A 4 x 8 APD array, consisting of two monolithic silicon wafers, coupled to a 32-channel LSO matrix for high-resolution PET

Avalanche photodiodes (APDs) offer new design options for high-resolution positron emission tomography (PET) detectors. The goal of this study was the design of a very compact PET detector module with two layers. For this purpose, new monolithic arrays of 4 x 8 densely packed APDs (each with a 1.6 x 1.6 mm(2) sensitive area) were developed and evaluated for their suitability to read out 32 small (2 x 2 x 6 mm(3)) lutetium oxyorthosilicate (LSO) crystals. The gain of the APDs was about 100. Its standard deviation within an array was less than 12%. The average dark current was less than 45 nA per pixel, and the detector capacitance was about 12 pF. Energy resolution was 15% for gamma-rays with 511 keV. The time resolution was 2.5 ns (full-width at half-maximum). A high- reflectivity foil was used to hold all 32 individual crystals in a matrix. This matrix was glued directly on the APD array. The light crosstalk between the channels was only 7.9%. Together with a specially developed 16-channel low-noise JFET-CMOS preamplifier chip, a compact front-end detector module was built. The monolithic APD arrays proved to be stable and may, in combination with lutetium oxyorthosilicate (LSO) crystals, facilitate the design of various detector geometries, such as panels or multilayer modules.