Independent channel readout system for a 2×2 array of H8500 with SBA photocatode

The gamma cameras built on a LaBr3 crystal and Hamamatsu H8500 has a 49 mm by 49 mm FOV, equal to the detector active area. In order to increase the active area it is necessary to put more than one detector side to side. This can be done because the H8500 is designed do reduce as much as possible the dead area between two detectors. If we use a 2 by 2 matrix it is possible to use a single 10cm by 10 cm LaBr3 crystal. The readout system has in this case 256 independent channels, with low noise and wide dynamic. The system is managed by an FPGA that reads all the inputs in parallel, up to a maximum rate of 250 KSample/sec, and makes a pre-processing of data. The use of the last generation H8500 PSPMTs, having a SBA photocatode with better quantum efficiency but a lower gain, requires an in deep analysis on the readout electronics induced error, in order to increase the signal to noise ratio. Moreover the individual channel readout allows a better correction of the anodic gains, a particularly relevant problem when more than one PMT is used, and a better study of the light sharing among two PMTs by the use of more sophisticated algorithms with respect to standard the CoG. This approach is mandatory in order to reduce the dead zone between two PMTs.