Event-Resolved Analysis of Neutron Detection by Organic Scintillators: Simulation and Analytical Solution

Organic scintillators, in both liquid and plastic form, are commonly used in systems for the detection of nuclear materials in nonproliferation and homeland security applications. Neutron detection in this type of detector occurs by multiple scatterings on hydrogen (H) and carbon (C), the main constituents of the scintillator. The analysis of the statistics of neutron collisions is important to understand the mechanism of neutron detection, and to perform subsequent unfolding procedures aimed at determining the incident neutron spectrum. We describe the Monte Carlo simulation of neutron interactions with the detector material for varying detector sizes and varying incident neutron energies. The simulations are performed using the code MCNP-PoliMi, which allows event-resolved predictions of the interactions of neutrons with the detector material. A subsequent post-processing of the simulation results allows us to determine the number of elastic collisions that the neutrons undergo with the nuclei of hydrogen and carbon atoms, together with the amount of energy that is deposited as a function of the number of collisions. The light output generated by proton recoils in hydrogen and carbon nuclei as a result of collisions with energetic neutrons from sources such as californium-252 (Cf-252) is also modeled, and the total detector efficiency is determined as a function of the incident neutron energy. When the neutron energy exceeds 4.4 MeV, inelastic scattering of neutrons from carbon nuclei can occur, generating subsequent gamma rays. This effect is taken into account and its contributions to the light output are discussed. An analytical model is developed to describe the amplitude probability distribution of the light output generated in the scintillator for a monoenergetic flux of neutrons, taking into account the multiple collisions and the conversion of the deposited energy per collision to light pulses. The analytical model can be solved quantitatively by numerical quadrature. The possibility of including in the model neutron inelastic scattering and light generation by inelastic gamma rays is also investigated.