Unambiguous Identification of Three $\beta$-Decaying Isomers in ${}^{70}\mathrm{Cu}$

Using resonant laser ionization, $\beta$-decay studies, and for the first time mass measurements, three $\beta$-decaying states have been unambiguously identified in ${}^{70}\mathrm{Cu}$. A mass excess of $-62976.1(1.6)\mathrm{keV}$ and a half-life of 44.5(2) s for the $(6^{-})$ ground state have been determined. The level energies of the $(3^{-})$ isomer at 101.1(3) keV with $T_{1/2}=33(2)\mathrm{s}$ and the $1^{+}$ isomer at 242.4(3) keV with $T_{1/2}=6.6(2)\mathrm{s}$ are confirmed by high-precision mass measurements. The low-lying levels of ${}^{70}\mathrm{Cu}$ populated in the decay of ${}^{70}\mathrm{Ni}$ and in transfer reactions compare well with large-scale shell-model calculations, and the wave functions appear to be dominated by one proton–one neutron configurations outside the closed $Z=28$ shell and $N=40$ subshell. This does not apply to the $1^{+}$ state at 1980 keV which exhibits a particular feeding and deexcitation pattern not reproduced by the shell-model calculations.

Figure 1

Top: Intensity of the 101.1 keV (●) and 141.3 keV (▲) internal transitions summed with the associated $\beta$-delayed $\gamma$ rays as a function of laser frequency. The $\gamma$ rays associated with the decay of the $(6^{-})$ ground state are indicated by (■). Bottom: Time-of-flight resonance curves as a function of cyclotron frequency for the laser settings are marked with arrows $(a,b,c)$ in the top figure. The solid lines are fits of the theoretically expected line shape to the data points.

Figure 2

Left: Schematic presentation of the experimental quasiparticle energies of the orbitals involved for ${}^{69}\mathrm{Cu}$ and ${}^{69}\mathrm{Ni}$ [6], the schematic $p\mathrm{\text{−}}n$ coupling schemes, and the GT $\beta$-decay modes of ${}^{70}\mathrm{Ni}$. Middle: large-scale shell-model calculations. The grey area on top of the $3^{-}–6^{-}$ multiplet indicates the contribution of the proposed $\pi 2p_{3/2}\nu 1g_{9/2}$ configuration, the white area indicates the $\pi 2p_{3/2}\nu (1f_{5/2}^{-2}1g_{9/2}^3)$ configurations, and the total neutron $2p\mathrm{\text{−}}2h$ admixture represents $33\%-38\%$. Right: Partial decay scheme of ${}^{70}\mathrm{Ni}$. The $(5^{-})$ state is from 7.