Comparison of pulse shape discrimination methods for phoswich and CsI:Tl detectors

Comparison of different pulse shape discrimination (PSD) methods was performed under two different experimental conditions and the best PSD method was identified. Beta/gamma discrimination of ⁹⁰Sr/⁹⁰Y and ¹³⁷Cs was performed using a phoswich detector made of BC400 (2.5 cm O.D.×1.2 cm) and BGO (2.5 cm O.D.×2.5 cm) scintillators. Alpha/gamma discrimination of ²¹⁰Po and ¹³⁷Cs was performed using a CsI:Tl (2.8×1.4×1.4 cm³) scintillation crystal. The pulse waveforms were digitized with a DGF-4C (X-Ray Instrumentation Associates, XIA) and analyzed offline with IGOR Pro software (Wavemetrics, Inc.). The four pulse shape discrimination methods that were compared include: digital constant fraction discrimination, rise time discrimination, constant time discrimination (CTD), and charge comparison methods. The CTD method is the ratio of the pulse height at a particular time after the beginning of the pulse to the time at the maximum pulse height. The charge comparison method resulted in a figure of merit (FoM) of 3.3 (9.9% spillover) and 3.7 (0.033% spillover) for the phoswich and the CsI:Tl detectors, respectively. The CTD method resulted in a FoM of 3.9 (9.2% spillover) and 3.2 (0.25% spillover), respectively. Inverting the pulse shape data typically resulted in a significantly higher FoM than conventional methods, but there was no reduction in spillover values. This outcome illustrates that the FoM may not be a good scheme for the quantification of a system to perform pulse shape discrimination.