Monte Carlo Calculations of the Detection Efficiency of Composite Scintillator Arrays for Fast and Moderated Neutrons, and for Gamma-Ray Spectroscopy

Fissionable materials can be distinguished from normal materials by their emission of fast neutrons and neutrons moderated by surrounding materials. Radioisotopes can often be identified by their gamma-ray spectra. In this work, we use the nuclear physics Monte Carlo package MCNP6.2 to compare the detection efficiencies of different detector designs for the triple-mode detection of fast and moderated neutrons, and gamma-ray spectroscopy. For each design, we tabulate the detection efficiencies for neutrons from 0.1 eV to 5 MeV and for the full absorption of gamma rays from 0.1 to 5 MeV. All designs use 100 scintillators 1 cm $\times $ 1 cm $\times $ 5 cm. Of these, 36 are Cs2 LiLaBr6:Ce (90% 6Li) (CLLB) and 64 are polyvinyl toluene (PVT). The first design uses separate arrays of $6\times 6$ CLLB and $8\times 8$ PVT scintillators. Three other designs are $10\times 10$ composite arrays using all 100 scintillators combined in different ways. PVT is a fast scintillator that can be used to detect fast neutrons by time-separated multiple proton recoils and discriminate against gammas. PVT in the three composite designs serves to moderate neutrons of all energies for capture by the 6Li in the CLLB. Gamma rays interact in the PVT and CLLB by Compton scattering and pair production; full energy absorption requires photoelectric absorption in the CLLB. The energy deposited by each gamma ray can be determined from a weighted sum of the CLLB and PVT signals from separate photodetectors. We find that for a fixed number of neutrons/cm2, one of the composite arrays has much higher 6Li detection efficiencies than the design that uses separate CLLB and PVT arrays, and has similar detection efficiencies for fast neutrons and gammas. We also find that in all cases, the average energy of the neutrons at capture by 6Li is well above the 25-meV thermal energy.