Passive neutron coincidence counting with plastic scintillators for the characterization of radioactive waste drums

This paper reports a numerical feasibility study of a passive neutron coincidence counting system for radioactive waste drums with plastic scintillators. The motivation is to replace 3He 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 3He. However, unlike 3He 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 241Am, focusing particularly on coincidences with multiplicity 2 and 3. 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 in paralyzing neighboring detectors. A significant improvement of the Pu to 241Am ratio for multiplicity 3 coincidences has thus been obtained, to the detriment of counting statistics but making the measurement feasible.