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Hyperfine induced transition rates in multiply charged beryllium like ions from storage ring experiments

Fachliche Zuordnung Optik, Quantenoptik und Physik der Atome, Moleküle und Plasmen
Förderung Förderung von 2011 bis 2020
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 192993699
 
Erstellungsjahr 2019

Zusammenfassung der Projektergebnisse

Hyperfine induced (HFI) transitions in atoms and ions are governed by the feeble interaction of the nuclear magnetic moment with the atomic shell. The corresponding lifetimes depend strongly on the nuclear charge and range from hundreds of years for light atoms to microseconds for the heaviest elements. These long lifetimes translate to narrow line widths which makes the associated atomic states attractive for establishing ultra-precise frequency standards. Traditionally, HFI lifetimes were experimentally studied for He-like ions using beam-foil spectroscopy. In He-like ions, however, the HFI transition competes with other one-photon transitions and the lifetimes are comparatively short. A more clear cut case are Be-like ions, where the 2s 2p 3 P0 level is the first excited level. In isotopes with nuclear spin I = 0 this level cannot decay by a one-photon transition. The next higher-order process is a very slow E1M1 transition. In isotopes with nonzero nuclear spin, however, the hyperfine interaction quenches the 2s 2p 3 P0 level. The associated lifetimes are difficult of measure by photon spectroscopy, since the numbers of photons emitted per second are very small. Corresponding experiments have failed to produce results up to now. In the present project another experimental approach was taken which was based on electron-ion collision spectroscopy in a heavy-ion storage ring. There, mixtures of 2s 2p 3 P0 metastable and 2s2 1 S0 ground-level ions were stored for several minutes. HFI lifetimes were measured by tuning the electron-ion collision energy to a recombination resonance associated with the 2s 2p 3 P0 level and monitoring the recombined ion yield as a function of storage time. This method is very efficient since practically all recombined ions can be collected. A HFI lifetime of 10.4 ± 0.4 s was measured for Be-like 33 S12+ ions in agreement with the latest theoretical results. Such an agreement was not reached in a previous experiment with 47 Ti19+ ions. Despite additional theoretical investigations of the role of the external electric and magnetic fields that were present in the storage-ring experiment, the reason for this discrepancy remains unclear. In experiments with Be-like 29 Si10+ and Mg-like 57 Fe14+ it turned out, that the recombinationresonance structure was less favourable than for 33 S12+ , and no quantitative results for the HFI lifetimes could be obtained. Nevertheless, the hyperfine quenching could be exploited for preparing pure-ground state ion beams which were then used for measuring atomic collision cross sections on an absolute scale. The resulting absolute cross sections for electron impact ionization and electron-ion recombination of Si10+ and Fe14+ have spectroscopic quality. Moreover they are very useful for applications in astrophysics and plasma physics where most of the required atomic data come from largely untested atomic theories. In principle, the here established storage-ring method for measuring HFI lifetimes could be used in combination with state-of-the-art theoretical calculations to determine nuclear magnet moments. This would be particularly interesting for short-lived nuclear isotopes from a radioactive-beam facility. A corresponding proposal was made in connection with the effort of establishing a heavyion storage-ring at the ISOLDE facility at CERN, and similar perspectives open up at the FAIR facility in Darmstadt. The method can also be used to measure lifetimes of other long-lived states. For example, within the present project, the feasibility of measuring the E1M1 lifetime in heavy Be-like ions with zero nuclear spin was already explored. In the near future, such ions will become available in the low-energy heavy-ion storage ring CRYRING of the FAIR facility. A corresponding beamtime request has already been granted.

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