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Testing scintillation transport models with photoelectron yields measured under different surface finishes

The UNIFIED, POLISH, and GROUND surface models of the optical photon transport program DETECT are tested for their capacity to predict the photoelectron yields measured with crystals of different surface finishes and reflective coats. Here, two BGO crystals of 5.6/spl times/12.8/spl times/29.7 mm, with respectively a polished and a corrugated surface finish are considered. Stylus profilometer scans are first taken to quantify their surface roughness. The crystals are then prepared in three different surface coat configurations to subsequently measure the photoelectron yield collected when they are excited by a beam of 511 keV photons. These measurements are used to confront the models' predictions on absolute ground. The results indicate that the transport of scintillation photons internally trapped within the volume of a highly polished crystal is well accounted for. However, significant discrepancies are noted between simulations and measurements when considering a corrugated finish or when the surface is coated by a diffuse reflector. Possible explanations are discussed and call for further investigations.