We present developments at the electron cooler of the CRYRING@ESR storage ring aimed at improving its performance as internal target in low-energy electron–ion collision experiments. We describe recent modifications of the high-voltage and vacuum systems which lead to improved electron energy control and lower background count rates during such measurements. In a benchmark experiment on dielectronic recombination of F ^6+ , we demonstrate that a spectroscopic resolution equivalent to transverse and longitudinal thermal electron energy spreads of ≈ 1 meV and ≈ 45 µeV, respectively, can be achieved under optimum conditions. This surpasses system performance observed previously in similar experiments.

The electron cooler of the CRYRING@ESR storage ring at the GSI-FAIR accelerator complex is a unique instrument, which not only provides beam cooling of the stored ions, but also serves as a low-energy electron target for Dielectronic Recombination experiments. The minimisation of vacuum contamination and the response to rapid energy changes are key requirements of the cooler in electron target mode. Therefore, a test bench was prepared to study the outgassing behaviour of the electron gun components and a setup was constructed to evaluate the drift-electrode-modulation of the acceleration voltage of the cooler. The vacuum studies showed that the electron gun cathode was severely malfunctioning and resulted in its replacement. The drift-electrode-modulation of the acceleration voltage showed significant improvements compared to direct modulation of the terminal voltage of the cooler.

Using novel metallic magnetic calorimeter detectors at the CRYRING@ESR, we recorded X-ray spectra of stored and electron cooled helium-like uranium (U^90+) with an unmatched spectral resolution of close to 90 eV. This allowed for an accurate determination of the energies of all four components of the Kα transitions in U^90+. We find good agreement with state-of-the-art bound-state QED calculations for the strong-field regime. Our results do not support any systematic deviation between experiment and theory in helium-like systems, the presence of which was subject of intense debates in recent years.

We present a recently established setup for laser spectroscopy at CRYRING@ESR at the GSI Helmholtz Centre for Heavy Ion Research. Here, laser spectroscopy can be performed on stored and cooled ion bunches and coasting beams. First spectra of ^24,25Mg^+ ions are presented that were recorded by classical Doppler-limited fluorescence spectroscopy as well as Λ-spectroscopy using counter- and copropagating laser beams that are Doppler-shifted by several nm.

We have tested operation of FAIR’s low-energy ion storage ring CRYRING@ESR with uncompensated electron cooler solenoid. With its standard working point on the lowest-order difference resonance, a second solenoid is normally used to cancel betatron coupling introduced by the cooler’s magnetic field. In operation with a D + test beam, we found that omission of the compensation solenoid did not lead to a notable deterioration of beam intensity, quality, or cooling time, though the expected coupling of betatron motion is then clearly observed.

In the last decade nuclear reaction measurements using heavy ion storage rings became an important tool for nuclear astrophysics studies. The new CRYRING Array for Reaction MEasurements (CARME), recently commissioned at the low energy CRYRING@ESR storage ring (GSI/FAIR), is designed to take this novel approach one step further and perform direct nuclear reaction measurements at stellar energies, as well as indirect studies of nuclear properties of interest for nuclear astrophysics. CRYRING is unique worldwide in being able to store high quality, isotopically pure, radioactive beams produced in-flight at the low energies required for nuclear astrophysics. This paper describes the first in-beam reaction measurement with CARME at CRYRING, the first beam on (conventional) target measurement for FAIR Phase-0, and the data analysis approach required by this unprecedented, unique experimental approach.

In this report, we compare two filter algorithms for extracting timing information using novel metallic magnetic calorimeter detectors, applied to the precision X-ray spectroscopy of highly charged ions in a storage ring. Accurate timing information is crucial when exploiting coincidence conditions for background suppression to obtain clean spectra. For X-rays emitted by charge-changing interactions between ions and a target, this is a well-established technique when relying on conventional semiconductor detectors that offer a good temporal resolution. However, until recently, such a coincidence scheme had never been realized with metallic magnetic calorimeters, which typically feature much longer signal rise times. In this report, we present optimized timing filter algorithms for this type of detector. Their application to experimental data recently obtained at the electron cooler of CRYRING@ESR at GSI, Darmstadt is discussed.

