are manufactured by Radiance Medical Systems Inc. (Irvine, CA), and also for a set of seeds from the Novoste Corporation (Norcross, GA). The seeds are stainless-steel-encapsulated, ceramic-based ⁹⁰Sr-⁹⁰Y sources whose calibration factors were also derived from earlier destructive radionuclidic assays by NIST. (R. Collé)

  Destructive assays were also performed for a Belmont, California company, IsoStent, which has developed a stainless steel stent containing the beta emitter phosphorus-32. The stents were first intercompared in a NaI well crystal -detector, looking at the emitted bremsstrahlung. A subset of the stents were then slowly digested in a small volume of carrier solution, with the addition of a few drops of concentrated hydrofluoric acid. The expanded (k = 2) uncertainties on the activities were on the order of 1.1 % to 2.1 % for the digested stents and 1.5 % – 2.6 % for the undigested stents. (J.T. Cessna)

  The results of all of these calibrations were used by these companies for internal quality control and to transfer the NIST calibrations to other manufacturing facilities and to two calibration laboratories in Europe.

• Development of Radioactivity Standards for Alpha-Emitting Radiotherapy Nuclides. We are continuing to work with the National Institutes of Health PET Department and Clinical Center to develop a standard for a potential radiotherapy nuclide, the -emitter ²¹¹At. To date, a measurement protocol for determining contained activity has been developed using 4 (/EC) liquid scintillation counting with efficiency tracing, which will give an expanded (k = 2) uncertainity of about 1 %. A final set of experiments is planned that will determine a dose calibrator dial setting for this nuclide in at least one standard geometry. (B.E. Zimmerman, J.T. Cessna, and M.A. Millican)

• Development of ¹⁶⁶Ho-DOTMP Secondary Standard. The same decay scheme properties of the rare-earth nuclide ¹⁶⁶Ho that make it attractive as a therapeutic radionuclide also make it necessary to take care when measuring the radionuclide with an ionization chamber. In order to develop calibration factors for a variety of ionization chambers in specific, clinically relevant geometries, two sets of calibration experiments were performed on solutions of ¹⁶⁶Ho-DOTMP, which is the final the drug product form. These solutions were supplied by ABC Laboratories using materials from Missouri University Research Reactor (both located in Columbia, MO). NIST 5 mL ampoules were prepared for measurement on the NIST chamber "A," an NPL ionization chamber, the commercial dose calibrators residing at NIST, and commercial dose calibrators belonging to the companies involved in the calibration studies. Two different geometries at two different levels were also produced to determine calibration factors for the commercial dose calibrators. Standard samples of these geometries were also returned to the companies involved to check their in-house commercial dose calibrators. (J.T. Cessna, B.E. Zimmerman, M.P. Unterweger, L.R. Karam, and M.A. Millican)

• Cocktail Composition Effects on the Application of the CIEMAT/NIST Efficiency Tracing Method. Liquid scintillation (LS) counting continues to be the method preferred by most metrology laboratories for performing quantitative assays of solutions containing beta-emitting radionuclides. Under many circumstances, there is little need to be concerned about the composition of the analyte solution, as most changes in detection efficiency arising from chemical effects can be accounted for by efficiency tracing against an established standard. However, there can be cases in which the solution composition can affect the efficiency in ways that cannot be accounted for using efficiency tracing techniques. We have performed a series of experiments aimed at identifying composition variables that influence LS cocktail stability and subsequent assay results for solutions containing ¹⁷⁷Lu or ¹⁸⁸W. (B.E. Zimmerman and J.T. Cessna)

• Radionuclidic Microcalorimetry for Absolute Radioactivity Standardizations. This basic measurement capability has largely been re-established in the past year. Two different calorimeters have been put together and have been operating since October 2000; viz. a dual-compensated cryogenic calorimeter operating at a nominal 8 K (termed CAL hereafter) and a commercial isothermal microcalorimeter (IMC) operating at 303.5 K. (R. Collé and B.E. Zimmerman)

• Dissemination of National Standards of Radioactivity. The Radioactivity Group disseminated the National Standards of Radioactivity mainly through the sale of over 650 Radioactivity Standard Reference Materials (SRMs), over 200 comparative measurements and Reports of Traceability and over 50 calibrations of customer sources. (L.L. Lucas, J.T. Cessna, and L.R. Karam)

• Calibrations of Large-Area Beta Sources. Studies are continuing on the effects of beta-backscattering to develop a systematic method for determining the effective source thickness needed for relating the measured rate to activity. These determinations will be used to calibrate surface monitoring equipment in terms of radioactivity rather than emission rate. (M.P. Unterweger and P. Hodge)

• Reevaluation of the Half-Life of Tritium. A remeasurement of the half-life of tritium has also been done and a reevaluation of the half-life using all reported values has been published. An accurate value for the half-life is very important in extending the useful lifetime of the tritium standards. (L.L. Lucas and M.P. Unterweger)

• Holmium-166m. The large number of gamma rays, the wide energy range, and the long half life make ^166mHo a very desirable gamma-ray source for determining the detection efficiency of germanium detectors and for monitoring their long-term stability. High-purity stable ¹⁶⁵Ho was neutron irradiated to produce ^166mHo, which is now being calibrated in terms of activity and the gamma-ray emission probabilities and their uncertainties are being evaluated. (L.L. Lucas, B.E. Zimmerman, and L.R. Karam)

