Neutron Resonance Spectroscopy. III.Th232andU238

Abstract

The neutron total cross sections for ${\mathrm{Th}}^{232}$ and ${\mathrm{U}}^{238}$ have been measured from about 100 eV to 4 keV with ∼0.5 nsec/m resolution at the highest energies. The mean level spacings for these nuclei have been observed to be (17.5±0.7) eV and (17.7±0.7) eV. For ${\mathrm{U}}^{238}$ about 17% of the observed levels probably belong to $l=1\mathrm{}$ and are omitted from the statistical analysis of the data. $S$-wave strength functions of (0.69±0.07)×${10}^{-4\mathrm{}}$ and (0.90±0.10)×${10}^{-4\mathrm{}}$ are obtained for ${\mathrm{Th}}^{232}$ and ${\mathrm{U}}^{238}$, respectively. Detailed comparisons of the observed statistical aspects of the level spacings and neutron reduced width distributions have been made with the predictions of various theoretical models. The distributions of ${{\Gamma }_n}^0$ for both nuclei seem to be consistent with a single-channel Porter-Thomas distribution. The observed distributions of nearest- and next-nearest-neighbor level spacings agree quite well with the expected distributions for real symmetric Hamiltonian matrices with randomly distributed matrix elements. The correlation coefficients for ${\mathrm{Th}}^{232}$ and ${\mathrm{U}}^{238}$ are, respectively: (a) (-0.21±0.07) and (-0.26±0.08) for adjacent level spacings, (b) (-0.03±0.07) and (-0.17±0.07) for adjacent reduced neutron widths, and (c) (0.12±0.07) and (0.15±0.07) for the neutron reduced width of a level and the average of its spacing from adjacent levels.