$β$ -delayed fission of isomers in $^{188}$Bi

$β$-delayed fission ($β$DF) 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 $β$DF partial half-lives were determined: $T_{1/2\mathrm{p},β\mathrm{DF}} (^{188}\mathrm{Bi^{ls}})=5.6(8)×10^3$ s and $T_{1/2\mathrm{p},β\mathrm{DF}} (^{188}\mathrm{Bi^{hs}})=1.7(6)×10^3$ 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 $T_{1/2\mathrm{p},β\mathrm{DF}} (^{188}\mathrm{Bi^{hs}})$, the calculated fission barrier of $^{188}$Pb had to be reduced by ≈30%. After this reduction, the measured $T_{1/2\mathrm{p},β\mathrm{DF}} (^{188}\mathrm{Bi^{ls}})$ was in agreement with calculations for a few possible configurations for $^{188}\mathrm{Bi^{ls}}$. Theoretical $β$DF probabilities for these configurations were found to be lower by a factor of 4–9 than the $β$DF probability of $^{188}\mathrm{Bi^{hs}}$. 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 $β$DF for $^{190}$Bi is also reported.