A new method to calculate the response of the WENDI-II rem counter using the FLUKA Monte Carlo Code

The FHT-762 WENDI-II is a commercially available wide range neutron rem counter which uses a He-3 counter tube inside a polyethylene moderator. To increase the response above 10 MeV of kinetic neutron energy, a layer of tungsten powder is implemented into the moderator shell. For the purpose of the characterization of the response, a detailed model of the detector was developed and implemented for FLUKA Monte Carlo simulations. In common practice Monte Carlo simulations are used to calculate the neutron fluence inside the active volume of the detector. The resulting fluence is then folded offline with the reaction rate of the He-3(n,p)H-3 reaction to yield the proton-triton production rate. Consequently this approach does not consider geometrical effects like wall effects, where one or both reaction products leave the active volume of the detector without triggering a count. This work introduces a two-step simulation method which can be used to determine the detector's response, including geometrical effects, directly, using Monte Carlo simulations: A "first step" simulation identifies the He-3(n,p)H-3 reaction inside the active volume of the He-3 counter tube and records its position. In the "second step" simulation the tritons and protons are started in accordance with the kinematics of the He-3(n,p)H-3 reaction from the previously recorded positions and a correction factor for geometrical effects is determined. The three dimensional Monte Carlo model of the detector as well as the two-step simulation method were evaluated and tested in the well-defined fields of an Am-241-Be(alpha,n) source as well as in the field of a Cf-252 source. Results were compared with measurements performed by Gutermuth et al. [1] at GSI with an Am-241-Be(alpha,n) source as well as with measurements performed by the manufacturer in the field of a Cf-252 source. Both simulation results show very good agreement with the respective measurements. After validating the method, the response values in terms of counts per unit fluence were calculated for 95 different incident neutron energies between 1 meV and 5 GeV. (c) 2012 Elsevier B.V. All rights reserved.