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High-spin structures of holmium ( $Z=67$ ) and erbium ( $Z=68$ ) isotopes are presented near mass 160, a region where collective and single-particle modes of excitation compete in the generation of angular momentum at the highest experimentally attainable spins in atomic nuclei. The level schemes of the ${}^{156,157,158}\mathrm{Ho}$ isotopes have been significantly extended, reaching spins in the $40\hslash$ to $55\hslash$ range, through multiple experiments utilizing the Gammasphere spectrometer. In particular, several new noncollective band-terminating states are reported for these nuclei. The nature of these states, including both valence-space terminations and core-excited oblateconfigurations, is discussed in terms of proton particle-hole (p-h) excitations across the semimagic $Z=64$ shell closure. Cranked-Nilsson–Strutinsky (CNS) calculations have been used to interpret the nature of these noncollective states. The corresponding terminating states in ${}^{154–159}\mathrm{Er}$ are also included, providing a systematic description of band termination in this mass region. In addition, three weak rotational structures have been assigned to ${}^{157}\mathrm{Ho}$ at the highest spins. These collective bands are proposed to be based on a triaxial strongly deformed nuclear shape, involving neutron p-h excitations across the $N=82$ shell gap. 

The nuclear two-photon or double-gamma ( $2\gamma$ ) decay is a second-order electromagnetic process whereby a nucleus in an excited state emits two gamma rays simultaneously. To be able to directly measure the $2\gamma$ decay rate in the low-energy regime below the electron-positron pair-creation threshold, we combined the isochronous mode of a storage ring with Schottky resonant cavities. The newly developed technique can be applied to isomers with excitation energies down to $\sim 100\mathrm{keV}$ and half-lives as short as $\sim 10\mathrm{ms}$ . The half-life for the $2\gamma$ decay of the first-excited $0^{+}$ state in bare ${}^{72}\mathrm{Ge}$ ions was determined to be 23.9(6) ms, which strongly deviates from expectations. Published by the American Physical Society2024