Neutron detector designs for detecting fission neutrons in intense pulsed environments

In this paper, the MCNPX code is used to optimize the moderator materials and thicknesses for a neutron detector based on a ⁶Li-loaded glass scintillator. It is shown that the response of the ⁶Li detector to a delayed-neutron-fission spectrum is comparable to a ³He-based gas proportional counter that is configured with a similar active area and moderator materials. For a delayed-neutron spectrum, a polyethylene thickness between 4 cm and 5 cm optimizes the number of ⁶Li(n,α)³H reactions in the glass. Furthermore, when the polyethylene is surrounded by additional thickness of 2-cm of Flexi-boron the response of the detector to neutrons with energies of 1-100 eV (where the emission of delayed fission neutrons is small) is reduced by a factor of two or more while reducing the response at the peak of the delayed spectrum by only 20-30%. The addition of this thick Flexi-boron layer can be an effective way to reduce the unwanted signals from cosmic-ray neutrons and other background sources. A good neutron detector should also be able to distinguish between high-energy gamma-rays and neutrons. In a pulsed-power environment, the bremsstrahlung from the generator can also temporarily blind the detector. Lead shielding can be used to reduce these effects so that the detector can recover fast enough to measure delayed fission neutrons. We have used MCNPX to address the effects of lead shielding on the response of the ⁶Li detector to delayed fission neutrons. It is found that the addition of lead has little effect on the optimum polyethylene thickness. The addition of 5-cm-thick lead (10-cm) reduces the response to the delayed neutron spectrum by about 30% (50%) while reducing the effects of 1 MeV gamma-rays by a factor of 30 (1000).