Excited-State Spin Assignments Based on (n, γγ) Angular-Correlation Measurements on Medium-Weight Nuclei

The angular correlations of cascade gamma rays were measured for ${\mathrm{Ca}}^{45}$, ${\mathrm{Fe}}^{55}$, ${\mathrm{Ni}}^{59}$, ${\mathrm{Ni}}^{61}$, and ${\mathrm{Ni}}^{63}$ formed by thermal-neutron capture. The observation of large anisotropies allows an unambiguous assignment of $J^{\pi }={\frac{3}{2}}^{-}$ to be made for states at excitations of 2.24 MeV in ${\mathrm{Ca}}^{45}$, 0.870 MeV in ${\mathrm{Ni}}^{59}$, and 0.158 and 0.526 MeV in ${\mathrm{Ni}}^{63}$—all of which exhibit an $l=1\mathrm{}$ stripping pattern in ($d, p$) reactions. The data for the $l=1\mathrm{}$ levels at 0.413 MeV in ${\mathrm{Fe}}^{55}$, 0.470 MeV in ${\mathrm{Ni}}^{59}$, and 0.284 MeV in ${\mathrm{Ni}}^{61}$ indicate isotropy to within the statistical errors of less than 1.5%, a result that is consistent with an assignment of $J^{\pi }={\frac{1}{2}}^{-}$ for these levels. The levels at 1.310 MeV in ${\mathrm{Ni}}^{59}$ and 1.008 MeV in ${\mathrm{Ni}}^{63}$ have also been assigned $J^{\pi }={\frac{1}{2}}^{-}$ on the basis of an observed isotropy, although this assignment is somewhat uncertain because the statistical errors are about 12 and 7% respectively. The results definitely show that the alleged separation of levels associated with the $p_{\frac{3}{2}}$ and $p_{\frac{1}{2}}$ gross-structure groups is incomplete; $J=\frac{1}{2}$ levels exist within the $p_{\frac{3}{2}}$ group.