Absolute Spin Assignments of Dy161 and Dy163 Neutron Resonances and the Hyperfine Coupling Constants in Dy163

Abstract A method and equipment are described for measuring the spin dependence of slow neutron cross sections. The neutron beam from the High Flux Beam Reactor (HFBR) is polarized by Bragg reflection from a magnetized 92% Co-8% Fe single crystal which process also serves to select the neutron energy. Nuclei in the target are statically polarized in a field of ∼ 35 kOe at temperatures ranging from 0.05 to 0.10 K. Large nuclear polarizations are obtained in polycrystalline ferromagnetic dysprosium metal because of the large magnetic hyperfine interaction. The total angular momenta, /, of the compound states corresponding to the first several resonances in Dy161 and Dy163 were determined by measuring the transmission of the target with the neutron beam alternately polarized parallel and antiparallel to the nuclear polarization. Quantitative results at two resonances of opposite spin permit absolute /-value assignments (i. e., the assignments are independent of any knowledge of the absolute signs of the nuclear magnetic moments). Results were as follows: Isotope 163 161 161 161 161 161 161 161 161 163 161 161 E0 (eV) 1.71 2.72 3.69 4.35 7.75 10.40 10.87 12.65 14.3 16.25 16.7 18.5 J 2 3 2 2 3 2 3 2 (?) 2 3 (?) 2 The hyperfine interaction constants for Dy163, obtained by measuring the nuclear polarization as a function of temperature, were A/k = 0.100 ± 0.005 K, and P/k = 0.008 ± 0.001 K where A and P are, respectively, the magnetic and the electric hyperfine interaction constants.