A New Positioning Algorithm for Position-Sensitive Avalanche Photodiodes

We are using a novel position sensitive avalanche photodiode (PSAPD) for the construction of a high resolution positron emission tomography (PET) camera. Up to now most researchers working with PSAPDs have been using an Anger-like positioning algorithm involving the four corner readout signals of the PSAPD. This algorithm yields a significant non-linear spatial ldquopin-cushionrdquo distortion in raw crystal positioning histograms. In this paper, we report an improved positioning algorithm, which combines two diagonal corner signals of the PSAPD followed by a 45degC rotation to determine the X or Y position of the interaction. We present flood positioning histogram data generated with the old and new positioning algorithms using a 3times4 array of 2times2times3mm and a 3times8 array of 1times1times3 mm³ of LSO crystals coupled to 8times8 mm² PSAPDs. This new algorithm significantly reduces the pin-cushion distortion in raw flood histogram image.