Quantum electrodynamics (QED), the quantum field theory that describes the interaction between light and matter, is commonly regarded as the best-tested quantum theory in modern physics. However, this claim is mostly based on extremely precise studies performed in the domain of relatively low field strengths and light atoms and ions^1–6. In the realm of very strong electromagnetic fields such as in the heaviest highly charged ions (with nuclear charge Z ≫ 1), QED calculations enter a qualitatively different, non-perturbative regime. Yet, the corresponding experimental studies are very challenging, and theoretical predictions are only partially tested. Here we present an experiment sensitive to higher-order QED effects and electron–electron interactions in the high-Z regime. This is achieved by using a multi-reference method based on Doppler-tuned X-ray emission from stored relativistic uranium ions with different charge states. The energy of the 1s_1/22p_3/2 J = 2 → 1s_1/22s_1/2 J = 1 intrashell transition in the heaviest two-electron ion (U⁹⁰⁺) is obtained with an accuracy of 37 ppm. Furthermore, a comparison of uranium ions with different numbers of bound electrons enables us to disentangle and to test separately the one-electron higher-order QED effects and the bound electron–electron interaction terms without the uncertainty related to the nuclear radius. Moreover, our experimental result can discriminate between several state-of-the-art theoretical approaches and provides an important benchmark for calculations in the strong-field domain.

Using novel metallic magnetic calorimeter detectors at the CRYRING@ESR, we recorded x-ray spectra of stored and electron cooled heliumlike 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 K α 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 heliumlike systems, the presence of which was subject of intense debates in recent years.

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${}^{\circ }$ and 180${}^{\circ }$ 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.

We present a here a non-parametric analysis of x-ray spectra for experiment E125 performed at GSI-FAIR in Darmstadt. This experiment measured intra-shell transitions of highly charged uranium with a twin-arm Bragg spectrometer equipped with x-rays CCDs. The method consists in slicing the CCD image and calculating the centroids of the lines, then adjusting the slice size in order to remove background. Due to the low count rate, we reveal the limitations of this method which provides an accuracy of only a few pixels, far from the sub-pixel requirement.

Dans cette thèse, je présente des mesures d'énergies de transition dans des ions lourds très chargés, dans le domaine des rayons X, pour des atomes allant du soufre à l'uranium. Ces mesures permettent de tester l'Électrodynamique Quantique (QED), dans des états liés, et le problème à n-corps relativiste. La première mesure a été réalisée sur des transitions intracouche 2p-2s dans l'uranium à deux, trois et quatre électrons, sur l'anneau de stockage ESR du complexe d'accélérateurs GSI/FAIR à Darmstadt avec deux spectromètres à cristal courbe. Elle a permis de tester les effets de QED jusqu'au second ordre et les effets de corrélation grâce à la précision obtenue de 37 parties par million (ppm), dix fois meilleure que celle de la mesure précédente en excellent accord avec la théorie. Le second ensemble de mesures a été réalisé sur des transitions 2p - 1s dans des ions de soufre et d'argon à 5 électrons (boronoïdes), avec un trou dans la couche 1s. Ces mesures permettent de tester les calculs de problème à n-corps relativiste, et en particulier les effets de corrélation électroniques. Ces mesures, sans références externes, ont été effectuées à l'aide d'un spectromètre à double cristal plan, ce qui permet d'atteindre une précision inégalée de 4 ppm. Pour analyser ces spectres complexes, il a fallu développer de nouvelles techniques Bayésiennes afin de pouvoir déterminer le nombre, la largeur et l'énergie des transitions présentes dans les raies observées. Nous avons ainsi pu déterminer l'énergie et la largeur de plusieurs transitions non-résolues, couramment observée par les satellites X. La mesure précise de ces transitions de mieux comprendre les spectres astrophysiques.