Crystal optimization for a CsI(Tl)-PIN photodiode γ-ray probe by Monte Carlo method

Precise tumor localization with a gamma probe based on a CsI(Tl) crystal coupled to a photodiode needs to improve the energy resolution of the device. In indirect gamma detection, quality of energy resolution depends on quality of light collection. For this purpose, we developed a simulation based on the Monte Carlo method, which describes the path of visible photons produced in a scintillator crystal by a photoelectric gamma interaction at 140 keV. A set of simulations was performed in order to study the influence of crystal geometry (shape and dimensions) and side surface conditions on light collection. Amount of detected signal versus gamma photon interaction positions in the crystal was also studied. In conclusion, for a photodiode with 10 mm × 10 min sensitive area, the best compromise between amount of collected light and uniformity of response versus interaction position was obtained for a 5 mm × 5 mm parallelepipedic crystal having ground faces covered with white reflectors.