New laboratory lifetime measurements of UII for the uranium cosmochronometer

We present new measurements of radiative lifetimes for six energy levels of singly ionized uranium, U II, using laser-induced fluorescence technique. One of the levels, 5f(3)6d7p(6)M(13/2) at 26191 cm(-1), decays by a transit ion at 3859.6 Angstrom. This line has recently been observed in the spectru m of the metal-poor star CS1082-001, the first detection of uranium outside the solar system. The lambda 3859 line can be used as the presently most a ccurate cosmochronometer (Cayrel et al. 2001). Our value of the lifetime of the M-6(13/2) level is 18.6 +/- 0.7 ns, and it confirms the f-value used i n the Nature article by Cayrel et al. (2001), which is based on an experime ntal lifetime of 20 +/- 5 ns (Chen & Borzileri 1981). The new measurement a lso removes the doubt about the choice between that value and other f-value s in the literature, differing by a factor of 3. Adopting the same branchin g fraction as Chen & Borzileri (1981) for the 3859.6 Angstrom line, we deri ve a gf-value of 0.68, which is 8% higher than the value used by Cayrel et al. (2001). Of significance for the chronometer is also the reduced uncerta inty of the radiative lifetime, 4% compared to 25%, and consequently of the f-value, which should decrease the uncertainty in the determination of the stellar age considerably.