Exploring the sampling rate effect on digital time of flight response for fast scintillator detectors

Abstract

A systematic study is conducted to understand the digital time of flight (TOF) response for a pair of fast scintillator detectors; BaF2–LaBr3, BaF2-BC501A, and LaBr3-BC501A respectively. Coincidence signals from each detector pair, irradiated by 22Na source, are acquired by LeCroy HDO5000A digital oscilloscope. While operating the oscilloscope at 2.5 giga samples per second (GSPS) and 500 mega samples per second (MSPS) of sampling rates, over 10 k coincidence signals were collected for each pair of detectors. Data at different sampling rates; 1.66 GSPS, 1.25 GSPS, 833 MSPS, 625 MSPS, 333 MSPS, and 250 MSPS were generated by down-sampling method. Using anode signal, the photon arrival time marker is determined by digital constant fraction (DCF) algorithm for each detector pair on event-by-event basis. For a given rate, the algorithm is optimized at various delay and fraction values to get the minimum TOF dispersion. Mentioned pairs reveals the TOF resolution (FWHM) as; 0.58 ns, 0.79 ns, 0.907 ns and 0.53 ns, 0.94 ns, 1.08 ns at 250 MSPS and 2.5 GSPS respectively. While analysing the events, a saturation in the TOF width is observed from 500 MSPS onward. The saturated dispersion values for the aforementioned pairs are found to be 0.53 ns, 0.94 ns, and 1 ns respectively. Effect is being understood by calculating the optimum DCF transition region distribution for BC501A and LaBr3 detectors, reveals a constant curvature profile at different rates. To further explain the saturation effect on computational ground, a simple pulse fitting approach is adopted using Landau distribution. Event-by-Event processing of fitted pulses followed by TOF resolution calculation reproduces the experimental numbers.