Scintillator crystal optimization by Monte Carlo simulation for photodiode matrix detector

The design of gamma ray imaging probes based on silicon photodiodes and on a CsI(Tl) monocrystal is delicate and complex. Electronic and statistical noise deteriorate energy and spatial resolution. A Monte Carlo simulation is used to set the probe´s parameters in order to obtain the best compromise between spatial uniformity energy uniformity, spatial linearity and energy collection. The output distribution of light depends on the physical properties of crystal edges, crystal thickness and refractive index of the coupling grease. A 75×75 mm² squared CsI(Tl) crystal coupled to a 5 by 5 array of photodiodes (15×15 mm²) has been simulated. Energy and spatial characteristics of the probe were determined for crystal thickness varying from 2 mm and 20 mm and refractive index of coupling grease varying from 1.5 and 2.5. The influence of the aspect of the surfaces has also been studied. The best results were obtained with a crystal thickness of 8 mm, a grease refractive index of 1.9, and a crystal with polished edges and diffusing entrance face. These parameters minimize the statistic fluctuations of the light distribution on the array of photodiode leading to the best compromise between spatial linearity and energy collection