Properties ofγ-Ray Transitions inCo56fromNi56Decay andFe56(p, nγ)Co56

Abstract

The $^{56}\mathrm{Ni}$ $\ensuremath{\epsilon}$ decay and the $^{56}\mathrm{Fe}(p, n\ensuremath{\gamma})^{56}\mathrm{Co}$ reaction with beam energies between 5.5 and 8.4 MeV have been used with Ge(Li) spectrometers to study the properties of $\ensuremath{\gamma}$ rays from states of $^{56}\mathrm{Co}$ below 2.86 MeV excitation. From $^{56}\mathrm{Ni}$ $\ensuremath{\epsilon}$ decay both the $\ensuremath{\gamma}$-ray spectrum and $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidences were studied. $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidences, $\ensuremath{\gamma}$-ray excitation functions, $\ensuremath{\gamma}$-ray angular distributions, and absolute cross sections were measured for the $^{56}\mathrm{Fe}(p, n\ensuremath{\gamma})^{56}\mathrm{Co}$ reaction. An $\ensuremath{\epsilon}$ decay scheme for $^{56}\mathrm{Ni}$, which includes six $\ensuremath{\gamma}$ rays, and an energy-level diagram for $^{56}\mathrm{Co}$, which includes 35 $\ensuremath{\gamma}$ rays (14 of which are reported for the first time) from 20 excited states, are presented. Comparison of the data from $^{56}\mathrm{Fe}(p, n\ensuremath{\gamma})^{56}\mathrm{Co}$ with predictions of the statistical compound-nuclear model have resulted in spin assignments (in parentheses) for the following states (energies in keV) of $^{56}\mathrm{Co}$: 158.4(3), 576.6(5), 829.7(4), 970.3(2), 1009.2(5), 1114.6(3), 1450.8(0), and 1720.3(1). Branching ratios are presented for 14 $\ensuremath{\gamma}$ rays from these eight states and multipole mixing ratios are given for 12 of these $\ensuremath{\gamma}$ rays (10 are predominantly $M1$). The data are consistent with a spin-4 assignment to the ground state. Contrary to previous suggestions, evidence from all experiments indicates that only one state (believed to be the antianalog of the $^{56}\mathrm{Fe}$ ground state) exists in $^{56}\mathrm{Co}$ in the neighborhood of 1451 keV excitation. The level energies, $\ensuremath{\gamma}$-ray multipole mixing ratios, and $\ensuremath{\gamma}$-ray branching ratios agree, in general, with shell-model predictions of McGrory.