Prompt Neutron and Gamma Ray Emissions from Fission

Binary fission is the process by which a nucleus splits into two excited fission fragments. These fragments are subsequently de-excited by the emission of prompt neutrons and gamma rays. Prompt neutrons are emitted within approximately 10-14 seconds after fission while the emission of prompt gamma rays typically follows prompt neutron emission (approximately 90 percent of the prompt gamma rays are emitted within 1 ns following fission, with the rest appearing within approximately 100 ns). Many previous experiments focus on the multiplicity and energy of prompt emissions averaged over all detected events. However, neutron and gamma-ray emissions are correlated with the fission fragments and with each other. The correlations depend on whether the fragments arise from spontaneous fission or neutron-induced fission and, in the case of neutron-induced fission, the energy of the incident neutron. Experiments that can measure the direction and energy of these prompt emissions, as well as the identity of the fission fragments, can study these correlations. Several experiments have been performed to capture this information, but more are needed. Prompt emissions from fission are important for applications because these emissions can escape the fissile sample and be detected. For example, in nuclear safeguards prompt emissions are used to determine the mass and composition of a fissile sample. In this presentation, I will discuss recent experiments performed at Los Alamos National Laboratory to study correlated neutron and gamma-ray emission from the spontaneous fission of Cf-252. Specifically, we show the measured neutron and gamma-ray multiplicity distributions and the neutron energy distributions as a function of neutron multiplicity. We compare these measured quantities to predictions obtained with the physics-based complete event fission models, CGMF and FREYA (Fission Reaction Event Yield Algorithm), and the transport code MCNPX-PoliMi.