Speaker
Description
The neutron emission is an important fingerprint of the excitation-energy sharing in the nuclear fission process. Highly excited, neutron-rich nuclei release their energy by emitting prompt neutrons and γ-rays. Improved determination of particle-emission characteristics enhances the understanding of fission dynamics. The nuclear data libraries lack data which are crucial for various applications, especially the evolution of nuclear observables as a function of excitation energy. For instance, the change in the prompt neutron multiplicity, $\bar{ν}$(A), as a function of excitation energy is one of many open questions, which leads to divergences between fission models. Earlier measurements, suggest that only the heavy fission fragments exhibit an increased average number of emitted neutrons, when the compound nucleus populates higher excitation energy. Data are however scarce, which necessitates systematic new studies and assessment on what bearings it has on fission modeling and on calculation codes for nuclear reactors, waste-management and other applications.
In this paper we report on a direct experimental determination of the so-called sawtooth shape, $\bar{ν}$(A), as a function of the nuclear excitation energy. Fission fragment from the $^{235}$U(n,f) reaction were measured, in coincidence with prompt fission neutrons, at the MONNET accelerator of the EC-JRC-GEEL. Most existing reference data are a result of indirect measurements, by detecting the fission fragment velocities and energies. We instead performed a direct coincidence measurement by using liquid scintillators and a Twin Frisch-grid ionization chamber. The experiments were conducted at thermal neutron energy as well as 5.5 MeV incident neutron energy. A fully digital data acquisition system was used which allowed detailed offline analysis on the raw signal traces. The harsh neutron background encountered from the neutron beam was significantly suppressed by the fission trigger from the fission chamber and by applying elaborated data cuts. We will present $\bar{ν}(A)$ and $\bar{ν}$(Total Kinetic Energy) for $^{235}$U at thermal and at 5.5 MeV incident neutron energy and discuss the overall features of the prompt fission neutron multiplicity, both in relation to earlier data and to evaluations and model calculations.