Speaker
Description
As a source of neutrons within the nuclear chain reaction, prompt fission neutrons are of decisive importance for nuclear physics applications. Their average neutron multiplicity impacts the amount of neutrons available for further fissions, while their energy distribution impacts the probability for neutrons to escape the core of a nuclear reactor and do structural damages to the vessel. New high precision measurements of prompt fission neutron spectra (PFNS) and average prompt neutron multiplicity have been done at the WNR facility of the Los Alamos Neutron Science Center. The PFNS of the major actinides $^{239}$Pu, $^{235}$U and $^{238}$U were measured as a function of the incoming neutron energy by double time of flight for incoming neutron energies above 1 MeV with two different methods. While results of two methods will be shown, we will detail one type of measurement, performed with respect to the well known PFNS and average neutron multiplicity of $^{252}$Cf spontaneous fission.
The setup consisted of a high efficiency and fast-timing fission chamber coupled to the liquid scintillators array Chi-Nu or VENDETA (VErsatile Neutron DETector Array). Prompt fission neutron energies between 150 keV and 12 MeV can be measured, for incident neutron energies from 0.7 to 800 MeV. Average neutron multiplicities and mean neutron energies were extracted from the PFNS measurement as a function of the incoming neutron energy. Up to 25 MeV of incoming neutron energies, relative uncertainties are below 0.5% on the average neutron multiplicity and below 0.8% on the mean energy. The energy range measured and the level of precision achieved allow us to observe the opening of alternative fission channels, such as second chance fission. Beyond the need for evaluated nuclear data libraries and nuclear applications, this level of precision also provide constrains on fission models such as GEF and CGMF, and on partial fission cross sections. The experimental method will be detailed and the results will be compared to existing data, evaluations and fission models for discussion.