This document describes a study of the cause of shadowing observed in certain beta-gamma plots produced with beta-gamma coincidence detectors for detection of radioxenon. Beta-gamma detectors must possess adequate resolution to allow for the metastable isotopes 131mXe and 133mXe to be distinguished from each other and from 133Xe. One effect that can minimize the ability to distinguish the metastables from 133Xe is the shadowing effect, in which a band of high-energy signal is present in addition to the primary signal. A study was performed to determine the cause of the shadowing, and how to minimize the effect of the shadowing on an individual beta cell.
Through this study, it is determined that the shadowing results from a difference in light transmission between the dome and end-cap portions of the beta cell. Consequently, it is not possible to remove the shadowing during the beta cell manufacturing process. The shadowing must be minimized by preferentially matching the light output from the end-cap and dome portion prior to the manufacturing process.