Time-of-flight neutron spectrum unfolding for mixed-oxide nuclear fuel and plutonium metal using a dual-particle imager

Abstract

1 Abstract-A dual-particle imager with sensitivity to both neutrons and photons has been developed for the detection of special nuclear material. This system has the capability to provide spectral information on detected sources that is useful for safeguard applications. In nuclear facilities with high photon background rates, neutron detection can aid in identification and verification capabilities. The dual-particle imager creates a reconstructed neutron spectrum by correlating neutron counts in pairs of liquid scintillators. The reconstructed spectrum, a combination of energy deposited and time-of-flight, contains resolution effects that blur the true, emitted source-spectrum. To reduce the impact of resolution effects, the reconstructed spectrum can be unfolded to create an estimated source spectrum. A statistical technique, maximum-likelihood expectation-maximization, has been shown to produce an unfolded result for Cf-252 and Am-Be. This technique requires a system response-matrix that contains reconstructed spectra for single neutron energies. Since measuring mono-energetic neutrons is difficult, the system-response-matrix is simulated. Monte Carlo modeling with MCNPX-PoliMi has been show to produce an accurate system response. This work expands on previous neutron unfolding results by showing results for mixedoxide fuel and plutonium metal measured with the dual-particle imager. These results also demonstrate the ability of the dualparticle imager to discrimination between spontaneous fission neutrons and ( ,n) reaction neutrons.

Angular-resolution and material-characterization measurements for a dual-particle imaging system with mixed-oxide fuel

“…For a single neutron event, both energy depositions must be above the detection threshold to be recorded by the system. While neutron spectra do not have easily recognizable features, the slope of the spectrum between 2.5 and 6 MeV can be used to discern a pure spontaneous fission source from a mixed source like the MOX canisters [18]. This characterization can also be done by looking at single-detector neutron pulse height distributions (PHD) [19].…”