Direct observation of $\beta$ and $\gamma$ decay from a high-spin long-lived isomer in $^{187}$Ta

TL;DR

This study reports the first direct observation of beta and gamma decay from a high-spin, long-lived isomer in $^{187}$Ta, revealing decay properties, half-life, and shape implications through advanced experimental techniques.

Contribution

It provides new experimental data on the decay modes, half-life, and shape interpretation of a high-spin isomer in $^{187}$Ta, expanding understanding of nuclear structure in this region.

Findings

Observed beta and gamma decay branches from the isomer.

Revised half-life of 136(24) seconds for the isomer.

Constraints on spin-parity and shape derived from hindrance factors.

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 $K^{\pi} = (25/2^-)$ 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 $\beta$-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.