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Issue A&A
Volume 372, Number 3, June IV 2001
Page(s) L50 - L52
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20010608



A&A 372, L50-L52 (2001)
DOI: 10.1051/0004-6361:20010608

New laboratory lifetime measurements of U II for the uranium cosmochronometer

H. Lundberg1, S. Johansson2, H. Nilsson2 and Z. Zhang1

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

(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, 5f36d7p6M13/2 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 $\lambda$3859 line can be used as the presently most accurate cosmochronometer (Cayrel et al. 1981). Our value of the lifetime of the 6M13/2 level is $18.6\pm0.7$ ns, and it confirms the f-value used in the Nature article by Cayrel et al. (2001), which is based on an experimental lifetime of $20\pm5$ ns (Chen & Borzileri 1981). 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 (1981) 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. (2001). 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.


Key words: atomic data -- stars: evolution -- Galaxy: evolution

Offprint request: H. Lundberg, hans.lundberg@fysik.lth.se




© ESO 2001


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