Investigation of the effects of scintillator pixel shape, surface treatment and optical coupling on the performance of Si-PM based BGO detectors

Positron Emission Tomography (PET) is based on the coincidence detection of annihilation photons, after their interaction with scintillator crystals. BGO produces less overall scintillation light compared to LSO and its variants. A considerable fraction of scintillation light becomes trapped and absorbed before it reaches the exit face of the BGO crystals. In order to improve the amount of optical photons that exit the crystal, we examined the performance of different scintillator geometries, surface treatments and interface material index of refraction for BGO crystals, coupled to solid state photodetectors. Simulations were performed to estimate the fraction of scintillation light detected by the photodetector (SiPM) as a function of the location of the source respect to the length of BGO crystals with different surface treatments and geometries (the standard rectangular block and crystals with an slanted face). In addition, the energy spectra for different crystal geometries and surface treatments were obtained through measurements of individual crystals coupled to a solid state photodetector, (Philips digital-SiPM DPC-3200). This work demonstrates that slight slanted geometry (120°) offers an advantage over conventional rectangular crystal shapes for light extraction for polished crystals, while there is no relevant difference in light extraction for non-polished crystals when changing the slanted angle.