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Featured researches published by A. Krämer.


Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms | 2001

Commissioning of a hydrogen cluster target at the experimental storage ring

A. Krämer; A. Kritzer; H. Reich; Th. Stöhlker

The internal jet target of the ESR storage ring is an extensively used experimental instrument where nitrogen, argon and even heavy noble gases such as xenon can routinely be used with densities ranging from 1×1012 up to . In order to broaden further on the spectrum of possible experiments at the jet target and in particular to meet the requirements of the deceleration technique, the jet target has now been optimized with respect to molecular hydrogen. By the installation of a liquid nitrogen cooling system, a density increase for molecular hydrogen by two orders of magnitude has been accomplished.


Physical Review A | 2000

Radiative recombination of bare Bi 83+ : Experiment versus theory

A. Hoffknecht; C. Brandau; T. Bartsch; C. Böhme; H. Knopp; S. Schippers; A. Müller; C. Kozhuharov; K. Beckert; F. Bosch; B. Franzke; A. Krämer; P. H. Mokler; F. Nolden; M. Steck; Th. Stöhlker; Z. Stachura

Electron-ion recombination of completely stripped Bi83+ was investigated at the Experimental Storage Ring (ESR) of the GSI in Darmstadt. It was the first experiment of this kind with a bare ion heavier than argon. Absolute recombination rate coefficients have been measured for relative energies between ions and electrons from 0 up to about 125 eV. In the energy range from 15 meV to 125 eV a very good agreement is found between the experimental result and theory for radiative recombination (RR). However, below 15 meV the experimental rate increasingly exceeds the RR calculation and at Erel = 0 eV it is a factor of 5.2 above the expected value. For further investigation of this enhancement phenomenon the electron density in the interaction region was set to 1.6E6/cm3, 3.2E6/cm3 and 4.7E6/cm3. This variation had no significant influence on the recombination rate. An additional variation of the magnetic guiding field of the electrons from 70 mT to 150 mT in steps of 1 mT resulted in periodic oscillations of the rate which are accompanied by considerable changes of the transverse electron temperature.


Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms | 1998

The absorption-edge technique at the gas-jet target of the ESR

A. Krämer; Th. Stöhlker; F. Bosch; T. Ludziejewski; H. F. Beyer; C. Kozhuharov; D. Liesen; P. H. Mokler; P. Rymuza; Z. Stachura; P. Swiat; A. Warczak

In a first pilot experiment the absorption-edge spectroscopy was applied at the gas-jet target of the ESR storage ring in order to measure the ground state transitions of H- and He-like gold-ions. The aim of this experiment was to test the potential of this technique for a precise study of QED-effects in high Z ions at the storage ring.


Physics Letters A | 1999

Two-photon decay in strong central fields observed for the case of He-like gold

H.W. Schäffer; P. H. Mokler; R. W. Dunford; C. Kozhuharov; A. Krämer; A. E. Livingston; T. Ludziejewski; H.T. Prinz; P. Rymuza; L Sarkadi; Z. Stachura; Th. Stöhlker; P. Swiat; A. Warczak

Abstract The photon energy differential shape of the second order matrix element for the two-photon (2E1) decay of the 1s2s 1 S 0 level in He-like gold has been measured. The results are in agreement with a recent fully relativistic calculation. The corresponding 2E1 matrix element deviates from those in lighter He-like systems due to the strong central field in a heavy two-electron ion.


Physical Review A | 2010

Shell- and subshell-resolved projectile excitation of hydrogenlike Au78+ ions in relativistic ion-atom collisions

A. Gumberidze; S. Fritzsche; F. Bosch; D. C. Ionescu; A. Krämer; C. Kozhuharov; Z. Stachura; A. Surzhykov; A. Warczak; Th. Stöhlker

The projectile excitation of high-Z ions has been investigated in relativistic ion-atoms collisions by observing the subsequent x-ray emission. The x-ray spectra from the projectile excitation have been separated from the x-ray emission following electron capture into the excited states using a novel anticoincidence technique. For the particular case of hydrogenlike Au{sup 78+} ions colliding with Ar atoms, Coulomb excitation from the ground state into the fine-structure-resolved n=2 levels as well as into levels with principal quantum number n{>=}3 has been measured with excellent statistics. The observed spectra agree well with simulated spectra that are based on Diracs relativistic equation and the proper inclusion of the magnetic interaction into the amplitudes for projectile excitation. It is shown that a coherent inclusion of the magnetic part of the Lienard-Wiechert potential leads to the lowering of the excitation cross section by up to 35%. This effect is more pronounced for excitation into states with high angular momentum and is confirmed by our experimental data.


