Cross-Talk Characterization in Passive Neutron Coincidence Counting of Radioactive Waste Drums With Plastic Scintillators

This paper reports on a numerical feasibility study of a passive neutron coincidence counting system for radioactive waste drums with plastic scintillators. The motivation is to replace $^{3}\text{He}$ gas counters generally used for this type of measurement. Indeed, plastic scintillators present several advantages for the measurement of neutron coincidences such as a good efficiency for detecting fast neutrons, short detection time, and low cost comparatively to $^{3}\text{He}$ . However, unlike $^{3}\text{He}$ counters, their high sensitivity to gamma rays and cross talk constitutes a drawback as parasite random and true coincidences are detected together with the useful signal of plutonium. Simulations are performed using the Monte Carlo transport code MCNPX-PoliMi v2.0 coupled to data processing algorithms developed with ROOT data analysis software. Performances of the coincidence counting system are studied for the case of a vitrified waste drum containing Pu and $^{241}\text{Am}$ , focusing particularly on multiplicity 1 and 2, i.e., 2 or 3 pulses recorded in a short time gate in different detectors. Cross talk induced by neutrons and gamma rays has been characterized in terms of time and distance between detectors, and strategies to limit this phenomenon are reported, consisting of ignoring neighboring detectors signal. A significant improvement of the Pu to $^{241}\text{Am}$ ratio for multiplicity 2 coincidences has thus been obtained, at the expense of counting statistics. Alternative case studies with organic and metallic matrixes of technological wastes are also reported, for which the part of useful signal of plutonium is significantly higher, showing the feasibility of the measurement method.