Strong neutron-$\gamma$ competition above the neutron threshold in the decay of $^{70}$Co

A. Spyrou, S. Liddick, F. Naqvi, B. P. Crider, A. C. Dombos, D. L. Bleuel, B. A. Brown, A. J. Couture, L. Crespo Campo, M. Guttormsen, A. C. Larsen, R. Lewis, P. Möller, S. Mosby, M. Mumpower, G. Perdikakis, C. J. Prokop, T. Renstrom, S. Siem, S. J. Quinn, S. Valenta

Published PRL 117, 142701 (2016)

The $\beta$-decay intensity of $^{70}$Co was measured for the first time using the technique of total absorption spectroscopy. The large $\beta$-decay Q value [12.3(3) MeV] offers a rare opportunity to study $\beta$-decay properties in a broad energy range. Two surprising features were observed in the experimental results, namely, the large fragmentation of the $\beta$ intensity at high energies, as well as the strong competition between $\gamma$-rays and neutrons, up to more than 2 MeV above the neutron-separation energy. The data are compared to two theoretical calculations: the shell model and the quasiparticle random phase approximation (QRPA). Both models seem to be missing a significant strength at high excitation energies. Possible interpretations of this discrepancy are discussed. The shell model is used for a detailed nuclear structure interpretation and helps to explain the observed neutron-$\gamma$ competition. The comparison to the QRPA calculations is done as a means to test a model that provides global $\beta$-decay properties for astrophysical calculations. Our work demonstrates the importance of performing detailed comparisons to experimental results, beyond the simple half-life comparisons. A realistic and robust description of the $\beta$-decay intensity is crucial for our understanding of nuclear structure as well as of $r$-process nucleosynthesis.

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experiment nuclear reactions

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