An array of position-sensitive avalanche photodiodes is provided in which output signal contacts from corners of each photodiode are grouped in common for readout. Each photodiode may be provided on a single wafer or chip, with photodiodes being separated by a trench or a groove. Corners of each photodiode include contacts for reading out signals resulting from gamma rays incident events in an adjacent scintillator. Grouping of the contacts into common output channels reduces the number of output channels, while still permitting localization of gamma ray incident events in the scintillator crystals. The photodiode array may be positioned adjacent to one face of a scintillator, with a photomultiplier tube to be positioned adjacent to another. The detector arrangement may be used for positron emission tomography imaging and similar techniques.