  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 the 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.
