Test results of multi channel readout system for high performance scintillation imaging

The recent improvements in the quantum efficiency of Hamamatsu H8500 position sensitive PMT can be exploited if a multi channel low-noise readout electronics is used. In order to test to its limits the capability of the H8500 and readout electronic system, the PSPMT is coupled with an integral line NaI(Tl) crystal 1.5 mm thick, a pixellated NaI(Tl) 2.0mm pitch, 6mm thick and a pixellated CsI(Tl), 1mm pitch, 1mm thick. The gamma camera with the different crystals is tested with gamma sources in the 30 to 660 keV energy range. The acquisition system used is made by four analog PCBs, each one makes the acquisition of the analog signal and the A/D conversion from 16 anodes at 14 bit resolution, and is connected by USB to a control PC. The aim of the measurements is to show that, as expected from poisson statistic, the spatial resolution and the size of the PSF (sigma of the distribution of light) are inversely proportional to the square root of the photocathode generated photoelectron number. The values measured in the two configurations (continuous integral line and pixelated crystal) are compatible with those expected. Particularly, for the pixellated NaI(Tl) we find a Gaussian light distribution with about 11 mm FWHM and a with a source at 140 keV we can estimate a photo electron yield equal to 600-800 per scintillating event, with an estimated resolution close to 0.35-0.50 mm. The gamma camera is able to correctly identify the light distribution also with 1 mm pixellated CsI(Tl) where the light distribution is virtually contained in a single anode (the anode dimension is 6 mm by 6 mm) and the assignment of the correct position is only estimated based on the very low levels of charge present on the anodes around the one that collected the most of the charge.