We present research program and project for high-resolution wavelength-dispersive spectrometer dedicated to low-energy X-ray spectroscopy at the electron cooler of the CRYRING@ESR storage ring, which is a part of the international Facility for Antiproton and Ion Research (FAIR) currently being built in Darmstadt. Due to the unique shape of the electorn-ion recombination X-ray source, resulting from the overlapping of the electron and ion beams in the electron cooler, the spectrometer can work in the specific asymmetric von Hamos (AvH) geometry. In order to completely eliminate the influence of Doppler effect on the measured X-ray energies, two asymmetric von Hamos spectrometers will be installed next to the dipole magnets on both sides of the electron cooler to detect blue/red (0°/180°) shifted X-rays, e.g. emitted in the radiative recombination (RR) process. The X-ray-tracing Monte-Carlo simulations show that the proposed AvH spectrometer will allow to determine with sub-meV precision, the low-energy X-rays (5–10 keV) emitted from stored bare or few-electron heavy ions interacting with cooling electrons. This experimental precision will enable accurate studies of the quantum electrodynamics (QED) effects in mid-Z H- and He-like ions.

We report on a new experimental approach for the Doppler correction of X-rays emitted by heavy ions, using novel metallic magnetic calorimeter detectors which uniquely combine a high spectral resolution with a broad bandwidth acceptance. The measurement was carried out at the electron cooler of CRYRING@ESR at GSI, Darmstadt, Germany. The X-ray emission associated with the radiative recombination of cooler electrons and stored hydrogen-like uranium ions was investigated using two novel microcalorimeter detectors positioned under 0∘ and 180∘ with respect to the ion beam axis. This new experimental setup allowed the investigation of the region of the N, M → L transitions in helium-like uranium with a spectral resolution unmatched by previous studies using conventional semiconductor X-ray detectors. When assuming that the rest-frame energy of at least a few of the recorded transitions is well-known from theory or experiments, a precise measurement of the Doppler shifted line positions in the laboratory system can be used to determine the ion beam velocity using only spectral information. The spectral resolution achievable with microcalorimeter detectors should, for the first time, allow intrinsic Doppler correction to be performed for the precision X-ray spectroscopy of stored heavy ions. A comparison with data from a previous experiment at the ESR electron cooler, as well as the conventional method of conducting Doppler correction using electron cooler parameters, will be discussed.

The exploitation of polarization degrees of freedom of hadron beams and/or targets offers a wealth of observables that are not accessible with unpolarized particles.These observables can be used to test the conservation or violation of fundamental symmetries like parity, charge conjugation, time reversal or combinations thereof.This paper describes some of the physics that can be pursued with polarized hadron beams or polarized targets using the CRYRING and the Experimental Storage Ring (ESR) at GSI/FAIR in Darmstadt after the completion of the experimental program with the Cooler Synchrotron COSY at Forschungszentrum Jülich.

Experimental determination of the cross sections of proton capture on radioactive nuclei is extremely difficult. Therefore, it is of substantial interest for the understanding of the production of the p-nuclei. For the first time, a direct measurement of proton-capture cross sections on stored, radioactive ions became possible in an energy range of interest for nuclear astrophysics. The experiment was performed at the Experimental Storage Ring (ESR) at GSI by making use of a sensitive method to measure (p,γ) and (p,n) reactions in inverse kinematics. These reaction channels are of high relevance for the nucleosyn-thesis processes in supernovae, which are among the most violent explosions in the universe and are not yet well understood. The cross section of the 118Te(p,γ) reaction has been measured at energies of 6 MeV/u and 7 MeV/u. The heavy ions interacted with a hydrogen gas jet target. The radiative recombination process of the fully stripped 118Te ions and electrons from the hydrogen target was used as a luminosity monitor. An overview of the experimental method and preliminary results from the ongoing analysis will be presented.

