Identification of the Lowest $T=2$, $J^{\pi }=0^{+}$ Isobaric Analog State in ${}^{52}\mathrm{Co}$ and Its Impact on the Understanding of $\beta$-Decay Properties of ${}^{52}\mathrm{Ni}$

Masses of ${}^{52g,52m}\mathrm{Co}$ were measured for the first time with an accuracy of $\sim 10\mathrm{keV}$, an unprecedented precision reached for short-lived nuclei in the isochronous mass spectrometry. Combining our results with the previous $\beta \text{−}\gamma$ measurements of ${}^{52}\mathrm{Ni}$, the $T=2$, $J^{\pi }=0^{+}$ isobaric analog state (IAS) in ${}^{52}\mathrm{Co}$ was newly assigned, questioning the conventional identification of IASs from the $\beta$-delayed proton emissions. Using our energy of the IAS in ${}^{52}\mathrm{Co}$, the masses of the $T=2$ multiplet fit well into the isobaric multiplet mass equation. We find that the IAS in ${}^{52}\mathrm{Co}$ decays predominantly via $\gamma$ transitions while the proton emission is negligibly small. According to our large-scale shell model calculations, this phenomenon has been interpreted to be due to very low isospin mixing in the IAS.

Figure 1

Part of the revolution time spectrum zoomed in at a time window of $608\mathrm{ns}\le t\le 619\mathrm{ns}$. The red and black peaks represent the $T_z=-1$ and $-1/2$ nuclei, respectively. The insert shows well-resolved peaks of the ground and $(2^{+})$ isomeric states of ${}^{52}\mathrm{Co}$.

Figure 2

Partial decay scheme of ${}^{52}\mathrm{Ni}$ (left) and theoretical level structure of ${}^{52}\mathrm{Co}$ (right). Excitation energies are in keV. The theoretical branching ratios (BRs) and log $ft$ values based on cd-GXPF1J are deduced from the present $Q$ value. The red levels are deduced from the ground-state mass of ${}^{52}\mathrm{Co}$ and the $\gamma$-ray energies from Ref. [26]. The black levels are determined from the $\beta \text{−}p$ data.