Guide to Nondestructive Assay Standards: Preparation Criteria, Availability, and Practical Considerations

For certification and measurement control, nondestructive assay (NDA) instruments and methods used for verification measurements of special nuclear materials (SNMs) require calibrations based on certified reference materials (CRMs), or working reference materials (WRMs), traceable to the national system of measurements, and adequately characteristic of the unknowns. The Department of Energy Office of Safeguards and Security is sponsoring production of a comprehensive guide to preparation of NDA standards. The scope of the report includes preparation criteria, current availability of CRMs and WRMs, practical considerations for preparation and characterization, and an extensive bibliography. In preparing the report, based primarily on experience at Los Alamos, we have found that standards preparation is highly dependent on the particular NDA method being applied. We therefore include sections that contain information specific to commonly used neutron and gamma-ray NDA techniques. We also present approaches that are alternatives to, or minimize requirements for physical standards. In this section, we describe instrument cross-calibration, where one member of a family of nearly identical detectors is carefully calibrated with WRMs (defining the calibration curve shape), and other detectors are normalized to the reference detector, using a single WRM, or an isotopic source. Another alternative, that is becoming increasingly viable, is the use of Monte Carlo simulations to define shape, and a single WRM, or isotopic source for normalization. Another cost- and effortsaving approach is characterize WRMS with the combination of calorimetry and plutonium isotopics (Cal/Iso). In many practical cases, Cal/Iso values are more accurate than can be obtained with destructive analysis and weighing. The guide is tailored to a wide audience, US DOE facilities, although considerable interest has been shown from those concerned with materials control and accountability in Russia, Europe, Japan and South America.