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Electrons bound in highly charged heavy ions such as hydrogen-like bismuth 209 Bi 82+ experience electromagnetic fields that are a million times stronger than in light atoms. Measuring the wavelength of light emitted and absorbed by these ions is therefore a sensitive testing ground for quantum electrodynamical (QED) effects and especially the electron–nucleus interaction under such extreme conditions. However, insufficient knowledge of the nuclear structure has prevented a rigorous test of strong-field QED. Here we present a measurement of the so-called specific difference between the hyperfine splittings in hydrogen-like and lithium-like bismuth 209 Bi 82+,80+ with a precision that is improved by more than an order of magnitude. Even though this quantity is believed to be largely insensitive to nuclear structure and therefore the most decisive test of QED in the strong magnetic field regime, we find a 7- σ discrepancy compared with the theoretical prediction. Precision measurements provide a sensitive test of fundamental constants and their uncertainties. Here the authors precisely measure the hyperfine structure splitting in bismuth ions, and report significant discrepancy with the theoretical prediction of quantum electrodynamics.

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.

Highly charged heavy ions at rest offer a wide spectrum of precision measurements. The GSI Helmholtzzentrum für Schwerionenforschung GmbH is able to deliver ions up to U92+. As the production of these heavy, highly charged ions requires high kinetic energies, it is necessary to decelerate these ions for ultimate precision. The broad energy distribution, which results from the deceleration in the HITRAP linear decelerator, needs to be reduced to allow for further transportation and experiments. The HITRAP cooling trap is designed to cool, i.e., reduce, this energy spread by utilizing electron cooling. The commissioning of this trap is done with Ar16+-ions from a local EBIT ion source. By analyzing the signal of stored ions after ejection, properties such as ion lifetime, charge exchange, and ion motions can be observed. Here, we provide an overview of the recent results of the commissioning process and discuss future experiments.

Collinear laser spectroscopy in the 1s2s3S1→1s2p3P0,2 transitions of helium-like 11B3+ was performed using the HITRAP beamline at the GSI Helmholtz Centre. The ions were produced in an electron beam ion source, extracted, and accelerated to a beam energy of 4 keV/q. Results agree with previous measurements within uncertainty. Thus, it was demonstrated that the metastable state in He-like ions is sufficiently populated to carry out collinear laser spectroscopy. The measurement is a pilot experiment for a series of measurements that will be performed at a dedicated collinear laser spectroscopy setup at TU Darmstadt with light helium-like ions.

Quantum electrodynamics has been tested to accuracies below the parts-per-trillion level in light-mass systems. However, tests in heavy-mass systems with a large nuclear charge have not yet reached similar accuracy. Here we report the hyperfine-structure splitting in the 1 s ground state of radioactive hydrogen-like 208 Bi 82+ . We produced the isotope in a nuclear reaction and injected the beam into a storage ring to perform laser spectroscopy on samples of 10 5 ions of Bi 82+ that have only a single remaining electron, which experiences extreme magnetic-field strengths. Our result for the hyperfine splitting is in excellent agreement with the most accurate prediction based on a combination of quantum electrodynamics calculations with an empirical treatment of the hyperfine-structure anomaly ratio extracted from laser spectroscopy on neutral atoms of 209 Bi and 208 Bi. This achievement paves the way for the most stringent test of quantum electrodynamics in strong magnetic fields and demonstrates the feasibility of laser spectroscopy on other exotic ions with low production yields. Hyperfine splitting was measured using the laser spectroscopy of accelerator-produced hydrogen-like bismuth ions. This demonstrates the feasibility of such measurements with other exotic ions with low production yields in a storage ring.

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.

