Prompt proton decay of a well-deformed rotational band in⁵⁸Cu

An excited well-deformed rotational band has been observed in the N=Z odd-odd nucleus ⁵⁸Cu. The first excited state in this band decays via γ emission to the spherical states associated with the first minimum in the potential, thus providing for its unambiguous assignment to ⁵⁸Cu. In contrast, its bandhead decays via emission of a prompt 2.4(1) MeV proton to an excited state in the daughter nucleus ⁵⁷Ni. This is the first observation of proton decay from states associated with a deformed secondary minimum in the potential. Self-consistent Hartree-Fock calculations reproduce well both the large collectivity of this band and the general trend of its moment of inertia.