Total kinetic energy release in the fast neutron-induced fission of $^{235}$U

TL;DR

This study measures the total kinetic energy release in neutron-induced fission of uranium-235 across a wide energy range, revealing a decrease in TKE with increasing neutron energy and insights into fission modes.

Contribution

First absolute measurement of TKE in $^{235}$U(n,f) over 2-100 MeV neutron energies using the 2E method with benchmarking at thermal energies.

Findings

TKE decreases non-linearly from 169 to 161 MeV between 2 and 100 MeV neutron energies.

The decrease in TKE is due to the fade out of asymmetric fission modes.

TKE associated with different fission modes remains constant across neutron energies.

Abstract

We have measured the total kinetic energy (TKE) release for the $^{235}$U(n,f) reaction for $E_{n}$=2-100 MeV using the 2E method with an array of Si PIN diode detectors. The neutron energies were determined by time of flight measurements using the white spectrum neutron beam at the LANSCE facility. To benchmark the TKE measurement, the TKE release for $^{235}$U(n$_{th}$,f) was also measured using a thermal neutron beam from the Oregon State University TRIGA reactor, giving pre-neutron emission $E^*_{TKE}=170.7\pm0.4$ MeV in good agreement with known values. Our measurements are thus absolute measurements. The TKE in $^{235}$U(n,f) decreases non-linearly from 169 MeV to 161 MeV for $E_{n}$=2-100 MeV. The multi-modal fission analysis of mass distributions and TKE indicates the origin of the TKE decrease with increasing neutron energy is a consequence of the fade out of asymmetric fission, which is associated with a higher TKE compared to symmetric fission. The average TKE associated with the superlong, standard I and standard II modes for a given mass is independent of neutron energy. The widths of the TKE distributions are constant from $E_{n}$=20-100 MeV and hence show no dependence with excitation energy.