Geometry dependence of the light collection efficiency of BGO crystal scintillators read out by avalanche photo diodes

Light collection efficiency from BGO crystal scintillators of various sizes and shapes was measured by reading them using 1×1 cm2 avalanche photo diodes. When the crystals have simple geometry, the light collection efficiency was found to depend on their size, shape and the read-out position through a rather well-defined empirical scaling relation. The light collection efficiency of tapered crystals was seen to depend on both the position of γ-ray irradiation, and the read-out position of the avalanche photo diodes. Using optical Monte-Carlo simulations, the relation was reproduced assuming plausible proper parameters for surface conditions and the attenuation length. This results were reproduced with a reasonable accuracy by optical Monte-Carlo simulations. Simple physical explanations are given to these geometrical effects. -- Highlights: • We studied light output of different shapes and size BGO crystals with an APD. • With a Monte-Carlo simulation, we successfully reproduced measured light outputs of BGO. • We derived a scaling law of a rectangle crystal volume and surface area APD attached. • We measured a dependence of the light output on a position with non-rectangle crystal