Despite nuclear fission being a prominent feature of physics, there are still several fundamental open questions about this process. One of which is the energy distribution of neutrons emitted immediately after fission, in particular the relative distribution of neutrons above 8MeV. Our experiment measured the prompt neutron energy spectrum of 235U from 3-10MeV. The measurement took place at Los Alamos National Lab and used a double-time-of-flight technique to measure the neutron beam and the fission neutron kinetic energy. Fission event timing was measured by a parallel plate avalanche counter while 2m plastic scintillation neutron detectors were used to measure neutron time of flight. By combining this information with a known flight path the kinetic energy of the neutron can be obtained. To eliminate backgrounds various time of flight and energy cuts were imposed and an accidental time of flight background was subtracted. An MCNP simulation, including the 2m neutron detector geometry, was done using the Madland and Nix model as the input energy distribution for the simulated neutrons. Finally we compare our measured energy spectrum with the MCNP simulated 235U spectrum, with an uncertainty of 40% for the 9-10MeV energy bin.

Host: Mike Kovash