A study of the light collection for avalanche photo diode readout of CsI(Tl)

In order to read out the large volume scintillation crystal by such a small photo-sensor as an avalanche photodiode, we have examined the light collection properties of different scintillator-to-sensor interfaces. A spiraled fiber attached on the readout plane of a 4 cm cubic CsI(Tl) scintillator was tried first as an application of the scintillating tile/fiber technique. A light collection buffer of a clear acrylic plate or a wavelength shifter plate was also tested. Its performance and the condition for the minimum enhancement of the light collection are discussed with reference to a Monte Carlo simulation result. The surface reflectivities of the crystal and the interface were the most effective parameter in the light collection.