SINGLE-PARTICLE AND CORE-COUPLED STATES IN 57CO FROM DECAY OF 57NI

The decay of 36.0-h Ni57 produced by the Fe54(α,n)Ni57 reaction has been studied to obtain information on the level structure of Co57. γ-ray spectra were obtained with high-resolution Ge(Li) detectors, a Compton-suppressed Ge(Li) system, and Ge(Li)-NaI(Tl) γ-γ coincidence spectroscopy. The γ-ray energies in keV (and the relative photon intensities) observed in this investigation are as follows: 127.1±0.1 (200), 161.8±0.3 (0.22), 252.5±0.6 (≤0.4), 380.0±0.2 (0.96), 673.4±0.2 (0.58), 906.8±0.3 (1.1), 1046.4±0.2 (1.6), 1223.5±0.4 (1.1), 1377.6±0.2 (1000), 1730.6±0.3 (0.72), 1757.6±0.2 (77), 1896.5±0.4 (0.28), 1919.5±0.2 (170), 2132.9±0.3 (0.47), 2730.6±0.2 (0.3), 2803.9±0.2 (1.7), and 3176.9±0.3 (0.24). On the basis of Ge(Li)-NaI(Tl) coincidence experiments, energy sums, and earlier nuclear reaction data, levels in Co57 were assigned at 0 (72-), 1223.5±0.4 (92-), 1377.6±0.2 (32-), 1504.7±0.2 (1/2-), 1757.6±0.2 (32-), 1896.5±0.4 (72-), 1919.5±0.2 (52-), 2132.9±0.3 (52-), 2730.6±0.2 (52-, 32-), 2803.9±0.2 (32-, 52-), 3108.2 (≤52-), and 3176.9±0.3 (52-) keV. The spin assignments are based on data from nuclear reaction studies, logft values, and relative photon intensities. An attempt was made to explain the experimental results and the structure of Co57 in terms of single-particle states and particle-plus-core states based on coupling the 2+ collective core vibration to the f72 odd proton hole. Previous work on Co57 and Co59 indicates that levels in Co57 at 1223.5 (92-), 1683 (112-), 1757.6 (32-), and 2132.9 (52-) keV belong to a core-coupled multiplet. Our γ-ray branching ratios are in agreement with this characterization of the states at 1223.5 and 1757.6 keV. © 1969 The American Physical Society.