Operating characteristics of avalanche photodiodes for PET systems

Results from recently developed avalanche photodiodes of the beveled edge type for use in low-light level applications such as PET systems are presented. Starting from a neutron transmutation doped (NTD) Si wafer, an avalanche photodiode with an active area of 100 mm² was produced by deep Ga diffusion, followed by boron and phosphor diffusion. The gain and dark current are dependent on external voltage, and gains up to 400 are obtained at 1950 V without much excess noise, breakdown occurs at 1950 V. The detector exhibits low noise and fast response, necessary for use in PET systems. The energy resolution from a ⁵⁷Co gamma-ray source was measured to be 942 eV at 6.5 keV, which corresponds to 111 r.m.s. noise electrons at the input. The authors measured the energy resolution of the detector coupled a 3×3×3 mm³ BGO scintillator from a ²²Na gamma ray source. The energy resolution at the 511 keV line was 12.5%, which is comparable to photomultiplier tubes coupled to a scintillator of the same site. Calculation of the electric field and width of the depletion region was done to estimate the thickness of the undepleted front region and to optimize the quantum efficiency for detection of BGO light