Scintillation Detection with Large-Area Reach-Through Avalanche Photodiodes

Silicon avalanche photodiodes of the "reach-through" type have been used as scintillation detectors with device performance closely approaching that of PMT's. The device consists of a 6 × 6mm diode with a 5 × 5mm photosensitive area in a 6.7 × 6.7 × 2.0mm package which is suitable for mounting in an array. Typical characteristics at room temperature are: quantum efficiency ¿50% at ¿=415nm; surface dark current <200nA; bulk dark-current (before gain) <1nA; noise current (at optimum gain of 20 to 50) <1pA/Hz¿; response time <5ns; capacitance ¿21pF; biasing voltage ¿200-400 Volts. When coupled to a NaI(Tl) scintillator (9 × 9 × 38mm) photo-peak resolutions (FWHM) for 662keV radiation of 10.4% at 21°C and 9.8% at 0°C were measured. The measured noise equivalent primary charge of one detector at a gain of 50 was 121 electrons (51 electrons rms) at room temperature. The significance of this new detector to high resolution imaging systems and detector arrays is discussed.