Low-energy investigations on rare ion beams are often limited by the available intensity and purity of the ion species in focus. Here, we present the first application of a technique that combines in-flight production at relativistic energies with subsequent secondary beam storage, accumulation and finally deceleration to the energy of interest. Using the FRS and ESR facilities at GSI, this scheme was pioneered to provide a secondary beam of ^118Te^52+ for the measurement of nuclear proton-capture at energies of 6 and 7 MeV/u. The technique provided stored beam intensities of about 10^6 ions at high purity and brilliance, representing a major step towards low-energy nuclear physics studies using rare ion beams.

For decelerated bare lead ions at a low beam energy of 10 MeV/u, the x-ray emission associated with radiative recombination (RR) at threshold energies has been studied at the electron cooler of CRYRING@ESR at GSI, Darmstadt. In our experiment, we observed the full x-ray emission pattern by utilizing dedicated x-ray detection chambers installed at 0??? and 180??? observation geometry. Most remarkably, no line distortion effects due to delayed emission are present in the well-defined x-ray spectra, spanning a wide range of x-ray energies (from about 5 to 100 keV), which enables us to identify fine-structure resolved Lyman, Balmer, and Paschen x-ray lines along with the RR transitions into the K, L, and M shells of the ions. For comparison with theory, an elaborate theoretical model is established taking into account the initial population distribution via RR for all atomic levels up to Rydberg states with principal quantum number n = 165 in combination with time-dependent feeding transitions. Within the statistical accuracy, the experimental data are in very good agreement with the results of rigorous relativistic predictions. Most notably, this comparison sheds light on the contribution of prompt and delayed x-ray emission (up to 70 ns) to the observed x-ray spectra, originating in particular from yrast transitions into inner shells.

The low-energy heavy ion storage ring CRYRING was transported from Stockholm to Darmstadt, modernized and reconfigured, and recommissioned as CRYRING@ESR. The machine is now in operation with all installations in service and is available as a user facility for experiments proposed through the SPARC collaboration. During the 2020–2022 period, we brought a number of experimental installations into service and used them to measure first data: the ultra-cold electron cooler for merged-beam electron–ion collisions, the gas jet target for atomic collisions, a next-generation microcalorimeter-based X-ray spectroscopy setup, and others. Ions can be injected either in low charge states from a local ion source through a 300 keV/u RFQ linac, or in high charge states from the GSI accelerator chain through ESR. This allows for very broad access to ions across the entire periodic table. CRYRING@ESR is able to de- or accelerate ions and cool and store beams of isotopically pure species in a desired charge state. While the analysis is still largely ongoing, the first experimental data already show that the machine reached its expected performance level, and our high expectations regarding achievable resolution in spectroscopy experiments have been fulfilled. With access to new classes of ions available through ESR injection and a new generation of experimental instrumentation, CRYRING@ESR is a unique facility for experiments with heavy, highly charged ions. Here, we will review our present setup and machine performance, discuss the data from our first commissioning experiments and briefly preview the upcoming new installations for the coming years.

We report on the first integration of novel magnetic microcalorimeter detectors (MMCs), developed within SPARC (Stored Particles Atomic Physics Research Collaboration), into the experimental environment of storage rings at GSI 6 6 GSI Helmholtzzentrum für Schwerionenforschung GmbH, Planckstraße 1, 64 291 Darmstadt., Darmstadt, namely at the electron cooler of CRYRING@ESR. Two of these detector systems were positioned at the 0° and 180° view ports of the cooler section to obtain high-resolution x-ray spectra originating from a stored beam of hydrogen-like uranium interacting with the cooler electrons. While previous test measurements with microcalorimeters at the accelerator facility of GSI were conducted in the mode of well-established stand-alone operation, for the present experiment we implemented several notable modifications to exploit the full potential of this type of detector for precision x-ray spectroscopy of stored heavy ions. Among these are a new readout system compatible with the multi branch system data acquisition platform of GSI, the synchronization of a quasi-continuous energy calibration with the operation cycle of the accelerator facility, as well as the first exploitation of the maXs detectors’ time resolution to apply coincidence conditions for the detection of photons and charge-changed ions. GSI Helmholtzzentrum für Schwerionenforschung GmbH, Planckstraße 1, 64 291 Darmstadt.

