Determining the absolute photoelectron yield of LSO with APDs

Avalanche photodiodes (APDs) are used as readout devices for LSO and many other types of scintillating crystals. In order to optimize the signal from the APD to the front end readout electronics, it is necessary to maximize the photoelectron yield from the APD crystal combination. We have studied this optimization for the RatCAP detector, which uses a 32 element pixel array of 2times2times5 mm³ LSO crystals read out with a Hamamatsu S8550 APD array. The absolute light output of several small single LSO crystals was determined using a calibrated photomultiplier tube. These crystals were then used to measure the photoelectron yield with an APD. It was found that the measured yield was -22% lower than would be expected from the LSO light output combined with the overall light collection and APD quantum efficiencies. We believe this effect is due to uncertainty in the gain measurements and additional light collection factors associated with the APD. We have also found that the apparent gain of the APD, which is used in determining the photoelectron yield, is dependent on whether the input signal is visible light or X-rays and the internal structure of the APD.