Observation of the hyperfine transition in lithium-like Bismuth 209Bi80+^{209}\text{Bi}^{80+}: Towards a test of QED in strong magnetic fields

We performed a laser spectroscopic determination of the $2s$ hyperfine splitting (HFS) of Li-like $^{209}\text{Bi}^{80+}$ and repeated the measurement of the $1s$ HFS of H-like $^{209}\text{Bi}^{82+}$. Both ion species were subsequently stored in the Experimental Storage Ring at the GSI Helmholtzzentrum f\"ur Schwerionenforschung Darmstadt and cooled with an electron cooler at a velocity of $\approx 0.71\,c$. Pulsed laser excitation of the $M1$ hyperfine-transition was performed in anticollinear and collinear geometry for $\text{Bi}^{82+}$ and $\text{Bi}^{80+}$, respectively, and observed by fluorescence detection. We obtain $\Delta E^{(1s)}= 5086.3(11)\,\textrm{meV}$ for $\text{Bi}^{82+}$, different from the literature value, and $\Delta E^{(2s)}= 797.50(18)\,\textrm{meV}$ for $\text{Bi}^{80+}$. These values provide experimental evidence that a specific difference between the two splitting energies can be used to test QED calculations in the strongest static magnetic fields available in the laboratory independent of nuclear structure effects. The experimental result is in excellent agreement with the theoretical prediction and confirms the sum of the Dirac term and the relativistic interelectronic-interaction correction at a level of 0.5% confirming the importance of accounting for the Breit interaction.Comment: 5 pages, 2 figure

Commissioning of the HITRAP Cooling Trap with Offline Ions

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 of Helium-like ¹¹B³⁺

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

Towards an Intrinsic Doppler Correction for X-ray Spectroscopy of Stored Ions at CRYRING@ESR

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

Setup of a Penning trap for precision laser spectroscopy at HITRAP

Ion traps have been established as a powerful tool for ion cooling and laser spectroscopy experiments since a long time ago. SpecTrap, one of the precision experiments associated to the HITRAP facility at GSI, is implementing a Penning trap for studies of large bunches of externally produced highly charged ions. The extremely strong electric and magnetic fields that exist around the nuclei of heavy elements drastically change their electronic properties, such as energy level spacings and radiative lifetimes. The electrons can therefore serve as sensitive probes for nuclear properties such as size, magnetic moment and spatial distribution of charge and magnetization. The energies of forbidden fine and hyperfine structure transitions in such ions strongly depend on the nuclear charge and shift from the microwave domain into the optical domain. Thus, they become accessible for laser spectroscopy and its potentially high accuracy. A number of such measurements has been performed in storage rings and electron beam ion traps and yielded results with relative accuracies in the 103 to 104 region. This work presents the construction and commissioning of a Penning trap used for capturing highly charged ions in-flight and cooling them nearly to rest, thus reducing Doppler broadening and increasing the possible accuracy. As an important step-stone towards that goal, singly charged Mg ions were produced in an electron impact ion source, transported to the trap and laser cooled to sub-K temperatures. Indications of Coulomb crystals formation are reported. Non-destructive detection of the stored ions was performed both electronically, with the attached cryogenic resonant circuits, and optically, by observing the fluorescence directly from the trap centre. The designed electronic components can also be used for resistive cooling, which becomes particularly efficient for highly charged ions. It was demonstrated that large ion clouds can be created in the trap, formed by accumulating many consecutive ion source shots. Ions were stored with lifetimes of the order of 100 s and cooled to temperatures lower than 60 mK. The same technique can be used in future experiments for mixing laser-cooled Mg+ with highly charged ions delivered by HITRAP. Thus, sympathetic cooling of highly charged ions to sub-K temperatures will become possible. This is sufficient for reducing the Doppler width of a transition in a heavy ion to several MHz and enables laser spectroscopy experiments with a relative accuracy up to 108. vii

Preparation of Low-Energy Heavy Ion Beams in a Compact Linear Accelerator/Decelerator

High precision tests of fundamental theories can often unfold their full potential only by using highly charged ions (HCI) at very low energies. Although in light of the envisaged energies at FAIR, experiments in the keV to MeV range may sound like backpedaling, these two techniques are in fact complementary, since the production of heavy HCI is virtually impossible without prior acceleration and electron stripping. However, subsequent preparation, transport, storage and detection of low-energy HCI bring new, surprising sets of problems and limitations. Here we will give an overview of the CRYRING@ESR local injector and the HITRAP linear decelerator. These two facilities consist out of one or two accelerator or decelerator stages, with a total length of around 10 meters, making them 'compact' in comparison to other GSI accelerators. The following sections describe their main design parameters, the achieved ion numbers, challenges of beam detection, as well as some special features such as multi-turn injection and single-shot energy analyzers. The conclusion will present the current status and will also give an outlook of the planned applications of low-energy ions at the FAIR facility