100 Years of Nuclear Isomers

The change of the shell structure in atomic nuclei, so-called nuclear shell evolution, occurs due to changes of major configurations through particle-hole excitations inside one nucleus, as well as due to variation of the number of constituent protons or neutrons. We have investigated how the shell evolution affects Gamow-Teller (GT) transitions, which dominate the $\beta$ decay in the region below the doubly magic nucleus $^{132}$Sn, using the newly obtained experimental data on a long-lived isomer in $^{127}$Ag. The experiment has been carried out at the RIBF facility as part of the EURICA decay spectroscopy campaign. The $T_{1/2}=67.5(9)$ ms isomer has been identified with a spin and parity of $(27/2^+)$ at an excitation energy of $1942^{+14}_{-20}$ keV, and found to decay via an internal transition of an $E3$ character, which competes with the dominant $\beta$-decay branches towards the high-spin states in $^{127}$Cd. In this presentation, the underlying mechanism of a strong GT transition from the $^{127}$Ag isomer is discussed in terms of configuration-dependent optimization of the effective single-particle energies in the framework of a shell-model approach. Besides, I will introduce a new project of decay spectroscopy at RIBF with a highly efficient fast-timing LaBr$_3$(Ce) array IDATEN.