Active voltage divider for improved estimation of interacting radiation energy with photomultiplier tubes coupled to high light yield scintillators

New materials with very high light yield and short scintillation decay time have been developed in the past few years: e.g. Cerium doped Lanthanum Bromide (LaBr₃:Ce), Europium doped Strontium Iodide (SrI₂: Eu). While providing exceptionally good energy and time measurements (e.g. LaBr₃ has 3% FWHM energy resolution and better than 500 ns FWHM time resolution for 661 keV gamma ray interaction), they also pose severe constraints on the working condition of photomultipliers. They have indeed to operate at very large instantaneous current values with minimum distortion of output pulses and also to preserve gain stability over large variation of counting rate of events. We analyzed the effects of output signal saturation and gain variation in photomultiplier tubes as a function of the high voltage level and the amount of energy released in the scintillator crystal. In order to improve the performance of photomultiplier tubes we developed a new voltage divider network completely based on active semiconductor devices. Preliminary tests with only 2 LaBr₃ detectors (3.5" × 8" in size), up to 17 MeV energy interactions, have already shown considerable improvement (by an order of magnitude) as far as linearity of energy estimation is concerned. Additional tests with an array of 10 detectors (3.5" × 8" in size) are planned during forthcoming experimental campaigns at the Legnaro National Laboratory of INFN Italy.