Experimental study of nuclei in the vicinity of the “island of inversion” through the fusion-evaporation reaction

Abstract

We report the observation of high-spin states in nuclei in the vicinity of the ``island of inversion,'' populated via the $^{18}\mathrm{O}+^{18}\mathrm{O}$ fusion reaction at an incident beam energy of $34$ MeV. The fusion reaction mechanism circumvents the limitations of other reactions used so far to populate excited states in these nuclei. The level structures of $^{33,34}\mathrm{P}$ and $^{33}\mathrm{S}$ have been investigated and their level schemes were substantially extended. The intensities and branching ratios of the $\ensuremath{\gamma}$ transitions were measured. The observed $\ensuremath{\gamma}$ coincidence intensity anisotropy and polarization asymmetry were analyzed for a consistent assignment of the spins and parities of the nuclear levels. The $1876$-keV transition in $^{34}\mathrm{P}$ that had an ambiguous multipolarity assignment has been confirmed to be a mixed transition. Truncated (1p-1h) shell model calculations within the ``$\mathit{sdpf}$'' valence space were performed. These calculations reproduce the experimental excitation energies of $^{34}\mathrm{P}$ but fail to explain the observed mixing ratio for the $1876$-keV transition.