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We have investigated the relation of forward emission cusp electron spectra and bremsstrahlung for 90AMeV U88+ + N2at the internal supersonic gas jet target of the ESR storage ring of GSI. We find that x ray photons measured in coincidence with cusp electrons originate from the short wavelength limit of the electron nucleus bremsstrahlung.

In this contribution, we present an experimental study carried out at GSI Darmstadt devoted to investigation of quantum electrodynamical (QED) effects for the ground states in hydrogen- and helium-like uranium. In the experiment, X-ray spectra following radiative recombination of free electrons with bare and H-like uranium ions (U92+, U91+) were measured at the electron cooler of the ESR storage ring. Utilizing clean and favorable experimental conditions present at the electron cooler, we were able to obtain very accurate values for the ground-state binding energies from the observed X-ray transitions. When compared with theory, our results provide the most stringent test of bound-state QED for one- and two-electron systems in the strong-field regime.

27th International Conference on Neutrino Physics and Astrophysics, NEUTRINO2016, London, England, 4 Jul 2016 - 9 Jul 2016; Journal of physics / Conference Series 888, 012192 - (2017). doi:10.1088/1742-6596/888/1/012192, Published by IOP Publ., Bristol

Significant multiple electron capture has been found in collisions of 46 MeV/u Pb81+ ions with thin carbon foils. An increase in the many-electron capture probability has been observed for the thinnest targets, which is contrary to the normally assumed process of subsequent single-electron capture in the bulk. This shows the significant contribution of the surface for multiple electron capture. Further evidence is found for a strongly reduced capture probability from the exit surface as compared to the entrance surface. Absolute yields for electron capture and projectile ionization are presented and discussed within a Monte-Carlo simulation.

The bremsstrahlung process in the domain of strong Coulomb fields has been investigated for and Ar target electrons colliding with He-like uranium ions at . The differential cross sections for bremsstrahlung were measured at laboratory observation angles of , , and . Substantial discrepancies were found between the experimental cross sections and the first-order Born approximation calculations. The reported data provide a new testing ground for non-perturbative treatment of the coupling between radiation and matter in the presence of strong fields.

The heavy ion synchrotron/storage ring facility at GSI, SIS/ESR, provides intense beams of cooled, highly-charged ions up to naked uranium (U92+). By electron capture during ion-atom collisions in the gas target of the ESR or by recombination at ion-electron encounters in the "electron cooler" excited states are populated. The detailed structure of very heavy one-, two-and three-electron ions is studied. The different mechanisms leading to the excited states are described, as well as the new experimental tools now available for a detailed spectroscopy of these interesting systems. Special emphasis is given to x-ray transitions to the groundstates in H- and He-like systems. For the heaviest species the groundstate Lambshift can now be probed on an accuracy level of better than 10% using solid-state x-ray detectors. Applying dispersive x-ray analysing techniques, this accuracy will certainly be improved in future. However, utilizing the dielectronic recombination resonances for a spectroscopy, the structure in Li-like heavy ions can already be probed now on the sub eV level.

The Lyman and Balmer series in hydrogen-like Dy65+, along with radiative electron capture (REC) into bare Dy66+, have been observed. The X-ray spectra were measured in coincidence with one-electron capture in a beam of 292 MeV amu-1 Dy66+ stored in the ESR storage ring and interacting with an internal argon gas target. The REC line profile was studied in detail and was in good agreement with theory, thus suggesting additional possibilities for spectroscopy.

By applying the deceleration technique for bare uranium ions at the ESR storage ring we studied the Radiative Electron Capture process in collisions with low-Z target atoms. This allowed us to extend the current information about the time-reversed photoionization process of highly-charged heavy ions to much lower energies than those accessible for neutral heavy elements in the direct reaction channel. The angular differential data obtained prove theoretical predictions that at high-Z higher-order multipole contributions and magnetic corrections play an important role even at energies close to the threshold.

Processes associated with double electron capture into bare U92+-ions have been observed under single collision conditions. Regions of very low cross sections, close to mbarn, have been explored successfully. In particular, an attempt to register photons with twice the energy of single K-REC photons has been performed. Moreover, K-REC spectra associated with double charge exchange have been analysed in terms of their angular distribution. As a result, evidence for correlated electron capture has been found

