Lifetimes of excited states in 229Th, populated in the β decay of 229Ac, have been measured using the advanced time-delayed βγ γ (t) method. Half-lives of 14 states have been determined including 11 of them for the first time. Twenty-seven new γ lines have been introduced into the β-decay scheme of 229Ac based on results of γγ coincidence measurements. Reduced transition probabilities have been determined for more than 70 γ transitions in 229Th. Average |D0| values of 0.029(1), 0.077(3), and 0.024(5) e fm have been deduced for the lowest Kπ =1/2±, 3/2±, and 5/2± parity partner bands, respectively. Excited states in 229Th 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 (β2, β3) plane for the lowest single quasiparticle configurations in 229Th have been calculated using the Strutinsky method.

This work has been supported in part by Grant No. 1 P03B 059 27 from the Polish Committee for Scientific Research(KBN) (for theoretical calculations), the Swedish Research Council, the research plan MSM0021620834 provided by the Ministry of Education of the Czech Republic, and the Spanish CICYT under Project No. FPA2002-04181-C04-02. Support by the European Commission through the HPRI-CT-1999-00018 project and by the ISOLDE Collaboration are also acknowledged. Fast timing detectors and electronics have been provided by the Fast Timing Pool of Electronics. One of us (E.R.) would like to thank the OSIRIS group for its generous hospitality and for financial support during her stay in Studsvik.

17 pages, 9 figures, 6 tables.-- PACS nrs.: 21.10.Tg; 21.10.Re; 21.10.Ky; 27.90.+b.

Peer reviewed