Performance of 8 × 8 pixel LaBr₃: Ce and Gd ₂SiO₅: Ce scintillator arrays coupled to a 64-channel multi-anode PMT

We have developed a LaBr₃ :Ce scintillator array consisting of 8 × 8 pixels with a size of 5.8 mm × 5.8 mm × 15.0 mm, which serves as an absorber of scattered gamma rays with energies from 0.1 to 1 MeV in a Compton camera. The pixels were cut from two pieces of LaBr₃ :Ce crystal with a diameter of 38 mm and a length of 38 mm with full width at half-maximum (FWHM) energy resolutions of 4.1 ± 0.1 % and 3.0 ± 0.1 % at 356 and 662 keV, respectively, measured with a single anode photomultiplier tube (PMT). The crystal had the following volumetric uniformities: light outputs with a difference of 0.5% (standard deviation: SD) and energy resolutions with that of 2% (SD) at 356 keV. In contrast, for each pixel in the array, the average and SD FWHM energy resolutions over 64 pixels, measured with a single-anode PMT and a collimator, were 5.8 ± 0.9% at 356 keV. The array was then coupled to a 64-channel multi-anode PMT (Hamamatsu H8500), the anode pitch of which was the same as the LaBr :Ce pixel pitch of 6.1 mm. When the 64 anodes were read out from four channels in a resistor chain by the charge division method, the FWHM energy resolution of all 64 pixels was 7.0 ± 0.5 % at 662 keV, whereas that of the inner6 × 6 pixels was 5.8±0.4% at 662 keV. In addition, we measured a Gd₂SiO ₅ :Ce (GSO:Ce) scintillator array consisting of 8×8 pixels with a size of 5.9 mm × 5.9 mm × 13.0 mm to compare its performance with that of the LaBr₃ :Ce array. The FWHM energy resolution of all 64 GSO:Ce pixels was 10.8 ± 0.3% at 662 keV. With these energy resolutions, FWHM angular resolutions of the Compton camera using the LaBr₃ :Ce and GSO:Ce arrays are expected to be 4.6° and 5.3°, respectively, at 662 keV.