Evidence for Gamow-Teller Decay of ${}^{78}\mathrm{Ni}$ Core from Beta-Delayed Neutron Emission Studies

The $\beta$-delayed neutron emission of ${}^{83,84}\mathrm{Ga}$ isotopes was studied using the neutron time-of-flight technique. The measured neutron energy spectra showed emission from states at excitation energies high above the neutron separation energy and previously not observed in the $\beta$ decay of midmass nuclei. The large decay strength deduced from the observed intense neutron emission is a signature of Gamow-Teller transformation. This observation was interpreted as evidence for allowed $\beta$ decay to ${}^{78}\mathrm{Ni}$ core-excited states in ${}^{83,84}\mathrm{Ge}$ favored by shell effects. We developed shell model calculations in the proton $fp{g}_{9/2}$ and neutron extended $fp{g}_{9/2}+d_{5/2}$ valence space using realistic interactions that were used to understand measured $\beta$-decay lifetimes. We conclude that enhanced, concentrated $\beta$-decay strength for neutron-unbound states may be common for very neutron-rich nuclei. This leads to intense $\beta$-delayed high-energy neutron and strong multineutron emission probabilities that in turn affect astrophysical nucleosynthesis models.

Figure 1

Left: $\beta$-delayed ${}^{83}\mathrm{Ga}$ (a) and ${}^{84}\mathrm{Ga}$ (b) neutron time-of-flight (TOF) spectra. The minimum ${\chi }^2$ fit (red line) was obtained using 16 (${}^{83}\mathrm{Ga}$) and 10 (${}^{84}\mathrm{Ga}$) neutron quasiresonances, in blue (see the text for more details). The inset shows the neutron time-of-flight spectrum in coincidence with the 1348 keV line in ${}^{82}\mathrm{Ge}$. Right: Gamow-Teller (GT) strength distribution above the neutron separation energy in the decay of ${}^{83}\mathrm{Ga}$ (c) and ${}^{84}\mathrm{Ga}$ (d). The data are shown in a solid black line and the grey shaded parts of the histogram indicate the uncertainties of the strength distribution. The strength distributions extracted from $\beta$-delayed gamma-ray spectroscopy are shown by blue lines, enhanced by factors of 20 and 10 for ${}^{83}\mathrm{Ga}$ and ${}^{84}\mathrm{Ga}$, respectively [29, 30]. The dashed histogram shows the Gamow-Teller strength from our shell model calculation using the ${}^{56}\mathrm{Ni}$ core.

Figure 2

Schematic of $\beta$ decay of ${}^{84}\mathrm{Ga}$ into ${}^{84}\mathrm{Ge}$. The GT decay proceeds via transformation of neutrons in the ${}^{78}\mathrm{Ni}$ core orbitals ($f_{5/2}$, $p_{3/2}$, $p_{1/2}$, and $g_{9/2}$) into protons occupying their spin-orbit partners outside the $Z=28$ core. These transitions are coupled with very strong matrix elements. The alternative decay mode, through the first forbidden (FF) $d_{5/2}\to f_{5/2}$ transformation, populates neutron bound states in ${}^{84}\mathrm{Ge}$.

Figure 3

(Top) Half-life predictions for $N>50$ gallium (solid line) and nickel (dashed line) isotopes. (Bottom) One ($P_{1n}$, solid line) and two ($P_{2n}$, dashed line) $\beta$-delayed neutron branching ratio predictions for gallium isotopes. For both panels GT-only predictions are blue diamonds, $\mathrm{GT}+\mathrm{FF}$ predictions are red circles, and the data are black. Experimental values were taken from [29, 30, 45, 47].