Multiple Time-Over-Threshold Readout Electronics For Fast Timing And Energy Resolving In A Sipm-Based Positron Annihilation Lifetime Spectrometer

Fast timing and energy resolving capabilities are crucial for a silicon-photomultiplier-based positron annihilation lifetime (SiPM-PAL) spectrometer. Time-over-threshold (ToT) offers an attractive method for combining timing and energy encoding with the advantage of simplicity in electronics implementation and SiPM readout. However, single ToT is not appropriate for a SiPM-PAL spectrometer because of its poor energy linearity and limited energy range. In this work, a multi-ToT readout circuit for a SiPM-PAL spectrometer is developed. Each readout channel uses only a pre-amplifier and a comparator for timing, three comparators for energy resolving, resistors, and capacitors. A simple pulse area estimation method using multi-ToT pulse widths and fixed threshold voltages for energy resolving is presented and evaluated. The energy performance and pulse area integral nonlinearity (INL) are investigated using Na-22, Co-60, and Cs-137 gamma sources with a SiPM-based scintillation detector. For pulse area spectra using the proposed method, multiToT (triple and dual threshold cases) resolves gamma energies much better than does single ToT. Using triple-ToT, the pulse area linearity in INL in the energy range of 511-1333 keV is superior than 4.21%, which is at least 25% better than that with dual-ToT. Excellent coincidence resolving times of 161.5 +/- 1.4 ps and 162.7 +/- 1.3 ps in full width at half maximum are achieved using tripleToT and dual-ToT, respectively, for 3 mm x 3 mm x 5 mm lutetium-fine-silicate scintillators. The quantitative energy and timing results show that the multi-ToT circuit is suitable for the readout electronics of a SiPM-PAL spectrometer.