Abstract
The optical decay rates of the 3s3p 3Po2 level in Mg-like ions of 59Co, 58Ni and 63, 65Cu have been measured at a heavy-ion storage ring. The measurement is sensitive to M1, M2 and hyperfine-induced decay rates. The measurement precision is just about high enough to detect the difference in the isotope effect on the level lifetime in the ions 63, 65Cu17+. We also discuss our findings when trying to measure the hyperfine-induced decay of the lowest triplet level, 3s3p 3Po0, in the two Cu isotopes.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. Hyperfine structure in atomic spectra reflects nuclear properties that are probed by the atom's own electrons. Such effects are usually tiny, unless competing processes are blocked by, for example, atomic symmetries. In ions with four electrons (two in the valence shell), recent measurements of a particularly sensitive transition rate (employing dielectronic recombination) disagreed considerably—by a deviation of 80%—with atomic theory calculations, which has subsequently reduced to a deviation of 60% because of greater attention to detail. Ions with 12 electrons also have two valence electrons, and theory probably requires a similar improvement here.
Main results. Using a heavy-ion storage ring as an ion trap, we attempted to measure the hyperfine-sensitive decay rates of two excitation levels in ions with 12 electrons. Nickel ions (without hyperfine effects) served as the reference for measurements on isotopes of cobalt and copper. For the most hyperfine-sensitive level, new calculations (by us and others) indicate changes by some 30% from earlier numbers, but the experiment proved too complex to yield reliable results as of yet. For the other excited level and the isotopes Cu-65 and Cu-67, the measurement resulted in accurate data that determine the hyperfine contribution (of some 10%) to within one twentieth. The data are just sensitive to the 1% difference in the total decay rate between the two isotopes.
Wider implications. This experiment employing passive photon observation of stored ions with 12 electrons validates the experimental technique and provides hyperfine-sensitive decay rate data as benchmarks for modern calculations.