Signal conditioning technique for position sensitive photodetectors to manipulate pixelated crystal identification capabilities

We analyze performance of position sensitive detectors such as Position Sensitive Avalanche Photodiodes (PSAPDs) used to readout pixelated scintillation crystal arrays. Crystal identification abilities are determined by analyzing the flood histogram. A good flood histogram has peaks which are as well separated and as narrow as possible since this enables crystal identification, and ultimately spatial resolution. We present a signal conditioning circuit for the readout of PSAPDs and investigate how the tradeoff between the parameters of the signal conditioning circuit components, the PSAPD bias voltage, and the dynamic range and noise slope of the front-end preamplifier influence the flood histogram. The signal conditioning circuit involves adding a resistor and a capacitor between the four spatial channels of the detector and the preamplifier. This technique was verified and studied both experimentally and by simulation using PSAPDs coupled to 8 × 8 arrays of 1mm³ scintillation crystals. Results showed that the flood histogram is not sensitive to the exact value of the resistor in the signal conditioning circuit, but in general a resistance about twice the sheet resistance of the position sensitive detector is recommended. The capacitor in the signal conditioning circuit should be chosen to balance preamplifier dynamic range and noise, as well as detector noise. The technique was implemented using digitally programmable circuits, resulting in a solution that facilitates dynamic adjustment of signal conditioning parameters based on process variations and real-time monitoring of variables such as temperature and PSAPD gain.