A Non-Destructive Investigation of Plutonium Reference Items Used for Calibration

The calibration of Non-Destructive Assay (NDA) equipment relies on the availability of certified items of known content and construction. Increasing use is being made of calculational tools to create calibration data and so representative standards are no longer always needed. However, even with this approach it is invaluable to benchmark the tools against the measured response under well known conditions and to apply the Measured: Calculated ratio as a scaling factor. Reference sources for Pu are typically doubly encapsulated for safety reasons and contain Pu of well known chemical form, elemental composition, relative isotopic composition and mass. Destructive analysis techniques are used to characterize the materials and so these attributes are usually known with far greater accuracy than that achievable by the NDA methods to which they are being applied. Construction details are also usually provided in order to permit attenuation and related factors to be estimated. This work concerns the empirical investigation of a set of encapsulated PuO{sub 2} powder standards. The characterization and fabrication of the items is adequately documented with the exception of fill height. The fill height governs the powder density and in turn both the self attenuation of photons and the self multiplication of neutrons,more » consequently this is an important omission. Initially the location and dimensions of the internal plunger cup was used as a basis to estimate the packing density, but later records of plunger positions made at the time of filling were found and significant revisions followed. As a consequence of discrepancies observed in measurements designed to evaluate a new lump correction algorithm we were led to investigate the powder density and distribution directly by gamma-ray scanning. In some cases this resulted in revised density estimates. Equally importantly it was discovered that for the smallest mass items, the powder was not held fixed in the form of a uniform disc by the spring loaded plunger but rather could flow inside the inner capsule. In this paper we describe the nature of the investigation, report the salient findings and explain the implications for modeling the sources for the calibration of NDA equipment. (authors)« less