β -delayed fission of isomers in Bi188

Abstract

β-delayed fission (βDF) decay of a low-spin (ls) and a high-spin (hs) isomer in 188 Bi was studied at the ISOLDE facility at CERN. Isomer-selective laser ionization and time gating were employed to investigate each isomer separately and their βDF partial half-lives were determined: T 1/2p,βDF (188 Bi hs) = 5.6(8) × 10 3 s and T 1/2p,βDF (188 Bi ls) = 1.7(6) × 10 3 s. This work is the first βDF study of two states in one isotope and allows the spin dependence of low-energy fission to be explored. The fission fragment mass distribution of a daughter nuclide 188 Pb, following the β decay of the high-spin isomer, was deduced and indicates a mixture of symmetric and asymmetric fission modes. Experimental results were compared with self-consistent mean-field calculations based on the finite-range Gogny D1M interaction. To reproduce the measured T 1/2p,βDF (188 Bi hs), the calculated fission barrier of 188 Pb had to be reduced by ≈30%. After this reduction, the measured T 1/2p,βDF (188 Bi ls) was in agreement with calculations for a few possible configurations for 188 Bi ls. Theoretical βDF probabilities for these configurations were found to be lower by a factor of 4-9 than the βDF probability of 188 Bi hs. The fission fragment mass distribution of 188 Pb was compared to the scission-point model SPY and the calculations based on the finite-range liquid-drop model. The first observation of βDF for 190 Bi is also reported.

Description of magnetic moments within the Gogny Hartree-Fock-Bogolyubov framework: Application to Hg isotopes

“…The Gamow-Teller response of even and odd 90−94 Zr isotopes [16] and β − -decay of 83 Ga [34] have also been successfully studied within the same approach. In the same framework, β + and electron-capture (EC) decay rates have been estimated for 188 Bi [35]. In the present study, we follow the same approach and estimate the β + and EC decay half-lives of 182,184,186 Hg isotopes to choose between the oblate or prolate configurations.…”

Identification of excited states in Bi188 and Po188

“…Two predominantly α-decaying states are known in 188 Bi: the 1 (+) ground state (gs) based on the presumed π 1/2 − [530] f 7/2 ⊗ ν1/2 − [521]p 3/2 configuration [5,8], and the (10 − )isomerfromtheπ h 9/2 ⊗ νi 13/2 coupling [9]. Based on laser-spectroscopy studies [5], the 1 (+) gs was found to be strongly prolate deformed with β =+0.25 (7), whereas the (10 − ) isomer is nearly spherical.…”