Nuclear structure of $^{229}$Th

Lifetimes of excited states in $^{229}$Th, populated in the $\beta$-decay of $^{229}$Ac, have been measured using the advanced time-delayed $\beta\gamma\gamma$(t)-method. Half-lives of 14 states have been determined including 11 of them for the first time. Twenty-seven new $\gamma$-lines have been introduced into the $\beta$-decay scheme of $^{229}$Ac based on results of $\gamma\gamma$-coincidence measurements. Reduced transition probabilities have been determined for more than 70 $\gamma$-transitions in $^{229}$Th. Average ∣D$_{0}$∣ values of 0.029(1), 0.077(3) and 0.024(5) $e$ fm have been deduced for the lowest K$^{\pi}$=1/2$^{\pm}$ , 3/2$^{\pm}$ and 5/2$^{\pm}$ parity partner bands, respectively. Excited states in $^{229}$Th and experimental transition rates have been interpreted within the quasiparticle-plus-phonon model. The half-life of the 3.5 eV , 3/2$^{+}$ isomeric state is predicted to be about 10 h. Potential energy surfaces on the ($\beta_{2}\,,\,\beta_{3}$) plane for the lowest single quasiparticle configurations in $^{229}$Th have been calculated using the Strutinsky method.