Design of a transportable high efficiency fast neutron spectrometer

Abstract

A transportable fast neutron detection system has been designed and constructed for measuring neutron energy spectra and flux ranging from tens to hundreds of MeV. The transportability of the spectrometer reduces the detector-related systematic bias between different neutron spectra and flux measurements, which allows for the comparison of measurements above or below ground. The spectrometer will measure neutron fluxes that are of prohibitively low intensity compared to the site-specific background rates targeted by other transportable fast neutron detection systems.To measure low intensity high-energy neutron fluxes, a conventional capture-gating technique is used for measuring neutron energies above 20MeV and a novel multiplicity technique is used for measuring neutron energies above 100MeV. The spectrometer is composed of two Gd containing plastic scintillator detectors arranged around a lead spallation target. To calibrate and characterize the position dependent response of the spectrometer, a Monte Carlo model was developed and used in conjunction with experimental data from gamma ray sources. Multiplicity event identification algorithms were developed and used with a Cf-252 neutron multiplicity source to validate the Monte Carlo model Gd concentration and secondary neutron capture efficiency. The validated Monte Carlo model was used to predict an effective area for the multiplicity and capture gating analyses. For incident neutron energies between 100MeV and 1000MeV with an isotropic angular distribution, the multiplicity analysis predicted an effective area of 500cm2 rising to 5000cm2. For neutron energies above 20MeV, the capture-gating analysis predicted an effective area between 1800cm2 and 2500cm2. The multiplicity mode was found to be sensitive to the incident neutron angular distribution.