The study of isotope shifts and hyperfine effects in dielectronic recombination (DR) resonance spectra strikes a conceptually new path for investigations of nuclear properties such as charge radius, spin, magnetic moment of nuclei or lifetimes of long-lived excited nuclear states. A series of DR experiments with heavy three-electron ions (Li-like) was performed at the heavy-ion storage ring ESR of the GSI Helmholtzzentrum fur Schwerionenforschung, Darmstadt, Germany. In a pilot experiment the two stable isotopes A=142 and A=150 of neodymium ANd57+ were investigated. From the displacement of DR resonances the isotope shifts δE142,150(2s – 2Pl/2) = 40.2(3)(6) meV and δE142,150(2s – 2p3/2) = 42.3(12)(20) meV for 2s – 2Pj transitions of the Li-like ions have been obtained. An evaluation within a full QED framework yielded a change in the mean-square charge radius of 142,150δ(r2) = −1.36(1)(3) fm2. At GSI, in addition to stable isotopes, in-flight synthesized radioisotopes can be studied as well. The production of radioisotopes of interest, the subsequent separation in the storage ring ESR and first DR experiments with the exotic nuclei 237U89+ and 234Pa88+ (Z=91) are presented. The paper is concluded with a brief outlook at future DR experiments with heavy radioisotopes at the ESR.

Schottky mass spectrometry was applied to stored and cooled neutron-deficient 152Sm projectile fragments at the experimental storage ring (ESR) facility at GSI Darmstadt, Germany. 311 different nuclides were identified by means of their revolution frequency spectra. The measured frequencies were used to investigate the shape of the momentum-compaction-factor of the ESR.

It is demonstrated that the effect of the electric dipole moment of electron and of the P,T-odd electron–nucleus interaction can be well distinguished in the experiments with H-like (or Li-like) highly charged ions in storage rings. These two effects cannot be separated in the experiments with a certain atom or molecule. Possible experiments for the search of P,T-odd effects with H-like ions in storage rings are discussed and the parameters of these experiments (observation time, field control) are estimated.

Absolute rate coefficients for dielectronic recombination (DR) of Be-like 136Xe50+ have been measured at the heavy-ion storage ring ESR. The experimental results are compared with relativistic distorted-wave calculations employing the multiconfiguration Dirac-Fock method. Based on the DR measurements, multiple intra-L-shell excitation energies were determined.

Absolute rate coefficients for dielectronic recombination (DR) of Li-like 136Xe51+ have been measured at the heavy-ion storage ring ESR. The experimental results are compared with relativistic distorted-wave calculations employing the multiconfiguration Dirac-Fock method. Based on the DR measurements the 2s-2p1/2 and 2s-2p3/2 excitation energies in Li-like Xe51+ were determined with a relative accuracy of ~400 ppm.

A resonant Schottky pickup was built into the experimental storage ring at GSI Darmstadt in 2010 and a similar one in the CSRe storage ring at IMP Lanzhou in 2011. Single-ion sensitivity was achieved at 400 MeV u−1. The pickup has also been used in many storage ring experiments ever since. A brief description of the pickup and its application in single-ion lifetime spectroscopy is provided in this work.

The utilization of the resonant atomic electron–ion collision process of dielectronic recombination (DR) as a tool to probe nuclear properties via isotope shifts and hyperfine effects is discussed. Based on DR, this resonance reaction spectroscopy at electron coolers of heavy-ion storage rings denotes a versatile approach to access nuclear parameters such as charge radius, spin, magnetic moment or lifetimes of long-lived excited nuclear states (isomers). The high sensitivity of DR allows for experiments with artificially synthesized rare isotopes and isomers. Recent experimental progress in the preparation of such exotic species at the ESR storage ring in Darmstadt is presented. The DR technique is exemplified for the case of 234Pa88+ (Z = 91).

Electron capture cross sections have been estimated from the projectile incoming inner shell vacancy dependent K and L x-ray emission cross sections of the projectile in 69.2 MeV/u Biq+-Au collisions (77≤ q ≤ 82) measured at the GSI heavy ion synchrotron SIS18. These estimations proved to be in agreement with the values deduced from the charge exchange cross sections obtained from the target thickness dependent charge state distribution of the emerging ions. The Eikonal approximation indicates trends in consonance with the measured values and predictions according to Schlachter et al. are in reasonable agreement too.

Observed enhancement of K-shell radiative recombination (RR) of bare uranium ions with cooling electrons is interpreted in terms of of distant transverse collisions in magnetized electron-cooler plasma described within the semiclassical geometrical model (SGM), which was recently proposed. The Monte Carlo simulations based on the proposed approach explains the enhancement measured in RR of U^{92+} ions with cooling electrons for the K-shell.

Absolute rate coefficients for dielectronic recombination (DR) of hydrogenlike U91+ have been measured at GSI's heavy-ion storage ring ESR in the center-of-mass energy range 63–90 keV. We find an excellent agreement between the measured DR rate coefficients and theoretical results from multi-configuration Dirac-Fock theory which includes the Breit interaction. The latter contributes nearly one half to the total resonance strengths of the KL1/2L1/2 and KL1/2M1/2 resonance groups.

Dielectronic recombination measurements with Li-like in-flight produced actinides were performed at the storage ring ESR of the GSI Helmholtzzentrum fur Schwerionenforschung employing the ring's cooler as a target for free electrons. The extension of the DR technique for the investigation of isotope effects towards shortlived nuclei (> 10 s) is a major advance and enables new types of experiments at the interface of atomic and nuclear physics.

