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

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.