Evaluation of hamamatsu H8500 new series MA-PMTs for readout of high-resolution LaBr3:Ce scintillation crystal

In recent years a great deal of attention was given to the Hamamatsu Position Sensitive PMTs, due to their suitability for building compact and high resolution gamma cameras. In this framework a great number of solution was proposed for the readout, that vary from the analog readout made with a resistive chain, in some cases including a hardware threshold, to fully digital readouts that read each of the PSPMT anodes individually. In order to obtain the best performance from the PSPMT, a readout that reads each channel individually must be chosen, but a great care must be also taken in designing and tuning the calculation of the point in which the gamma photon has actually interacted with the crystal. This paper describes a 5 cm by 5 cm, gamma camera, based on the Hamamatsu H8500 PSPMT and a continuous La Br3 : Ce crystal, that makes the readout of each channel individually, up to a sampling frequency of 250K Sample/sec, and uses a new algorithm, based on mean square estimation, to reconstruct the parameters of the light distribution produced by the interaction with the gamma photon. The system has many advantages: it does not suffer from reduction of the useful field of view of the detector, that is observed in the center of gravity based approach due to the truncation of the light distribution, determined by the limited dimension of the scintillating crystal. This characteristic allows the full exploiting of the detector surface (at the price of a slight lowering of the resolution a the very borders of the crystal); moreover it is easy to compensate for the gain variations in the PSPMT anodes and in the analog front-ends, avoiding spatial distortion of the gamma image; finally, for very high rate sources, the algorithm is inherently able to discriminate for double hits.