Photodetector and scintillation crystals requirements for gamma-ray imaging

The diffusion of the PET and SPET techniques in different applications, like investigation on small organs and tissues or animal imaging, has induced in the past years the researchers to develop modular scintillation cameras to have compactness and versatility in order to obtain dimensions and configurations suitable to the particular application. To this purpose different photodetectors have been studied, as an alternative to the photomultiplier tubes (PMT) based on semiconductor technology. At the same time new scintillating crystals have been tested to match the requirements like high light yield or fast decay time, needed for SPET and PET application, respectively. In this paper we have investigated the photodetector and scintillation crystals requirements to optimize a gamma-ray imager based on scintillation crystals. To this aim we show results about the principal parameters characterizing a gamma-ray imaging, like energy and spatial resolution. The performances of a continuous LaBr3: Ce crystal (49 x 49 x 4mm(3) + 3mm glass window) are compared to the ones from a pixellated and continuous NaI: Tl crystal, coupled to multi-anode photomultiplier tube (Hamamatsu H8500 MA-PMT). Furthermore the results are supported with Monte Carlo simulations. With the lanthanum detector, we obtain 1.1mm of intrinsic spatial resolution, comparable with that predicted by the MC simulations. We test also the new ultra bialkali PMT Hamamatsu R7600200 with a QE = 42%, obtaining an improvement in terms of energy resolution of about 25%, respect to a standard PMT, with a LaBr3: Ce cylinder (1/ 2 ''phi f x 1/ 2 '' thickness).