HMS-4 Taking Another Look at Correction Algorithms

Holdup Measurement System-4 (HMS-4) is a portable, ruggedized, nondestructive assay system comprised of a shielded, gain-stabilized NaI(Tl) detector, a multi-channel analyzer (MCA), and a handheld computer with built-in bar-code reader used as the field measurement controller (or simply referred to as the controller). Gamma-ray spectra are acquired, stored, and analyzed with the HMS-4 software that resides on the controller. The Generalized Geometry Holdup (GGH) model is used to simplify the measure of holdup by assuming that all deposits are configured as a point, line, or area deposit. Some deposits may be intermediate between two of these, and in those cases, analyzing them both ways and taking an average often gets you a more accurate and defensible answer. The choice between these options is usually left to the operator and is determined by the time allotted to the operator as well as what the deposit appears to be. While this can be an accurate way of characterizing the held up material, there are many corrections that need to be taken into account for each measurement. The main issue with measurements taken by HMS-4 is infinite thickness. When the depth of the sample along the collimation axis is much larger than the mean free path of the 186-keV photons in the sample being measured, and the deposit exceeds the amount that can be corrected with self-attenuation, the sample is said to be infinitely thick. When a deposit is flagged as infinitely thick, accurate measurements cannot be taken because the associated error will be too high. A correction needs to be analyzed in order to get the most accurate measurement possible. This paper will describe research being done to mitigate the error associated with holdup measurements by looking specifically at algorithms used to calculate the gram quantity of material.