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

A. Spyrou, et al.

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.