Speaker
Description
$^{187}$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 predicited to take place. Using high mass resolving power at the Experimental Storage Ring at GSI, the ground state and two long-lived isomers in $^{187}$Ta were previously identified with their masses, which were translated into excitation energies of 1789(13) keV and 2933(14) keV for the first and second isomers, respectively [1]. In our work, the $^{187}$Ta isomers have 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 (KISS) [2] at RIKEN. A preceding analysis on the first isomer and a rotational band to which the isomer decays revealed that the isomer has K$^{\pi}$ = (25/2$^-$) and axial symmetry is slightly violated in this nucleus [3]. In this presentation, we will focus on new experimental findings obtained for the second isomer, such as the internal $\gamma$- and external $\beta$-decay branches and a neutral-atom half-life of 136(24) s [4]. The spin-parity assignment constrained by the evaluated hindrances for K-forbidden transitions and its interpretation based on configuration-constrained potential-energy surface calculations will be discussed.
Reference:
[1] M. W. Reed et al., Phys. Rev. Lett. 105(2010) 172501; Phys. Rev. C 86(2012) 054321.
[2] Y. Hirayama et al., Nucl. Inst. Meth. B353, 4(2015), and B412, 11(2017).
[3] P. M. Walker et al., Phys. Rev. Lett. 125(2020) 192505.
[4] J. L. Chen et al., Phys. Rev. C (accepted for publication).
