Year
2016
Abstract
This work expands our understanding of uncertainty treatment for a class of nuclear safeguards measurement techniques. The focus of the present experiment was a calibration standard of highly enriched uranium metal which is used as the basis for calibrating detectors for nuclear materials holdup measurements in the piping and equipment of facilities that process nuclear materials. The experiment involved the use of a high-purity germanium gamma ray detector to take precise measurements of 185.7 keV, 205.3 keV, 163.4 keV, and 143.8 keV gamma rays at various angles of rotation of the disc source material relative to the detector. The differential attenuation as a function of angle is sensitive to the areal density of the source and the encapsulation. Results indicated that the effects of self-attenuation by the highly enriched uranium source material and attenuation by the stainless steel casing which houses the material were symmetric about the axis of rotation of the source – consistent with a uniform source. A quantitative comparison between observations and expectations based on the physical description of the source showed good agreement proving empirical confirmatory evidence that the description is accurate. We conclude that the source is reliable for calibrating holdup measurements. Importantly, these measurements confirm the significant self-attenuation factor of this source and how the self-absorption factor (SAF) varies significantly with angle. The uncertainty on the SAF is reasonably modest however (a few %), but what traditionally (under the HMS-4 protocol) is not accounted for is the angular dependence and how it affects the calibration factor for an extended (area) source determined by mapping out the field of view.