Direct observation of ${\hat{I}}^2$ and ${\hat{I}}^3$ decay from a high-spin long-lived isomer in ${}^{187}$Ta

Abstract

${}^{187}$Ta ($Z=73$, $N=114$) is located in the neutron-rich $A\approx 190$ region where a prolate-to-oblate shape transition via triaxial softness is predicted to take place. A preceding work on the $K^{\ddot{I}}€=(25/2^{-})$ isomer and a rotational band to which the isomer decays carried out by the same collaboration revealed that axial symmetry is slightly violated in this nucleus. This paper focuses on a higher-lying isomer, which was previously identified at 2933(14) keV by mass measurements with the Experimental Storage Ring at GSI. The isomer of interest has been populated by a multi-nucleon transfer reaction with a ${}^{136}$Xe primary beam incident on a natural tungsten target, using the KEK Isotope Separation System at RIKEN. New experimental findings obtained in the present paper include the internal and external ${\hat{I}}^2$-decay branches from the high-spin isomer and a revised half-life of 136(24) s. The evaluated hindrances for $K$-forbidden transitions put constraints on the spin-parity assignment, which can be interpreted as being ascribed to a prolate shape with a five-quasiparticle configuration by model calculations.

9 pages, 5 figures