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Current Directions

• 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) that serves as the national standard for this quantity. The WAFAC is an automated, free-air ionization chamber with a variable volume, allowing corrections to be made for passage of the beam through non-air-equivalent electrodes. Seeds of seventeen different designs from twelve manufacturers have been calibrated using the WAFAC. Quantitative information on the spectrum of emitted photons, obtained for a seed from each submitted batch using HPGe spectroscopy, is used for quality control and to refine correction factors needed for WAFAC measurements. Following the primary-standard measurement of air-kerma strength, the responses of several well-ionization chambers to the various seed sources are determined. The ratio of well-chamber response to air-kerma strength yields a response factor for the well-ionization chamber for a given seed type. A new NIST ionization chamber, specifically designed to characterize prostate brachytherapy seeds more efficiently than commercially available chambers, is currently being tested.

• Internet-Based Calibration Services for the Radiation-Processing Industry. In cooperation with industry, academia and other government agencies, an Internet-based system is being constructed for fast remote calibration of high-dose radiation sources against the U.S. national standard gamma-radiation source. The new service will deliver immediate calibration results to the industry 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. 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.

• 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 data from populations with chronic exposure is of special interest (e.g., 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 tissue sample for EPR analysis, and into the analysis and interpretation of the EPR results. The long-term objectives of this work are to validate the EPR dose assessment methods for enamel and dentin.

• NIST Mammography and Conventional X-Ray Proficiency Testing. The mammography calibration facility is being prepared for the continuation of NIST mammography proficiency testing. The characterization of the new Mo anode x-ray tube is in the final stages with the verification of the standard correction factors. All NIST reference chambers to be used for NIST proficiency testing in mammography beams are currently being calibrated using the new Mo anode. The data-acquisition software upgrade is in the final testing stage. After a major renovation of the NIST conventional (W-anode) x-ray range, the x-ray facilities are available for calibrations and proficiency testing using 75 NIST and ISO beam qualities.

• Neutron Interferometry and Optics. The Neutron Interferometer and Optics Facility is now in full operation as a national user facility with a busy schedule of experiments. Large new interferometer crystals of NIST design can operate over a wavelength range of roughly 0.2 nm to 0.45 nm, with fringe visibility as high as 88 % at the shorter wavelengths. Experiments include applications for materials science as well as fundamental physics measurements. Very substantial advances are being made in neutron scattering length measurements. Neutron optics developments include phase contrast imaging and high-resolution radiography.

• Neutron Fields for Materials Dosimetry and Personnel Dosimetry. A diverse array of well-characterized and documented neutron fields is maintained for calibrations and for development of methods for materials dosimetry and personnel dosimetry. Neutron source calibration facilities have undergone a major renovation and in the past year we have intercompared NBS-I, the NIST RaBe source, with three standard neutron sources obtained from the BIPM. The application of calorimetry to absolute neutron counting is being pursued to validate and improve on traditional methods of fluence measurement and neutron source calibration.

• Symmetries of the Weak Nuclear Force. The end station on cold neutron guide NG-6 is also operated as a national user facility for investigation of the symmetries and parameters of the nuclear weak interaction. Two very different neutron lifetime experiments, a search for time-reversal asymmetry in neutron beta decay, and measurements of parity non-conserving spin rotation are competing for beam time.

• Laser Polarization of Neutrons. Both the spin-exchange and metastable optical pumping approaches continue to appear promising and worthy of further development. Our high-pressure cells and diode laser arrays are being used as spin filters and/or analyzers in neutron experiments at NIST and Los Alamos. Our low-cost, compact compressor for use with the metastable ³He optical pumping system is already achieving polarization levels adequate for medical magnetic resonance imaging applications. Additional collaborations in the medical applications are getting started.

• Standards, Calibrations and Instrumentation for Environmental Monitoring. The measurement of environmental surface contamination, particularly around nuclear sites and in environmental remediation has posed an important and difficult problem. This program addresses the metrological needs in this area. Three systems that are under study and evaluation are (i) imaging plate technology, (with efforts in developing response calibration curves for measuring apparent activity); (ii) glow-discharge resonance ionization mass spectrometry (with improvements in sensitivity and selectivity based on several factors such as lowering of the background, careful choice of excitation scheme, development of a graphite furnace source, incorporation of a non-axial-beam geometry); and (iii) thermal ionization mass spectrometry (with special efforts in bioassay material measurements).

• Radionuclide Standards for Nuclear Medicine. The development of a standard for the potential -emitting radiotherapy nuclide ²¹¹At will be the highest-priority project for the Radiopharmaceutical Standards Program during the present fiscal year. Other important projects include the investigation of geometrical effects in measuring nuclides such as ¹⁷⁷Lu and ¹⁶⁶Ho in vials and syringes using commercial dose calibrators. Ongoing projects will be the investigation of liquid scintillation cocktail effects in assaying solutions containing ⁹⁰Y, ¹⁶⁶Ho, ¹⁷⁷Lu, ¹⁸⁸W/¹⁸⁸Re, and ²¹¹At.

• Traceability for Low-level Radiochemistry Metrology. Traceability in low-level radiochemistry is being addresssed through publication of ANSI Standard N13.30 (Performance Criteria for Radiobioassay) and ANSI N42.22 (Traceability of Radioactive Sources to the National Institute of Standards and Technology and Associated Instrument Quality Control), and draft ANSI N42.23 (Measurement and Associated Instrumentation Quality Assurance for Radioassay Laboratories). These three consensus standards call for traceability testing programs that link the quality of operational measurements to the national standards. The Radioactivity Group maintains extensive traceability testing programs for low-level radiochemistry laboratories.