Dual-panel polyvinyl toluene scintillator application as a radiation portal monitor device for gamma and neutron detection

Abstract

Detection techniques for radiation portal monitors (RPMs) are being developed at Canadian Nuclear Laboratories. One of the purposes of RPM systems at the border ports of entry is to detect illegal transportation of special nuclear material (SNM), which can be used for radiation dispersal devices or nuclear weapons production.One of the most widely used detection technologies considered for RPMs is the polyvinyl toluene (PVT) scintillator. PVT is the most widely used material due to its low cost and its capability to be formed into different shapes and sizes. However, the primary limitation of PVT scintillators is their poor energy spectrum resolution, which limits the discrimination between neutron and gamma radiation.In the present work, a method that allows for the discrimination of fast neutron events from gamma events in a PVT-based RPM was investigated. This method uses two sets of plastic scintillators separated by an air gap. The gap increases the travel distances between scattering events to more easily distinguish pulses produced by neutron interactions in the scintillator from those produced by gamma rays. The time-of-flight (TOF) values between the coincident events in these two sets of scintillators were measured. The TOF for neutrons is significantly longer than the TOF for gamma rays. Another distinguishing characteristic of neutrons is that the flight time between these two scintillator panels will vary according to their energy.The results show that TOF measurements can distinguish neutron sources with sufficiently different energies. Therefore, this method is capable of identifying radioactive material based on neutron energy.In addition to the experimental work, Monte Carlo simulations were carried out using MCNP6.2. The simulation showed that the detector setup had an efficiency (which is the ratio of the number of counts recorded by the detector to the number of neutrons that are incident on the detector) of approximately 12%. Also, the MCNP results showed that the thickness of the second panel had a minimal effect on the signal and no effect on the gamma/neutron peak separation, and therefore could be any conveniently available thickness. However, increasing the air gap between panels increased the peak separation distance but reduced the signal, with a gap distance between 50 cm and 100 cm producing acceptable results.