Simple charge division readouts for imaging scintillator arrays using a multi-channel PMT

Three simple charge division circuits were assembled and tested as 2-D position encoding readouts for multi-channel photomultiplier tubes (MC-PMT). They were evaluated with an 8x8 array of individual scintillators (2x2x10 mm EGO) coupled to a 64 channel MC-PMT (Philips XP1722) via 25 cm long, 2 mm diameter, double clad, optical fibers (Kuraray). This type of gamma-ray imaging detector has many potential applications in medical and industrial imaging. Though independent channel readout would allow for the discrimination of scatter within the array, and higher count rates, it would also require an excessive amount of supporting electronics. This is especially true for systems comprised of many MC-PMTs. In this study, the number of channels being read out was reduced from 63 to 4 using three different simple resistor networks. These circuits take advantage of the discretized nature of the scintillator array, the low interchannel crosstalk of the MC-PMT and low input impedance current-sensitive preamplifiers. For each circuit, the scintillator identification accuracy was compared. The identification accuracy as a function of deposited energy was also determined by exposure to various gamma-ray emitters. It was found preamplifier circuit noise contributed the most degradation of the detector's spatial response so several low noise op amps were evaluated. It was also determined that keeping the preamplifier input impedance small was necessary for accurate positioning. The coincidence timing resolution of the detector (15 ns) is only slightly degraded by the readout circuit.