Charge Multiplexing Readout for Position Sensitive Avalanche Photodiodes

Abstract

A technique to charge-multiplex multiple position sensitive avalanche photodiodes (PSAPD) has been studied. We have observed that with multiplexing, the spatial resolution is slightly degraded, but the energy and time resolution are not affected. We are building high resolution PET systems that use numerous PSAPDs, and we would like to use multiplexing to reduce the number of electronic readout channels and complexity. This charge multiplexing technique will reduce the number of readout channels required by a factor of two. A PSAPD is a new silicon semiconductor photodetector that may be used to replace a photomultiplier tube in PET or SPECT systems. It is a planar avalanche photodiode with a resistive coating that allows continuous positioning over the entire active area. The resistive sheet of the PSAPD splits the charge signal amplified by an avalanche process of the diode into four spatial channels. The top contact contains the hole signal which is equal in magnitude to the sum of the 4 spatial channels. The PSAPD can be multiplexed in a way that sums the spatial channels but leaves the top channel independent. PSAPDs are currently fabricated with active areas that range from 8times8 mm2 and higher. The proposed PET system detectors comprise 1 mm lutetium oxyorthosilicate (LSO) crystals coupled to extra thin PSAPDs. Non-multiplexed, these detectors have achieved excellent spatial resolution (<1 mm2), energy resolution (<10%), and time resolution (<2 ns). Unlike a position-sensitive PMT, the electronic amplification of a PSAPD is only on the order of 1000, which makes it challenging to charge multiplex multiple devices