Identification of excited states in ₃₁⁶¹Ga ₃₀: Mirror nuclei in the upper f p shell

In the fusion-evaporation reaction ⁴⁰Ca + ²⁴Mg at 104 MeV beam energy, excited states have been observed for the first time in the isotope ⁶¹₃₁Ga₃₀. The experimental setup comprised the Ge array CLARION, a recoil mass spectrometer and, in its focal plane, an ionization chamber. Five transitions in ⁶¹Ga are identified, out of which a cascade of three transitions has been established by means of recoil-γγ coincidences. The strong transitions at 271 keV in ⁶¹Ga and 124 keV in ⁶¹Zn are viewed as the "mirror" 5/2^- → 3/2^- ground-state transitions. The rather large energy difference of 150 keV is suggested to arise from Coulomb monopole contributions. Shell-model calculations support this interpretation.