Identification of excited states in Ga316130: Mirror nuclei in the upper fp shell

Abstract

In the fusion-evaporation reaction $^{40}\mathrm{Ca}+^{24}\mathrm{Mg}$ at $104\phantom{\rule{0.3em}{0ex}}\text{MeV}$ beam energy, excited states have been observed for the first time in the isotope ${^{31}_{61}\mathrm{Ga}}_{30}$. The experimental setup comprised the Ge array CLARION, a recoil mass spectrometer and, in its focal plane, an ionization chamber. Five transitions in $^{61}\mathrm{Ga}$ are identified, out of which a cascade of three transitions has been established by means of recoil-$\ensuremath{\gamma}\ensuremath{\gamma}$ coincidences. The strong transitions at $271\phantom{\rule{0.3em}{0ex}}\text{keV}$ in $^{61}\mathrm{Ga}$ and $124\phantom{\rule{0.3em}{0ex}}\text{keV}$ in $^{61}\mathrm{Zn}$ are viewed as the ``mirror'' $5/{2}^{\ensuremath{-}}\ensuremath{\rightarrow}3/{2}^{\ensuremath{-}}$ ground-state transitions. The rather large energy difference of $150\phantom{\rule{0.3em}{0ex}}\text{keV}$ is suggested to arise from Coulomb monopole contributions. Shell-model calculations support this interpretation.