The design and performances of a newly built electrostatic charge state analyzer constructed to act as a spectrometer for keV/u ions are reported. It consists of two 90∘ curved electrodes enclosed by Matsuda electrodes. This setup was recently tested using Ar9+ and Ar12+ ion beams at an energy of 10 keV per charge unit. This spectrometer achieves a good separation of different charge states formed by electron capture processes during collisions between primary ions and the residual gas. Thanks to these first tests, we have identified up to three different background contributions on the detector that need to be reduced or suppressed.

P.-M. Hillenbrand , 2 S. Hagmann , Y. S. Kozhedub , E. P. Benis , C. Brandau , 5 R. J. Chen, D. Dmytriiev , 6 O. Forstner , 7 J. Glorius , R. E. Grisenti , 2 A. Gumberidze , M. Lestinsky , Yu. A. Litvinov , 6 E. B. Menz , 7, 8 T. Morgenroth, 7, 8 S. Nanos , 4 N. Petridis, Ph. Pfäfflein , 7, 8 H. Rothard, M. S. Sanjari , 11 R. S. Sidhu , 6 U. Spillmann , S. Trotsenko, I. I. Tupitsyn , L. Varga, 6 and Th. Stöhlker 2, 7, 8 Institut für Kernphysik, Goethe-Universität, 60438 Frankfurt, Germany GSI Helmholtzzentrum für Schwerionenforschung, 64291 Darmstadt, Germany Department of Physics, St. Petersburg State University, 199034 St. Petersburg, Russia Department of Physics, University of Ioannina, 45110 Ioannina, Greece I. Physikalisches Institut, Justus-Liebig-Universität, 35392 Giessen, Germany Fakultät für Physik und Astronomie, Ruprecht-Karls-Universität, 69117 Heidelberg, Germany Institut für Optik und Quantenelektronik, Friedrich-Schiller-Universität, 07743 Jena, Germany Helmholtz-Institut Jena, 07743 Jena, Germany Tandem Accelerator Laboratory, INPP, NCSR “Demokritos”, 15310 Agia Paraskevi, Greece Centre de Recherche sur les Ions, les Matériaux et la Photonique CIMAP, Normandie Université, ENSICAEN, UNICAEN, CEA, CNRS, 14000 Caen, France Aachen University of Applied Sciences, 52066 Aachen, Germany (Dated: January 26, 2022)

In this paper, we present an experimental and theoretical study of excitation processes for the heaviest stable helium-like ion, that is, He-like uranium occurring in relativistic collisions with hydrogen and argon targets. In particular, we concentrate on angular distributions of the characteristic Kα radiation following the K → L excitation of He-like uranium. We pay special attention to the magnetic sub-level population of the excited 1s2lj states, which is directly related to the angular distribution of the characteristic Kα radiation. We show that the experimental data can be well described by calculations taking into account the excitation by the target nucleus as well as by the target electrons. Moreover, we demonstrate for the first time an important influence of the electron-impact excitation process on the angular distributions of the Kα radiation produced by excitation of He-like uranium in collisions with different targets.

Transverse electron cooling of heavy molecular ions has been studied at the Test Storage Ring (TSR). Electron beams from a cold GaAs:(Cs,O) photocathode, with kinetic energies down to 31 eV, have been used for cooling of singly-charged ions of masses up to 41 u. We believe that these are the heaviest singly-charged ions for which successful electron cooling has been reported so far. Transverse ion-beam emittances << 1 mu m were reached after typically several seconds of cooling time. The measured transverse cooling rates agree with a simple binary-collision model, assuming a transverse electron temperature of approximately 1 meV/k(B). The results serve as benchmark for electron cooling at the new Cryogenic Storage Ring, which uses the same photocathode electron source and is targeting singly-charged ions of even higher mass.