Pulse Shape Discrimination Algorithms, Figures of Merit, and Gamma-Rejection for Liquid and Solid Scintillators

Pulse shape discrimination (PSD) in scintillators is useful to distinguish between signals from neutrons and gamma rays. A standard algorithm is to divide the integral of the tail end of the pulse by the total pulse integral, which itself yields the deposited energy. When the PSD ratio is plotted against the energy, two bands emerge for scintillators that respond differently to neutrons and gammas. Often, a figure of merit (FOM) is defined as the distance between the two bands in a particular energy range, divided by the sum of the full-widths at half-maximum of the PSD ratio bands. A high FOM is usually interpreted as a good indicator of how well a certain scintillator device can distinguish between neutrons and gammas. This, however, ignores the actual shape of the two bands, which may not have a Gaussian shape in the direction of the PSD ratio axis, especially in the presence of pulse pileup. Thus, the FOM may not say much about the gamma rejection capability of the detector, and can therefore be a little misleading. Here, we describe the results of research and development performed on a stilbene detector borrowed from the Lawrence Livermore National Laboratory, but the algorithms described were also successfully used on a number of plastic and liquid scintillation detectors. The standard algorithm described above was compared with a newly developed wavelet-based algorithm that is also used to reject pileup, as well as a pulse shape fitting approach which is shown to be capable of significantly improving the gamma-rejection ratio.