Absolute rate coefficients for dielectronic recombination of Be-like Xe50+ have been measured by employing the electron-ion merged beam technique at GSI's heavy-ion storage ring ESR in the center-of-mass energy range 0 – 600 eV. In addition to ground state ions, resonances from metastable Xe50+ (2s2p3P0) parent ions have been observed. These may be exploited in future measurements of the E1M1 two photon 2s2p3P0 → 2s2 1S0 transition rate.

Theoretical concepts in the production and controlling of polarized highly charged ion beams in storage rings are investigated. A scheme for the control of the degree of ion beam polarization, based on the observation of the linear polarization of emitted x-ray photons in hyperfine quenched (HFQ) decay of excited levels of polarized He-like ions, is presented. One of the important motivations for the production of polarized ion beams is the possibility of observing the parity nonconservation (PNC) effects in HFQ transitions in He-like highly charged ions, where these effects can reach an unprecedented high value for atomic physics. The measurement of these effects requires online diagnostics of the degree of ion beam polarization. The possible schemes for such experiments as well as estimates of the magnitude of the observable PNC effects are presented.

A pioneering experiment was recently performed at the Experimental Storage Ring (ESR) at GSI. Fully stripped ions of 96Ru were injected into the storage ring and slowed down to a few MeV per nucleon. The 97Rh ions from the 96Ru(p,γ) reaction at a newly developed hydrogen jet target were detected with Double Sided Silicon Strip Detectors (DSSSD) mounted inside a pocket. The experiment and the status of the analysis at a beam energy of 11 MeV per nucleon will be presented. © 2010 IOP Publishing Ltd.

The observation of the enhancement of radiative recombination (RR) into the K-shell for U92+ ions interacting with cooling electrons in x-ray RR experiment performed at the ESR storage ring is reported. From the measured intensities of K-RR x-rays for the electron relative energies in the range 0-1000 meV a clear indication of the recombination enhancement, with respect of the fully relativistic RR calculations, was found for the K-shell for the zero relative electron energy. This results add new aspects to existing interpretations of the enhancement effect emphasizing a role of recombination into high Rydberg states.

The hyperfine quenching (HFQ) mechanism of metastable states in polarized He-like heavy ions is considered. The lifetime dependence of these states on the ion polarization in an external magnetic field is established. This dependence is presented for the 23P0 state of the europium (Z = 63) ion and is proposed as a method for the measurement of the ion polarization in the experiments for the search of parity violating effects.

We report on simultaneous measurements of the continuum momentum distribution for RECC cusp electrons and bremsstrahlung in 90AMeV U88++ N2 → U88++ {N2+*} + ecusp(00) + hv (RECC) collisions. We show that x-ray photons which appear coincident with RECC cusp electrons stem from the short-wavelength limit of the electron-nucleus bremsstrahlung. The observed pronounced asymmetric cusp shape is in good agreement with theory within the relativistic impulse approximation. We expect that when exploiting the full imaging properties of our forward electron spectrometer, fully differential cross sections at the tip region of the bremsstrahlung spectrum with near 10−4 electron energy resolution are possible in the near future.

Accurate investigations of the structure of one- and few-electron ions in the high-Z regime provide unique possibilities for testing fundamental theories underlaying our present understanding of of the physics of extremely electro-magnetic strong fields. In this review, we concentrate on x-ray spectroscpic investigations of the ground-state transition energies in H-like uranium (heaviest stable element available) by using the intense beams of cooled heavy ions provided by the storage ring ESR at GSI. Such experiments allow for a precise study of the ground-state binding-energies in high-Z H-like ions where relativistic and QED effects are strongest. The most recent experiment is presented where the deceleration capability of the ESR storage ring was exploited for x-ray spectroscopy at the ESR electron cooler. In addition, we discuss the ongoing developments for a new generation of ground-state Lamb shift experiments aiming on a precision of 1 eV or even better. In particular, emphasis will be given to the dedicated crystal spectrometer (FOCAL) in combination with state of the art 2D position-sensitive solid state detectors, allowing for energy and time resolved x-ray imaging.

A Monte Carlo simulation program of x-ray spectra produced in collisions of fast ions with a gas-jet target of a storage ring is presented. The calculated spectra are compared with experimental data obtained at the ESR storage ring of GSI in Darmstadt.

The anisotropy of the Ll transition emission following heavy ion-atom collisions has been measured at a projectile velocity corresponding to 1.4 MeV N-1 kinetic energy. The atomic numbers of the collision partners range up to Z=92 covering a wide range of Z1/Z2 values. A discrepancy between the experiment and direct Coulomb excitation predictions has been observed and is considered to be due to quasimolecular excitation processes as well as to satellite line structures.