Structure of the Fission Transition NucleusU235

Abstract

Fission-fragment angular distributions for the ${\mathrm{U}}^{234}$ ($n, f$) reaction were measured for incident neutron energies of 200, 300, 400, 500, 600, 700, 843, 998, and 1184 keV. A novel $2\pi$ geometry Lexan (polycarbonate) detector was used to detect the fission fragments. The angular distributions show intermediate-angle peaks and change shape rapidly with changes in neutron energy. Based on a sophisticated Hauser-Feshbach analysis, which includes level-width fluctuation corrections of the energy variation of the fission cross section and angular distributions, a description of the highly deformed fission transition nucleus ${\mathrm{U}}^{235*}$ is given. Assignments of the quantum numbers ($K$, $\pi$) are made for three low-energy single-particle states, and they are ½+, $\frac{3}{2}+$, and $\frac{3}{2}-$. In addition, the presence of another $K=\frac{1}{2}$ state is demonstrated. Assignments of the energy $E$ and the barrier curvature $\hslash \omega$ of these states are also given.