TECHNICAL ACTIVITIES 1998 - NISTIR 6268

CURRENT DIRECTIONS

• Absorbed Dose Standards and Calibrations. NIST is preeminent in the direct realization of the absorbed dose in water, the quantity of interest for ionizing radiation, particularly in radiation therapy. Working with the medical physics community, we are implementing the direct transfer of the absorbed-dose-in-water calorimetric standard for ⁶⁰Co gamma rays through new calibrations of ionization chambers. The calibrations will be further disseminated by the secondary calibration network accredited by the American Association of Physicists in Medicine (AAPM), and will support the new AAPM protocol for dosimetry based on absorbed dose calibrations.

• New Standards and Calibrations for Low-Energy Photon Brachytherapy Sources. The recently developed Wide-Angle-Free-Air Chamber (WAFAC) is the basis for the new NIST standard for air-kerma strength for brachytherapy seeds containing low-energy photon-emitting radionuclides such as ¹²⁵I and ¹⁰³Pd. To support the consensus on providing the measurement that is more useful in therapy applications, the new standard and calibrations will discount the contribution to air kerma by characteristic x rays produced in the titanium encapsulation of such seeds. Extensive measurements and analyses have been carried out to firmly establish the new standard and to determine the relationship to the previous standard. A dedicated measurement laboratory has been constructed, new automated data-acquisition and analysis systems have been implemented, and a new, automated, variable-volume WAFAC has been designed and built. NIST is working closely with the medical physics community and manufacturers to introduce the new standard in 1999 so as to maintain complete continuity in therapy dosimetry planning. An enormous increase in demand for these brachytherapy sources has stimulated the emergence of new manufacturers and new seeds, making the transition to the new WAFAC standard especially timely and underscoring the care with which this transition must be conducted.

• X- and Gamma-Ray Source Facilities. An extensive program is near completion to upgrade the x-ray and gamma-ray facilities used in the NIST dosimetry standards and calibration program. A new 300 kV x-ray generator and a new 100 kV set have been installed in the W-anode x-ray calibration ranges. With the addition of 41 ISO beam qualities and two new U.S. beam qualities, NIST will offer 75 conventional x-ray beam qualities, as well as the 17 mammography (Mo- and Rh-anode) x-ray beam qualities recently established. New data-acquisition/ automation hardware and software systems are in development for control of measurements in the W-anode and mammography x-ray ranges, the protection-level horizontal-beam ⁶⁰Co and ¹³⁷Cs gamma-ray ranges, the therapy-level vertical-beam ⁶⁰Co and ¹³⁷Cs gamma-ray ranges, and the low-energy photon brachytherapy Wide-Angle-Free-Air-Chamber Facility. Preliminary planning has been done for the re-sourcing of two ⁶⁰Co sources: a therapy-level vertical beam, and a high-dose-rate Gammacell used in our radiation-processing dosimetry program.

• Standards, Calibrations, and Instrumentation for Environmental Monitoring. The measurement of environmental surface contamination, particularly around nuclear sites and in environmental remediation, poses an important and difficult problem. This program addresses the metrological needs in this area. Two systems under study and evaluation are (i) imaging plate technology and (ii) glow-discharge resonance ionization mass spectrometry.

• Radionuclide Standards for Nuclear Medicine. NIST, in collaboration with the Nuclear Energy Institute (NEI), is targeting a number of radionuclides for standard development/calibration this year. One of these is ¹⁷⁷Lu, a 6.7-day half life beta-particle emitter with has potential for use in radioimmunotherapy and bone palliation. We will also continue our efforts to compare short-lived nuclear medicine nuclides internationally, either through bi-lateral intercomparisons, or exchanges of calibration factors for well ionization chambers.

• Radionuclide Metrology Development. A pulse recording technique has been developed which will permit a given data set to be analyzed ex post facto. Intercomparisons of the various types of analytical reductions on the same sets of data will be possible and become routine, which will lead to reduced systematic uncertainties and very much faster measurements. Measurements of very short lived radionuclides will become significantly easier.

  There is also a need to compare liquid scintillation counting results for low energy γ -ray and β -particle emitting radionuclides with calibration values from other methods such as coincidence counting. Liquid scintillation has, in recent years, become the most used calibration method, combining speed, simplicity and high efficiency. However, it must be demonstrated to give results equivalent to primary methods for low energy x-ray emitters, such as palladium-103, and nuclides which decay both by beta-particle emission and electron capture, such as iridium-192.

• Traceability for Low-level Radiochemistry Metrology. Many tens of thousands of low-level radiochemical measurements are made annually to support environmental remediation and occupational health programs. The credibility of these measurements have been based on participation in regulation driven performance evaluation programs of limited scope. The fundamental flaw that the metrology community recognizes is that there is a lack of direct linkage to the national radioactivity standards. This situation is being addressed in the 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 has established such a traceability testing program for low-level radiochemistry laboratories such as: Westinghouse Carlsbad, University of New Mexico at Carlsbad, Sandia National Laboratory, and EPA Montgomery. It is anticipated that the program will eventually include both the environmental restoration and radiobioassay communities.

• 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.

• 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 percent 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 are undergoing a major renovation and in the next year we will be able to intercompare NBS-I, the NIST RaBe source, with three standard neutron sources obtained from the BIPM. A new generation of staff has taken over these activities with continuing guidance from emeritus staff members, who are serving on contract or as guest researchers.

• 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.

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TECHNICAL ACTIVITIES 1998 - Contents

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Online: April 1999