Large $\beta$-Delayed Neutron Emission Probabilities in the ${}^{78}\mathrm{Ni}$ Region

The $\beta$-delayed neutron branching ratios ($P_{\beta n}$) for nuclei near doubly magic ${}^{78}\mathrm{Ni}$ have been directly measured using a new method combining high-resolution mass separation, reacceleration, and digital $\beta \mathrm{\text{−}}\gamma$ spectroscopy of ${}^{238}\mathrm{U}$ fission products. The $P_{\beta n}$ values for the very neutron-rich isotopes ${}^{76–78}\mathrm{Cu}$ and ${}^{83}\mathrm{Ga}$ were found to be much higher than previously reported and predicted. Revised calculations of the $\beta n$ process, accounting for new mass measurements and an inversion of the $\pi 2p_{3/2}$ and $\pi 1f_{5/2}$ orbitals, are in better agreement with these new experimental results.

Figure 1

The $\gamma$-ray spectra recorded for the $\beta n$ precursors ${}^{76}\mathrm{Cu}$ (a), ${}^{77}\mathrm{Cu}$ (b), and ${}^{78}\mathrm{Cu}$ (c) showing the decays of respective Zn isotopes used to determine the $P_{\beta n}$ values. For ${}^{76,77}\mathrm{Cu}$ the $\gamma$-ray singles spectra taken in saturation mode are shown. For ${}^{78}\mathrm{Cu}$ a $\beta$-gated spectrum taken using a 5.2 s MTC cycle is shown. For ${}^{76}\mathrm{Cu}$, the corresponding particle identification plot for the mass 76 isobars in the RO mode is shown in (d). Zinc ions are removed by the charge exchange cell while the higher-$Z$ components of the beam (${}^{76}\mathrm{Ga}$ and ${}^{76}\mathrm{Ge}$) are stopped by the ion chamber resulting in a pure ${}^{76}\mathrm{Cu}$ beam. The partial decay scheme for ${}^{77}\mathrm{Cu}$ indicating the significance of the 772-keV isomer in ${}^{77}\mathrm{Zn}$ to the $P_{\beta n}$ value analysis is shown in (e). The number in parentheses following the $\gamma$-ray energy is the measured absolute $\gamma$-ray intensity from comparison of the $\gamma$-ray peak areas to the number of implanted ${}^{77}\mathrm{Cu}$ ions.

Figure 2

The probability of $\beta$-delayed neutron emission as a function of the energy window for this decay mode. Results of this experiment (▲) are compared to those listed in Ref. 13 (■) and to our new theoretical estimates (◇); see text.