Strong Neutron-$\gamma$ Competition above the Neutron Threshold in the Decay of ${}^{70}\mathrm{Co}$

The $\beta$-decay intensity of ${}^{70}\mathrm{Co}$ 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 $\gamma$-neutron 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.

Figure 1

Experimental spectra for the decay of ${}^{70}\mathrm{Co}$ (solid red line) together with the best fit of the ${\chi }^2$ minimization analysis procedure (dashed black line). The top panel shows the TAS spectrum, the middle panel shows the segment spectrum, and the bottom panel shows the segment multiplicity in the SuN detector. All experimental spectra are gated on implanted ${}^{70}\mathrm{Co}$ ions, and the random correlation background was subtracted.

Figure 2

Cumulative $\beta$-decay intensity of ${}^{70}\mathrm{Co}$ versus excitation energy of the final states in ${}^{70}\mathrm{Ni}$. The solid black line and green-shaded area are the experimental results with uncertainties. The dashed red line is a shell-model calculation (see the text for details). The thick dotted blue line is a QRPA calculation under the assumption of spherical shape. The inset shows QRPA calculations under different deformation assumptions.

Figure 3

Cumulative $B(\mathrm{GT})$ of ${}^{70}\mathrm{Co}$ versus excitation energy of the final states in ${}^{70}\mathrm{Ni}$. The solid black line and green-shaded area are the experimental results with uncertainties. The dotted blue line is a QRPA calculation under the assumption of spherical shape. The dashed red line is a shell-model calculations (see the text for details). The inset presents the full spectrum of the shell-model calculation, including a quenching factor of 0.55, where the giant Gamow-Teller resonance can be observed around 23 MeV.