A targeted sparse readout for multi-anode photo-multipliers and optically isolated crystals

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Abstract
Multi anode photo-multipliers are able to localize the position of a light source, and measure its intensity. Normally these devices have their anode wires connected to a resistor network. If the light originates in an optically collimated, pixilated scintillation crystal array, only a few of the anode wires receive any significant signal. If only those wires whose signal is above the noise threshold are used to localize the light, the signal-to-noise ratio improves (same signal but less noise). We found that only about 6 wires on each axis have a significant signal when a 511 keV gamma ray is detected in a 2 mm/spl times/2 mm BGO crystal. Using a custom made integrated circuit which provides the binary code of the address corresponding to the biggest of 16 analog inputs, and a sample-and-hold on each anode wire we hope to direct a six channel ADC to sample the anode wires which provide the best positioning signal on each axis of the PMT. The shape of the signal envelope permits identification of crystals which would otherwise be beyond the PMT's field of view. This readout should enhance these PMTs in application like positron emission mammography, and small animal PET imaging.

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