• Resonance Ionization Mass Spectrometry. RIMS has been evaluated for measuring ¹³⁵Cs/¹³⁷Cs isotopic ratios. Spectroscopic measurements of 6s ²S_1/2(F = 4) 6p ²P_3/2(F = 5) transition frequency shifts for ¹³⁵Cs and ¹³⁷Cs confirmed existing values and demonstrated that it is possible to perform such measurements on sub-picogram samples. Optical isotopic selectivity of ~ 10³ for both ¹³⁵Cs and ¹³⁷Cs against stable ¹³³Cs was observed and, when combined with a quadrupole mass spectrometer, overall selectivity of greater than 10⁹ was demonstrated. (L. Pibida, L.R. Karam, and J.M.R. Hutchinson)

• International Equivalence. As part of NIST's efforts in the international arena of measurement activities, the Radioactivity Group has prepared a listing of calibration capabilities for Appendix C (list of calibration and measurement capabilities, or CMC's) of the Mutual Recognition Arrangement and submitted it to our RMO. (L.R. Karam and L.L. Lucas)

• Mass Spectrometry for Environmental and Bioassay Measurements. Accurate and precise analyses of low-level Pu isotopes are critical for environmental, radiobioassay, and nuclear safeguard programs. By actively producing Pu ions through thermal ionization rather than passively detecting Pu alpha decay, mass spectrometry not only has a capability of detecting ~10⁶ Pu atoms but also determines its origin.

  The production of Pu ions that may ultimately determine the measurement sensitivity, accuracy, and precision depends directly on sample deposition method. To optimize the efficiency, stability, and duration of Pu ion production, three deposition methods including direct deposition, electroplating, and resin bead loading were evaluated. The characteristics affecting the Pu ion production were compared for each method at a few picograms of ²³⁹Pu and ²⁴⁰Pu. The study showed that the best sample loading method for analyzing low-level Pu is to electroplate Pu onto Re filament surface. (Z.C. Lin and K.G.W. Inn)

• Second Intercomparison Study for Detecting &$181;Bq of ²³⁹Pu in Urine by Atom-Counting Techniques. To support the DOE's Marshall Island Resettling Program, we are assisting the DOE in identifying the most promising analytical techniques capable of quantifying Pu in urine at or below a level of ~20 µBq/L. Techniques including ICP-MS, FAT, AMS, and TIMS were evaluated for ²³⁹Pu analysis under more realistic conditions by introducing environmental level of ²⁴⁰Pu and natural U into urine samples. Also, more blank samples, a total of 8, are included to provide a better blank value estimate for ²³⁹Pu. The test samples were prepared, verified, and sent to University of Utah, LLNL, and LANL for intercomparison measurement. The results from FAT, TIMS, and AMS have been evaluated. With completion of ICP-MS analysis, a full assessment of the four techniques will be reported. (Z.C. Lin and K.G.W. Inn)

• Radioanalytical Traceability. By working with the American National Standards Institute's nuclear instrumentation N42 and N13 committees to establish criteria for radioanalytical traceability, three standards including ANSI N42.23, ANSI N42.22, and ANSI N13.30 have been published. Each of these standards were developed through consensus participation among industrial, commercial, utility, federal, state, national laboratory representatives to strengthen the credibility of national radioanalytical programs. ANSI N42.23 envisions the accreditation of reference laboratories that participate directly in a traceability-testing program with NIST, technical document reviews, and on-site assessments. These reference laboratories serve as traceability link between NIST and the service laboratories through their Performance Testing (PT) programs. The ANSI N42.22 standard provides additional and more specific criteria for source manufacturers and those reference laboratories producing PT materials. The criteria for the acceptance of testing results is |V_R - V_N| < 3 × (_N² + _R²)^1/2. That is the absolute bias between the reported value, V_R, and the NIST value, V_N, shall be less than or equal to three times the total propagated NIST uncertainty, _N, and the reported uncertainty, _R. It is the right hand side of the equation defines the "traceability limit" to the claimed traceability. ANSI N13.30 defines the traceability testing criteria for radiobioassay performance testing programs. It is anticipated that traceability testing for the reference laboratories supporting all aspects of traceability testing will be fully implemented in the future. (K.G.W. Inn, Z.C. Lin, Z.Y. Wu, and C. McMahon)

• NIST Radiochemistry Intercomparison Program (NRIP). The program was implemented to assure the traceability of low-level radioanalytical measurements as defined in ANSI N42.22. Four years of performance data from fifteen participants were evaluated for ⁹⁰Sr, ²³⁸U, ²³⁸Pu, ²⁴⁰Pu, and ²⁴¹Am at 0.03 Bq to 0.3 Bq per sample in water, air-filters, soil, synthetic urine, and synthetic feces. The relative mean difference from the NIST massic activity values across all matrices and radionuclides ranged from –1 % to –6 % with standard deviation of the means from 2 % to 7 %. Analysis of variance indicated that the analytical methods and sample matrix are the main factors causing bias in ²⁴¹Am and ⁹⁰Sr analyses, and ²³⁸U analysis, respectively. About 90 % of the results were acceptable per the ANSI N42.22 criteria and an improvement in the participant's performance has been observed since the inception of the NRIP program. The majority of the failures to pass the ANSI N42.22 criteria were due to unrealistically small estimates of measurement uncertainties. Workshops have organized in assisting the NRIP participants with estimating their measurement uncertainties. (K.G.W. Inn, Z.Y. Wu, C. McMahon, and Z. Lin)