New laboratory lifetime measurements of U II for the uranium cosmochronometer

¹ Department of Physics, Lund Institute of Technology, PO Box 118, 22100 Lund, Sweden
² Department of Physics, Lund University, PO Box 118, 22100 Lund, Sweden e-mail: sveneric.johansson@fysik.lu.se; hampus.nilsson@fysik.lu.se; zhang.zhiguo@fysik.lth.se

Corresponding author: H. Lundberg, hans.lundberg@fysik.lth.se

Received: 23 April 2001
Accepted: 26 April 2001

Abstract

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, 5f³6d7p⁶M_13/2 at 26191 cm^-1, decays by a transition at 3859.6 Å. This line has recently been observed in the spectrum of the metal-poor star CS1082-001, the first detection of uranium outside the solar system. The λ3859 line can be used as the presently most accurate cosmochronometer (Cayrel et al. [CITE]). Our value of the lifetime of the ⁶M_13/2 level is ns, and it confirms the f-value used in the Nature article by Cayrel et al. ([CITE]), which is based on an experimental lifetime of ns (Chen & Borzileri [CITE]). The new measurement also removes the doubt about the 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]) for the 3859.6 Åline, we derive a gf-value of 0.68, which is 8% higher than the value used by Cayrel et al. ([CITE]). Of significance for the chronometer is also the reduced uncertainty 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.