Designing position sensitive NaI(Tl) bar-detectors with an empirical model

In small position-sensitive bar-detectors, multiple internal reflection and the edge effect of scintillation light cause excessive light spread and lead to poor intrinsic spatial resolution. Surface treatment techniques have been used to limit light spread inside detectors and improve resolution performance. Since it is costly and time-consuming to build NaI(Tl) detectors to test for each variation of surface treatment, a practical model to predict the potential effectiveness of each technique could help guide detector design. An empirical model has been developed to evaluate the potential of different surface treatment techniques. In this model, a Lucite bar and a light-emitting diode are used to simulate light propagation and refraction in an NaI(Tl) bar-detector. Light sensors were suspended above the exit window with a small air gap for spatial recording of light refracted from the bar. A three-sided transverse sanded surface turned out to be the best compromise between light spread and light output among the nine different combinations of surface treatments evaluated. These results were later confirmed by experiments on NaI(Tl) bar-detectors