3″ × 3″ LaBr3:Ce position sensitivity with multi-anode PMT readout

The response of a cylindrical continuous 3" × 3" LaBr₃:Ce crystal coupled to four Position Sensitive photomultipliers (PSPMT) was studied using a 1 mm collimated beam of 662 keV gamma rays. The LaBr₃:Ce crystal is sealed in a capsule and has diffusive/reflective surfaces, the energy resolution is 21 keV (FWHM) at 662 keV, measured using a R6233-100sel PMT. The PSPMTs are HAMAMATSU H8500 and have 64 segments each. Two PSPMTS have 10 dynodes and a 52 mm flat square standard Bialkali photocathode while the other two have 8 dinodes and a SBA photocathode. We performed a scan of the crystal moving the collimated source. The 256 segments of the four PSPMTs were grouped into 4 or 16 elements. The Centroid position, FWHM and area of the full energy peak were extracted from the measured spectra. The data show a regular behavior which depends on the position of the incident gamma beam. The position of the source was also calculated event by event and the positional resolution was measured.