Interaction of 350-keV Polarized Neutrons with OrientedHo165Nuclei

Abstract

The interaction of polarized and unpolarized 350-keV neutrons with oriented and unoriented ${}_{}{}^{165}{}_{}{}^{}\mathrm{Ho}$ nuclei was investigated in order to study the effect of nuclear deformation and to search for a possible spin-spin interaction. The ORNL 3-MV pulsed and bunched Van de Graaff proton beam was used in conjunction with a time-of-flight spectrometer. The ${}_{}{}^7{}_{}{}^{}\mathrm{Li}(p, n)$ reaction at a laboratory angle of 51° provided a source of 55% polarized, 350-keV neutrons. Using the NBS transportable ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ refrigerator, a single crystal of ${}_{}{}^{165}{}_{}{}^{}\mathrm{Ho}$ metal was cooled to 0.34°K and a nuclear polarization of approximately 15% was obtained with a super-conducting split solenoid. The differential cross section for unpolarized 350-keV neutrons, elastically scattered from unoriented ${}_{}{}^{165}{}_{}{}^{}\mathrm{Ho}$ nuclei, was measured in a cylindrical geometry and is in good agreement with the results of a coupled-channel calculation. The total cross section of unoriented ${}_{}{}^{165}{}_{}{}^{}\mathrm{Ho}$ was measured between 300 and 400 keV. A coupled-channel calculation of the total cross section using the same optical-model parameters as those which fitted the angular distribution is found to agree very well with these data. To investigate the spin-spin interaction, measurements were made of the transmitted intensities with the target nuclei polarized alternately parallel and antiparallel to the direction of neutron polarization. The observed change in intensity, (-0.11±0.32)%, and the observed total cross section of 7.94 b imply a change in cross section of (+30±85) mb, where the plus sign corresponds to a larger cross section for the parallel orientation. Comparison of this result with that of the coupled-channel calculation indicates that if the spin-spin interaction in the optical-model potential is written as $-V_{\mathrm{ss}}\left(\sigma ·\mathrm{I}\right)$ times a Saxon form factor, then $V_{\mathrm{ss}}$ lies between -130 and +280 keV. Since the ${}_{}{}^{165}{}_{}{}^{}\mathrm{Ho}$ nucleus is highly deformed, a transmission measurement was made to determine the change in cross section between a highly oriented and a nearly unoriented target. The transmission of the target decreased by (1.3±0.4)% for the higher orientation value, which corresponds to a cross-section increase of 350±100 mb. This deformation effect on the total cross section is also in good agreement with that derived from the coupled-channel calculation. The deformation parameter $\beta$ used in the optical-model potential was +0.3, in agreement with other measurements.