Isomer shift and magnetic moment of the long-lived 1/2${}^{+}$ isomer in ${}_{30}^{79}$Zn${}_{49}$: signature of shape coexistence near ${}^{78}$Ni

Abstract

Collinear laser spectroscopy has been performed on the ${}_{30}^{79}$Zn${}_{49}$ isotope at ISOLDE-CERN. The existence of a long-lived isomer with a few hundred milliseconds half-life was confirmed, and the nuclear spins and moments of the ground and isomeric states in ${}^{79}$Zn as well as the isomer shift were measured. From the observed hyperfine structures, spins $I=9/2$ and $I=1/2$ are firmly assigned to the ground and isomeric states. The magnetic moment $\mu$ (${}^{79}$Zn) = $-$1.1866(10) ${\mu }_{\mathrm{N}}$, confirms the spin-parity $9/2^{+}$ with a $\nu g_{9/2}^{-1}$ shell-model configuration, in excellent agreement with the prediction from large scale shell-model theories. The magnetic moment $\mu$ (${}^{79m}$Zn) = $-$1.0180(12) ${\mu }_{\mathrm{N}}$ supports a positive parity for the isomer, with a wave function dominated by a 2h-1p neutron excitation across the $N=50$ shell gap. The large isomer shift reveals an increase of the intruder isomer mean square charge radius with respect to that of the ground state: $\delta ⟨r_c^2{⟩}^{79,79m}$ = +0.204(6) fm${}^2$, providing first evidence of shape coexistence.

Comments: 5 pages, 4 figures, 1 table, Accepeted by Phys. Rev. Lett. (2016)