APPLICATION OF ACCELERATORS IN RESEARCH AND INDUSTRY: 17TH International Conference on the Application of Accelerators in Research and Industry | 2003

Electron Loss from Fast, Low‐Charge‐State Ions Colliding with Gases

R. D. DuBois; A. C. F. Santos; Th. Stöhlker; F. Bosch; A. Bräuning-Demian; A. Gumberidze; S. Hagmann; C. Kozhuharov; R. Mann; A. Orsic Muthig; U. Spillmann; S. Tachenov; W. Barth; Ludwig Dahl; B. Franzke; J. Glatz; L. Gröning; S. Richter; D. Wilms; A. Krämer; K. Ullmann; Ottmar Jagutzki

Absolute cross sections for single and multiple projectile loss from MeV/u low‐charge‐state argon and xenon ions are presented. The impact energies were 0.74 and 1.4 MeV/u and the incoming charge states were 1,2 and 3. Using the present and data from the literature, it was shown that the multiple loss cross sections scale as (ΣBE)−n, where ΣBE is the sum of the binding energies required to remove a certain number of electrons. The power n was found to be velocity dependent, varying roughly from 2.5 to 1 for velocities ranging from approximately 0.2 to 20 (units of 108 cm/s).


X-RAY AND INNER-SHELL PROCESSES: 18th International Conference | 2000

Inverse photoionization studied via radiative electron capture into highly charged ions

Th. Stöhlker; O. Brinzanescu; A. Krämer; T. Ludziejewski; X. Ma; P. Swiat; A. Warczak

Differential aspects of the photoelectric effect for high-Z hydrogen-like and few-electron ions are studied by the time-reversed process in ion-atom collisions, i.e. by radiative capture of a quasi-free target electron. In this time-reversed situation the capture of an electron into a bound state of the ion is accompanied by the simultaneous emission of a photon, allowing for a direct and unambiguous experimental access to this process. Therefore, radiative capture provides a unique tool to investigate the details of the photoionization process even in the case of high-Z highly charged ions where such investigations are currently not accessible in the direct channel. At high-Z relativistic effects become important for which photon angular distributions turn out to be a very sensitive probe. This was demonstrated very recently, by an angular distribution study performed for electron capture into the 1s ground state of U92+. This particular experiment allowed us to identify spin-flip contributions to the ph...


Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms | 1999

Excitation of high-Z one- and two-electron ions in relativistic collisions with gaseous targets

T. Ludziejewski; Th. Stöhlker; H. F. Beyer; F. Bosch; S. Fritzsche; D. C. Ionescu; C. Kozhuharov; A. Krämer; D. Liesen; P. H. Mokler; P. Rymuza; Z. Stachura; P. Świat; A. Warczak

Abstract Progress in the studies of ground state excitation of heavy hydrogen-like and helium-like ions in relativistic collisions with atoms is reported. In particular the recent results obtained in experiments carried out at the internal gas-jet target of the ESR Storage Ring at GSI-Darmstadt are presented. Special emphasis is given to the sensitivity of the measured state selective cross sections to the magnetic effects occurring in relativistic heavy-ion–atom collisions.


Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms | 1998

Measurement of the spectral shape for the two-photon decay in heliumlike gold

H.W. Schäffer; P. H. Mokler; R. W. Dunford; C. Kozhuharov; A. Krämer; T. Ludziejewski; H.-Th. Prinz; P. Rymuza; L Sarkadi; Th. Stöhlker; P. Swiat; A. Warczak

Progress on a measurement of the spectral distribution of the two-photon decay of the 1s2s 1S0 level in the strong central fields of heavy heliumlike ions is reported. A measurement of the exact shape of the continuous spectrum of the two-photon decay in heavy heliumlike ions provides a sensitive test of the details of the complete structure of a heliumlike system. In our experiment at GSI, a beam of 106.6 MeV/u Au77+ ions was excited by a thin Aluminum foil and the subsequent decays were observed in an array of Ge(i) detectors.


Physical Review Letters | 1999

Angular Distribution Studies for the Time-Reversed Photoionization Process in Hydrogenlike Uranium: The Identification of Spin-Flip Transitions

Th. Stöhlker; T. Ludziejewski; F. Bosch; R. W. Dunford; C. Kozhuharov; P. H. Mokler; H. F. Beyer; O. Brinzanescu; B. Franzke; Jörg Eichler; A. Griegal; S. Hagmann; Akira Ichihara; A. Krämer; J. Lekki; D. Liesen; F. Nolden; H. Reich; P. Rymuza; Z. Stachura; M. Steck; P. Swiat; A. Warczak

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A. Warczak

Jagiellonian University

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T. Ludziejewski

Goethe University Frankfurt

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Z. Stachura

Polish Academy of Sciences

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P. Rymuza

Argonne National Laboratory

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R. W. Dunford

Argonne National Laboratory

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P. Swiat

Jagiellonian University

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