Attenuation Correction Procedures

The nondestructive assay (NDA) of nuclear material must deal with large sample sizes and high self-absorption. Typical containers range from 2-L bottles to 220-L drums, and even a small sample with a high concentration of nuclear material has significant self-attenuation. Although gamma-ray self-attenuation may frequently be ignored in the filter papers or small vials encountered in radiochemical applications, it usually cannot be ignored in NDA measurements of nuclear material. Because the size and shape of nuclekr material samples vary widely, it is difficult to construct appropriate calibration standards. In principle, calibration standards are not needed if the detector efficiency is accurately known as a function of source position and energy, if the counting geometry and the sample size and shape are accurately known, and if the gamma-ray emission rates are accurately known. However, it is tedious to characterize a detector efficiency with sufficient accuracy, and there are still significant uncertainties in the values of the specific activities for many important gamma rays. The use of calibration standards reduces or eliminates the need to accurately know the detector efficiency, the counting geometry, and the specific activities. The most authoritative guide for calibrating NDA systems, ANSI N15.20-1975 (Ref. 1), rather firmly insists that a calibration standard is “an item physically and chemically similar to the items to be assayed,” a restriction no longer necessary for gamma-ray assay. The guide also insists that calibration standards “must be chosen so that their contained masses of the nuclide(s) of interest span the mass range expected for the items to be assayed.” This restriction may be considerably relaxed. Relatively few standards are usually needed to calibrate gamma-ray assay systems for the accurate assay of items covering a wide range of size, shape, chemical composition, and mass. Sections 6.2 to 6.6 describe the nature and computation of the attenuation correction factor CF(AT). A more detailed discussion of this subject is given @ Ref. 2. Section 6.7 discusses calibration standards and Section 6.8 describes assay systems using transmission-corrected procedures.