Abstract
Fission modeling in general-purpose Monte Carlo transport codes often relies on average nuclear data provided by international evaluation libraries. As such, only average fission multiplicities are available and correlations between fission neutrons and photons are missing. Whereas uncorrelated fission physics is usually sufficient for standard reactor core and radiation shielding calculations, correlated fission secondaries are required for specialized nuclear instrumentation and detector modeling. For coincidence counting detector optimization for instance, precise simulation of fission neutrons and photons that remain correlated in time from birth to detection is essential. New developments were recently integrated into the Monte Carlo transport code TRIPOLI-4 to model fission physics more precisely, the purpose being to access event-by-event fission events from two different fission models: FREYA and FIFRELIN. TRIPOLI-4 simulations can now be performed, either by connecting via an API to the LLNL fission library including FREYA, or by reading external fission event data files produced by FIFRELIN beforehand. These new capabilities enable us to easily compare results from Monte Carlo transport calculations using the two fission models in a nuclear instrumentation application. In the first part of this paper, broad underlying principles of the two fission models are recalled. We then present experimental measurements of neutron angular correlations for 252Cf(sf) and 240Pu(sf). The correlations were measured for several neutron kinetic energy thresholds. In the latter part of the paper, simulation results are compared to experimental data. Spontaneous fissions in 252Cf and 240Pu are modeled by FREYA or FIFRELIN. Emitted neutrons and photons are subsequently transported to an array of scintillators by TRIPOLI-4 in analog mode to preserve their correlations. Angular correlations between fission neutrons obtained independently from these TRIPOLI-4 simulations, using either FREYA or FIFRELIN, are compared to experimental results. For 240Pu(sf), the measured correlations were used to tune the model parameters.
Highlights
Spontaneous fission is characterized by the emission of bursts of neutrons
The combination of the Monte Carlo radiation transport code TRIPOLI-4.10 with these fission generators enables users to directly model fission eventby-event and transport fission secondaries through complex detector geometries while keeping them fully correlated from birth to detection
A FIFRELIN calculation consists on one hand in sampling the initial state of a pair of fission fragments emerging from a binary fission event and on the other hand in deexciting both fragments [14]–[17]
Summary
Spontaneous fission is characterized by the emission of bursts of neutrons. Characterization of nuclear materials becomes possible by measurements of the fission neutron angular anisotropy. General-purpose Monte Carlo codes are available for modeling neutron transport. They have traditionally employed “average fission models” for modeling fission, characterized by uncorrelated secondary particle emission. The combination of the Monte Carlo radiation transport code TRIPOLI-4.10 with these fission generators enables users to directly model fission eventby-event and transport fission secondaries through complex detector geometries while keeping them fully correlated from birth to detection. We present experimental measurements of neutron-neutron angular correlations between 252Cf(sf) and 240Pu(sf) fission neutrons We compare these correlations to those obtained by simulations and discuss the differences between the two fission models. Model parameters are adjusted to best match the experimental data
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