Evaluation of the Constant Fraction Time-Over-Threshold (CF-TOT) method for neutron-gamma pulse shape discrimination

Abstract

The use of Time-over-Threshold (TOT) for the discrimination between fast neutrons and gamma-rays is advantageous when large number of detection channels are required due to the simplicity of its implementation. However, the results obtained using the standard, Constant Threshold TOT (CT-TOT) are usually inferior to those obtained using other pulse shape discrimination (PSD) methods, such as Charge Comparison or Zero-Crossing approaches, especially for low amplitude neutron/gamma-ray pulses. We evaluate another TOT approach for fast neutron/gamma-ray PSD using Constant-Fraction Time-over-Threshold (CF-TOT) pulse shape analysis. The CT-TOT and CF-TOT methods were compared quantitatively using digitized waveforms from a liquid scintillator coupled to a photomultiplier tube as well as from a stilbene scintillator coupled to a photomultiplier tube and a silicon photomultiplier. The quality of CF-TOT neutron/gamma-ray discrimination was evaluated using Receiver Operator Characteristics curves and the results obtained with this approach were compared to that of the standard CT-TOT method. The CF-TOT PSD method results in > 99.9% rejection of gamma-rays with > 80% neutron acceptance, much better than CT-TOT.

Generalized Method for the Optimization of Pulse Shape Discrimination Parameters

“…A relatively high TTR corresponds to pulses with an increased delayed fluorescence emission with respect to the prompt fluorescence emission. Despite the increasing number of alternative PSD approaches, e.g., based on zero-crossing [7,8,9,10,11], time-over-threshold [12] and machine-learning [13,14], the CI remains the most frequently-used method for PSD. While being simple to implement, CI requires a lengthy, source-and material-specific optimization of the pulse integration gate parameters, which is typically performed by evaluating the PSD figure-of-merit [15] for many iterations of such parameters.…”

Optimization and first electronic implementation of the Constant-Fraction Time-Over-Threshold pulse shape discrimination method

“…Existing multi-channel Application Specific Integrated Circuits (ASICs) are all based on the CT-ToT [15,16] method. In a recent publication [17] we analysed a different variant of ToT, using a constant fraction method (CF-ToT), demonstrating its superior performance in neutron/gamma PSD compared to CT-ToT, for energies in the range 100-1000 keVee. Since CF-ToT is independent of pulse amplitude and varies solely with the pulse shape, it can potentially serve as a better alternative for the design of future multi-channel PSD circuits that avoid pulse digitization.…”