The time shift between the neutron and γ-ray timestamps in organic stilbene scintillator

It is shown that the use of the standard constant fraction timing method for obtaining timestamps of signals from an organic stilbene scintillator leads to a systematic difference in timestamps when detecting neutrons and γ-rays. A detailed study of this effect was carried out using a digital time-of-flight spectrometer based on a stilbene detector and a pulsed source of quasi-monoenergetic neutrons based on the 2H(d,n)3He reaction. It is shown that this effect is present for the entire range of crystal light output values and is related to the difference in the rise time of signals induced by neutrons and γ-rays. It is demonstrated that this effect can lead to a systematic error in absolute measurements of the prompt fission neutron spectra using the time-of-flight method with conventional analog constant fraction timing. A method for compensating this systematic shift based on the analysis of the timestamps position for the digital signals at the different values of the constant fraction is suggested.