Fission fragment mass distributions and total kinetic energy release of 235-uranium and 238-uranium in neutron-induced fission at intermediate and fast neutron energies LA-UR-15-28829

Abstract

This Ph.D. dissertation describes a measurement of the change in mass distributions and average total kinetic energy ($\overline{TKE}$) release with increasing incident neutron energy for fission of $^{235}$U and $^{238}$U. Although fission was discovered over seventy-five years ago, open questions remain about the physics of the fission process. The energy of the incident neutron, $E_n$, changes the division of energy release in the resulting fission fragments, however, the details of energy partitioning remain ambiguous because the nucleus is a many-body quantum system. Creating a full theoretical model is difficult and experimental data to validate existing models are lacking. Additional fission measurements will lead to higher-quality models of the fission process, therefore improving applications such as the development of next-generation nuclear reactors and defense. This work also paves the way for precision experiments such as the Time Projection Chamber (TPC) for fission cross section measurements and the Spectrometer for Ion Determination in Fission (SPIDER) for precision mass yields. The dissertation discusses the motivations and underlying theory behind the study of fission. It also contains a description of the detector, a twin Frisch-gridded ionization chamber, and the neutron source at the Los Alamos Neutron Science Center (LANSCE) Weapons Neutron Research (WNR), which provides neutrons from the intermediate to fast region ($E_n =$ 100's of keV to 100's of MeV). The double energy (2E) analysis procedure is also described, which calculates fragment masses and energies pre- and post-prompt neutron emission. The results of these studies reveal clear structure in the energy-dependent $\overline{TKE}$ and its variance $\sigma_{TKE}$ for both uranium isotopes that correlate to multi-chance fission in the cross section. A low $E_n$ turnover in the $\overline{TKE}$ was observed for $^{235}$U which had only been hinted at in previous experiments. Correlated $TKE$ and masses are also presented.