Hamamatsu S8550 APD arrays for high-resolution scintillator matrices readout

The performance of Hamamatsu S8550 avalanche photodiode (APD) arrays for scintillator matrices readout has been evaluated. The S8550 device is a monolithic 8×4 pixels structure with an active area of 2.56 mm2 for each pixel. The device allows stable operation at gains up to 74, with a detection efficiency of about 60% for photons of 420 nm wavelength. It is characterized by a low noise equal to 27 electrons equivalent noise charge at room temperature. The energy resolution of 14.6%, for the 511 keV peak from a 22Na source has been recorded with a 2×2×10 mm3 LSO crystal coupled to one pixel. The number of electron–hole pairs produced by the 511 keV photopeak absorbed in LSO is equal to 4830±240 e–h/MeV. Coupling LSO scintillator crystals to individual pixels of the APD array a coincidence timing resolution of 3.0±0.2 ns FWHM has been measured for the 511 keV γ-rays from a 22Na source. Finally, we compared the characteristics and readout performance of the Hamamatsu array with the results measured earlier under the same conditions for the quadrant large area avalanche photodiodes of Advanced Photonics Inc.