Sorting fission from parasitic coincidences of neutrons and gamma rays in plastic scintillators using particle times of flight

This work addresses the use of plastic scintillators as an alternative to He-3 detectors for radioactive waste drum characterization. The time response of scintillators is three orders of magnitude faster than that of gas proportional counters and they offer similar neutron detection efficiency at lower cost. However, they are sensitive to gamma rays and the commonly used Pulse Shape Discrimination technique is not possible with basic PVT scintillators. This paper reports on an innovative data processing technique allowing to extract spontaneous fission events from parasitic coincidences, such as those from the (alpha,n) reactions accompanied by correlated gamma rays or from pure gamma-ray sources emitting correlated radiations. The proposed approach makes advantage of differences in the pulse detection times recorded in measurements with the Cf-252, AmBe and Co-60 sources. More precisely, a 2D histogram of time delays between the detected 2nd and 1st pulses, on the x-axis, and between the 3rd and 2nd pulses, on the y-axis, is found to allow for selection of a region of interest most relevant to spontaneous fission events.