Temperature and bias voltage studies of a large area position sensitive Avalanche Photodiode

We are constructing a 1 mm³ resolution, high sensitivity PET detector system with depth of interaction capability. The detectors are built from modules comprising LSO crystal arrays coupled to Position Sensitive Avalanche Photodiodes (PSAPDs). The entire system will have 4,608 densely packed dual LSO-PSAPD modules. The performance of the large area (1?1 cm²) PSAPDs in our system depends on bias voltage and temperature. Coincidence data was obtained by placing a ²²Na source between an LSO crystal coupled to a PMT and an LSO-PSAPD module. The bias voltage was varied between 1695 and 1780 V. The energy resolution remains constant around 14.1 ? 0.01 % (Standard Error - SE) FWHM at 511 keV between 1710 and 1780 V, and decreases by about 5% when the applied voltage is between 1695 and 1710 V for a specific sample. Crystal identification capability stays constant over the observed range. Optimal coincidence time resolution of 2.63 ? 0.02 ns (SE) FWHM was observed around 1740 V at room temperature. Coincidence time resolution decreases by about 10 % for a 20 V change. The gain increases by a factor of 2 for every 35 Volt increase and can be described by two exponentials. The point where those two exponentials intersect corresponds to the beginning of the avalanche breakdown. The module´s temperature was varied using a thermoelectric cooler coupled to a heatsink. In general, decreasing the temperature of a PSAPD improves performance. The coincidence time resolution improved from 5.88 ? 0.05 ns (SE) FWHM at 39?C to 1.96 ? 0.03 ns (SE) at 5?C. 511 keV energy resolution improved from 14.43 ? 0.01 % (SE) at 39?C to 11.82 ? 0.01 % 5?C. PSAPD gain increases by 25 % every 4.5?C increase. The rate of gain increase is even larger (10 % per degree) at the lowest temperatures. Most of the observed behavior is attributed to the PSAPD, since the light output of LSO varies only slightly with increasing temperature. In summary, we present the performance v- - ariation of a large area PSAPD as a function of temperature and bias voltage. These parameters are of extreme importance in densely packed systems needed for state-of-the art PET design.