Spin Dependence of theU235Low-Energy Neutron Cross Section

The interaction of polarized monochromatic neutrons with polarized ${\mathrm{U}}^{235}$ nuclei has been used to study the spin dependence of the total cross section of ${\mathrm{U}}^{235}$ at a number of neutron energies between 0.075 and 2.04 eV. Measurements were made primarily with a ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}_{0.2}^{}{}_{}{}^{}$ ${\mathrm{La}}_{0.8}$ ${\mathrm{Cl}}_3$ target in which appreciable nuclear polarization was produced. The results show that the 0.275-eV resonance is in the opposite spin state to the 1.14-eV resonance and to the major part of the thermal cross section. The 2.08-eV resonance is probably in the same spin state as the 0.275-eV resonance. If we assume that ${\mathrm{U}}^{235}$ has a negative magnetic moment, then $J=I-\frac{1}{2}=3\mathrm{}$ at 0.275 eV. None of the existing cross-section analyses are completely consistent with the observations for $E\le 1.14$ eV. In ${\mathrm{U}}^{235}$ metal a small "brute-force" nuclear polarization was detected, but there was no evidence of any hyperfine interaction. A somewhat larger nuclear polarization was observed in ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ ${\mathrm{Fe}}_2$, due to a negative hyperfine field of undetermined magnetude.