Performance of 8 x 8 Pixel LaBr3:Ce and Gd2SiO5:Ce Scintillator Arrays Coupled to a 64-Channel Multi-Anode PMT

We have developed a LaBr3: Ce scintillator array consisting of 8 x 8 pixels with a size of 5.8 mm x 5.8 mm x 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 LaBr3: 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 inner 6 x 6 pixels was 5.8 +/- 0.4% at 662 keV. In addition, we measured a Gd2SiO5 : Ce (GSO: Ce) scintillator array consisting of 8 x 8 pixels with a size of 5.9 mm x 5.9 mm x 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 degrees and 5.3 degrees, respectively, at 662 keV.