S. Toleikis

Double-electron capture in relativistic U92+ collisions at the ESR gas-jet target

Abstract Total cross-sections for radiative single- and double-electron capture are measured in collisions of fast highly charged ions with light target atoms. Cross-sections for non-correlated double capture are in accordance with theoretical predictions. No significant line structure at twice the single K-radiative electron capture (K-REC) photon energy was observed. Angular distributions single K-REC photons associated with single- and double-charge exchange exhibit the same pattern. The corresponding REC lines are almost by a factor of two broader for double-charge exchange than in the single capture case.

Impact parameter dependent electron capture by decelerated U91+ ions at 20 MeV/u using crystal channeling conditions

Abstract We present results of an experiment using decelerated U91+ ions, extracted from the GSI-ESR storage ring, and transmitted through a thin silicon crystal in channeling conditions. Charge state at emergence, secondary electron multiplicity and X-rays are measured simultaneously. These conditions allow to study the competition between mechanical electron capture (MEC) and radiative electron capture (REC) as a function of impact parameter. We observe that REC is the dominant charge exchange process for the best channeled ions, i.e. those travelling always far from the silicon target core electrons. For ions with high transverse energy, MEC into highly excited states is counterbalanced by enhanced ionization arising from a succession of close collisions along atomic rows.

Lifetime measurement of the 2 3P0 state in He-like uranium

In recent years for the fundamental theory of quantum electrodynamics, considerable progress in the evaluation of higher order corrections has been achieved—not only for hydrogen—but also for helium-like systems—up to very heavy nuclei. We were aiming at a more precise determination of the lifetime of the metastable 2 3P0 state in He-like uranium which has a calculated value of 57.3 ps [1, 2]. From the lifetime it is possible to derive the energy of the state. In October 2011 we were able to perform a first test experiment at GSI, Darmstadt to study the feasibility of a new experimental detection technique. This advanced set-up consists of two state-of-the-art energy-, time- and position-sensitive germanium detectors [3] in combination with collimators in a Soller-slit like assembly. A beam of U91+-ions at an energy of 290 MeV u−1 is passed through a thin nickel foil in the interaction chamber. From the decrease in intensity as a function of the target distance one may extract a decay curve from which the lifetime can be derived. The advantages of this new set-up, in comparison to former experiments [4, 5] will be discussed and the results of a preliminary data analysis will be presented.

Multiple electron capture from thin C-foils into 46 MeV/u U91+

Abstract Absolute yields for multiple electron capture as well as projectile ionization have been measured for 46 MeV/u U91+ ions traversing thin carbon foils with different surface topologies. Multiple electron capture yields vary strongly, depending on the target manufacturer. The samples including the one with the smoothest surface show a deviation of the yield from the expected scaling based on subsequent single capture for decreasing target thickness. This indicates a strong contribution of the surface, whose nature cannot be identified by the current data.

Impact parameter dependent charge exchange studies with channeled heavy ions

We use decelerated (below 20 MeV/u) H-like heavy ions extracted from the GSI-ESR storage ring to study electron capture processes such as Radiative Electron Capture (REC) and Mechanical Electron Capture (MEC) in channeling conditions. With the help of simulations, we show that MEC occurs at relatively large impact parameters into highly excited states. REC studies provide information about the electron gas polarization.

Magnetic Sublevel Population Studied for H- and He-like Uranium in Relativistic Ion–Atom Collisions

An experimental study for K-shell excitation of helium-like uranium in relativistic collisions with low-Z gaseous target is presented. Within this experiment information about the population of the magnetic sublevels has been obtained via a photon angular differential study of the decay photons associated with the excitation process. The preliminary results presented show, for the particular case of the 3P1 level, a surprisingly strong population of the magnetic sublevels with μ=±1.

Spectroscopy of Ly-α Lines at Storage Rings by Crystal Spectrometry and Absorption Edge Technique

Crystal spectrometry and absorption edge technique have the capability to overcome the gap in accuracy between experiment and theory in the strong field domain of QED. New results are presented which indicate the capacity of these methods to measure the energies of X-rays emitted by highly charged heavy ions at modern storage rings with a precision sensitive to second order corrections to the Lambshift in H-like very heavy ions.