EFFECT OF GEOMETRICAL MODIFICATIONS AND CRYSTAL DEFECTS ON LIGHT COLLECTION IN IDEAL RECTANGULAR PARALLELEPIPEDIC BGO SCINTILLATORS

The major loss of signal in ideal rectangular parallelepipedic scintillators results from the presence of permanent modes which cannot escape the crystal volume. When the photodetector is coupled with an index of nd=1.55, this value can be as high as 46% in BGO. The purpose of this study is to investigate some alternatives to this geometry with the aid of Monte Carlo simulations and direct measurements on a set of crystals. The study is focused on long narrow-shaped BGO scintillators designed to be used in conjunction with avalanche photodiodes in positron emission tomography. Results show that slightly unpolishing the crystal surfaces efficiently eliminates trapped modes but that very-high-reflectivity reflectors are needed to transport photons to the detection plane. Minor modifications to the geometry eliminating all symmetry from the crystal also efficiently eliminate trapped modes and generally lead to the highest light output. © 1990.