Measured and Simulated Prompt Fission Neutron and Photon Correlations

Abstract

An accurate understanding of fission is critical to characterization of special nuclear material (SNM) for nonproliferation and safeguards applications. Noninvasive and nondestructive techniques rely primarily on highly penetrating and relatively abundant fission emissions. Spontaneously and under particle interrogation, SNM emits neutrons and photons from fission, which are characteristic of the fissioning isotopes. Characteristic neutrons and photons are emitted from nuclear fission when a deformed, neutron-rich nucleus divides into two fragments that then de-excite. During de-excitation, neutrons are emitted first, followed by photons; this process gives rise to correlations. New, event-by-event, physics-based models, CGMF (Los Alamos National Laboratory) and FREYA (Lawrence Livermore National Laboratory), predict correlations in prompt fission emissions. Current safeguards and nonproliferation systems do not utilize angular or multiplicity correlations. Little data exist to validate these models; correlated quantities have been measured only for 252Cf(sf). My work provides measured correlation data to validate models useful for future system design.

Previous correlation measurements have been limited by the acquisition challenges of a many-detector array and therefore have used simple detector systems. Additionally, few detection methods exist that are simultaneously efficient to neutrons and photons. In this work, I show a many-detector array of pulse-shape-discrimination-capable organic scintillators, sensitive to both fast neutrons and photons, to measure correlations in neutron energy, photon energy, multiplicity, and emission angle. This work is achieved through MCNPX-PoliMi simulations and through use of time-synchronized, high-throughput, multiple-digitizer acquisition systems. I performed experiments sensitive to correlations with a large array of organic scintillators. I performed measurements of 252Cf(sf) at both the University of Michigan and the Los Alamos National Laboratory; and of 240Pu(sf) at the Joint Research Centre in Ispra, Italy, and at the Los Alamos National Laboratory.

I measured the 240Pu(sf) neutron-neutron angular distribution and found it to be less anisotropic than the 252Cf(sf) neutrons. 240Pu(sf) and 252Cf(sf) neutron-neutron angular distribution simulation results indicate that fission models capture the general trend of neutron anisotropy. 240Pu(sf) and 252Cf(sf) experimental multiplicity results suggest weak neutron-photon competition during fragment de-excitation. The measured correlations were compared with MCNPX-PoliMi simulations using the built-in model and two new event-by-event fission models, CGMF and FREYA, which predict correlations in prompt emissions from fission. Simulation results from CGMF and FREYA predict a stronger negative correlation than the experiment result.