New laboratory lifetime measurements of U II for the uranium cosmochronometer

Abstract:
We present new measurements of radiative lifetimes for six energy levelsof singly ionized uranium, U II, using laser-induced fluorescencetechnique. One of the levels,5f<SUP>3</SUP>6d7p<SUP>6</SUP>M<SUB>13/2</SUB> at 26191 cm<SUP>-1</SUP>,decays by a transition at 3859.6 Å. This line has recently beenobserved in the spectrum of the metal-poor star CS1082-001, the firstdetection of uranium outside the solar system. The lambda 3859 line canbe used as the presently most accurate cosmochronometer (Cayrel et al.cite{Cayrel}). Our value of the lifetime of the<SUP>6</SUP>M<SUB>13/2</SUB> level is 18.6+/-0.7 ns, and it confirms thef-value used in the Nature article by Cayrel et al. (cite{Cayrel}),which is based on an experimental lifetime of 20+/-5 ns (Chen &Borzileri cite{Chen}). The new measurement also removes the doubt aboutthe choice between that value and other f-values in the literature,differing by a factor of 3. Adopting the same branching fraction as Chen& Borzileri (cite{Chen}) for the 3859.6 Å line, we derive agf-value of 0.68, which is 8% higher than the value used by Cayrel etal. (cite{Cayrel}). Of significance for the chronometer is also thereduced uncertainty of the radiative lifetime, 4% compared to 25%, andconsequently of the f-value, which should decrease the uncertainty inthe determination of the stellar age considerably.