Precision experiments with thermalized exotic nuclei will be possible at the super-conducting fragment separator Super-FRS at the Facility for Antiproton and Ion Research (FAIR). In the Early Science/First Science programs of FAIR, they will be performed at the focal plane FHF1 of the Super-FRS (in front of the High-Energy Branch) and, at a later stage during First Science++, also at the Low-Energy Branch. Exotic nuclei will be produced in flight and separated in the Super-FRS and their momentum spread will be reduced by energy-bunching. The ions will be further slowed down in a homogeneous degrader, thermalized in a gas-filled stopping cell, and extracted and transferred to the experimental setups. The stopping cell is, thus, a key device for experiments with thermalized exotic nuclei, and its performance characteristics will have a strong impact on the range of nuclides available and their yields, since its stopping and extraction efficiencies and extraction times strongly influence the rate of the extracted nuclei and put a limit on their lifetimes. In combination with a multiple-reflection time-of-flight mass spectrometer, the stopping cell will enable the measurement of masses, branching ratios, e.g., β -delayed (multi-)neutron emission probabilities, and lifetimes, as well as the in-cell production of exotic nuclei by multi-nucleon transfer with primary and secondary beams. Moreover, it will be a tool for the absolute calibration of the particle identification in the Super-FRS. Furthermore, using the combination of accurate mass determination with the PID of the Super-FRS on an event-by-event basis, completely new experimental possibilities will become available, such as the identification of millisecond isomers at the Super-FRS and the measurement of the dependence of the isomer-to-ground-state ratios on the production mechanism. These studies will be pursued in the context of the Super-FRS Experiment Collaboration. At the Low-Energy Branch, high-accuracy mass measurements, in-trap conversion electron and alpha spectroscopy, and trap-assisted spectroscopy will be performed with MATS (Precision Measurements of very short-lived nuclei using an Advanced Trapping System for highly charged ions). Measurements of nuclear spins, magnetic dipole and electric quadrupole moments, and root-mean-square charge radii will be carried out using collinear laser spectroscopy on ions and atoms and beta-NMR experiments with LaSpec (Laser Spectroscopy of short-lived nuclei). These experiments will address a wide scientific field ranging from nuclear structure and nuclear astrophysics to tests of the weak interaction and of the Standard Model. For the Super-FRS, a stopping cell with an areal density of 20 mg/cm 2 is required, with a cross-section of the stopping volume of 200 cm 2 , a high extraction efficiency that is element-independent, an extraction time on the order of 10 ms, and a rate capability of 10 7 ions/s up to a nuclear charge of Z=92. Furthermore, the stopping cell needs to deliver bunches of ions with high purity. No existing stopping cell is capable of reaching these performance characteristics simultaneously. To fulfill these requirements, a novel concept for gas-filled stopping cells has been developed. It is based on the cryogenic stopping cell of the FRS Ion Catcher, and, in addition, it implements the two-stage extraction of the thermalized ions in a direction orthogonal to the incoming ion beam (high-areal-density orthogonal-extraction cryogenic stopping cell, HADO-CSC). This will boost all performance characteristics of the stopping cell and thus remove the performance bottleneck of present stopping cells for the thermalization of exotic nuclei produced at relativistic energies.

HITRAP is a facility for deceleration of highly charged ions (HCI) produced online by the GSI accelerator. It consists of two linear stages for deceleration down to several keV/charge and an ion trap for ion cooling down to sub-eV energies. We report on two recent developments within the facility: a new design for the HITRAP cooling trap in order to improve its stability and voltage rigidity, as well as the investigation of the so-called MIVOC method in the HITRAP EBIT for production of Sb35+, Fe21+, and B5+.

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.

Heavy few-electron ions are relatively simple systems in terms of electron structure and offer unique opportunities to conduct experiments under extremely large electromagnetic fields that exist around their nuclei. However, the preparation of highly charged ions (HCI) has remained the major challenge for experiments. As an extension of the existing GSI accelerator facility, the HITRAP facility was conceived as a multi-stage decelerator for HCI produced at high velocity. It is designed to prepare bunches of around 105 HCI and to deliver them at low energies to various experiments. One of these experiments is SpecTrap, aiming for laser spectroscopy of trapped, cold HCI. We present the latest results on deceleration of ions in a radio-frequency quadrupole, synchrotron cooling of electrons in a trap as a preparation step for the prospective electron cooling of the HCI decelerated in HITRAP, as well as laser cooling of singly charged Mg ions for sympathetic cooling of HCI in SpecTrap.