Bias Investigation of a 55-gallon Drum-sized Segmented Gamma Scanner

This paper addresses a series of studies undertaken while fulfilling the measurement requirements for certification of a 55-gallon drumsized segmented gamma scanner to be used at the plutonium facility (TA-55) at Los Alamos. These studies were initiated to investigate anomalies seen in the data as well as gain a quantifiable understanding of effects caused by using standards of considerably different geometric proportions than the assay items, oftentimes necessitated due to lack of adequate standards. The first investigation concerned effects caused by items not being exactly centered on the measurement table. The remaining studies involved establishing the magnitude of the bias induced by so-called \"end effects\" as well as by size differences between calibration standards and assay items. In order to unravel the above effects, can and drum parameter files were interchangeably employed in conjunction with comparisons of the instrument calibrations obtained with can-sized standards and 55-gallon drum standards. The paper will discuss the results and the magnitude of the observed effects.