Energy Resolution in CMOS SSPM Detectors Coupled to an LYSO Scintillator
SSPMs fabricated using CMOS technology consisting of arrays of 30- and 50-mum square pixels in 1.5 mm times 1.5 mm total area with high, 61%, and low, 29%, fill factors (packing density) were used to measure the photon intensity resolution for a pulsed laser light source. Different sources of noise (i.e., cross talk, dark counts, and electronic) have a deleterious effect on the energy resolution, and this work looks at the relative sizes of these contributions. Even though noise effects increase with larger fill factors and active area, this work examines the trade-off between these noise terms and the improvement in the detection efficiency with increased excess bias or fill factor. The energy resolutions from various gamma rays, including 122 keV, 511 keV, 662 keV, and 1275 keV, measured with an LYSO scintillation crystal (1.5 mm times 1.5 mm times 3 mm) were compared to the pulsed laser results to examine all noise contributions to the energy resolution. The SSPM response can be described by a binomial function. When greater than 70% of the pixels are triggered, the energy resolution contains a substantial contribution from the detector response that arises from a binomial detector response of the SSPM, which contains a finite number of pixel elements.