Experimental studies of the ground state QED : corrections in H- and He-like uranium

In summary, the cooled heavy-ion beams of the ESR storage ring offer excellent experimental conditions for a precise study of the effects of QED in the groundstate of high-Z one- and two-electron ions. This has been demonstrated within the series of experiments conducted at the electron cooler device as well as at the gasjet target. In this work we have used a recently developed experimental approach to obtain the first direct measurement of the two-electron contributions to the ground state binding energy of helium-like uranium. By employing our method, all one-electron contributions to the binding energy such as finite-nuclear size corrections and the one-electron self energy cancel out completely. Note, this is a distinctive feature of this particular kind of QED test and is in contrast to all other tests of bound state QED for high-Z ions such as 1s Lamb shift (in one-electron systems), g-factor of bound electrons, or hyperfine splitting. Compared to former investigations conducted at the superEBIT in Livermore we could already substantially improve the statistical accuracy and extend studies to the higher-Z regime. Moreover, our result has reached a sensitivity on specific two-electron QED contributions. Our value agrees with the theoretical predictions within the experimental uncertainty. Similar to the superEBIT experiment possible sources of systematic errors are essentially eliminated and the final result is limited only by counting statistics. For the case of the 1s Lamb shift in hydrogen-like uranium, the achieved accuracy of +- 4.2 eV is a substantial improvement by a factor of 3 compared to the most precise value up to now [44] (see Fig. 5.6). Our result already provides a test of the first-order QED contributions at the 1.5% level and only a slight improvement is required in order to achieve a sensitivity to QED contributions beyond first-order SE and VP.Für spektroskopische Untersuchungen an den schwersten Ein- und Zwei-Elektronensystemen bietet die Experimentier-Speicherringanlage für hochgeladene Ionen -ESR - bei GSI in Darmstadt ideale Voraussetzungen. Bei denen im Rahmen dieser Arbeit am ESR durchgeführten Experimenten lag das Hauptaugenmerk auf einer präzisen Vermessung des Beitrags der Elektron-Elektron-Wechselwirkung zum Ionisationspotenzial im He-ähnlichen Uran, d.h den schwersten Zwei-Elektronensystemen, die gegenwärtig Experimenten zur Verfügung stehen. Diese repräsentieren die einfachsten atomaren Vielteilchensysteme, in denen aber auf Grund der hohen Kernladung zugleich relativistische Effekte, Korrelationen und QED-Beiträge eine wichtige Rolle spielen. Auf Grund des gewählten Experimentieraufbaus konnten zudem die Grundzustandübergänge für H-artiges Uran vermessen werden, so dass ebenso präzise Informationen für die Grundzustandsenergie des Urans mit nur einem Elektronen gewonnen werden konnten. Für den Zwei-Elektronenbeitrag zum Ionisationspotenzial im He-ähnlichen Uran konnte aus dem Experiment ein Wert von 2248 eV gewonnen werden, wobei eine Genauigkeit von 9 eV erreicht wurde. Für den Grundzustand in He-ähnlichen Ionen stellt dieses Ergebnis die bislang genaueste Bestimmung des Zwei-Elektronenbeitrags dar, deren Sensitivität erstmals an die Größe der spezifischen Zwei-Elektronen-QED Beiträge heranreicht. Generell sei hier betont, dass es im Rahmen dieser Arbeit erstmals gelang, die vorliegenden Daten für das Ionisationspotenzial schwerer He-ähnlicher Ionen auf U90+ zu erweitern. Der Vergleich mit der theoretischen Vorhersage liefert eine gute Bestätigung für die Theorie der QED für Vielteilchensysteme im Bereich extrem starker Felder. Zudem erfolgte auch eine Bestimmung für die 1s-Lamb-Verschiebung für das H-ähnliche Uran. Hierzu diente vor allem eine Auswertung der sehr intensiven charakteristischen 2P3/2 -> 1s1/2 Ly-alpha1 Strahlung. Aus einem Vergleich der Messergebnisse mit den Vorhersagen der Dirac-Theorie für einen punktförmigen Urankern folgt für die 1s-Lamb-Verschiebung ein Wert von 460 eV mit einer Genauigkeit von 4.2 eV. Dieses sehr präzise Ergebnis, das frühere Ergebnisse um einen Faktor Drei an Genauigkeit übertrifft, befindet sich in sehr guter Übereinstimmung mit neuesten theoretischen Modellrechnungen. Insbesondere kann durch das vorliegende Ergebnis die 1s-Lamb-Verschiebung in wasserstoffähnlichem Uran auf dem Niveau von 1.5% als gesichert angesehen werden. Hierdurch erfährt die Theorie der Quantenelektrodynamik in sehr starken Coulomb-Feldern eine hervorragende Bestätigung

Alignment of Heavy Few-Electron Ions Following Excitation by Relativistic Coulomb Collisions

The Coulomb excitation of highly charged few-electron ions in relativistic collisions with protons and low- Z atoms is studied within the framework of first-order perturbation theory and the multiconfiguration Dirac-Fock method. Apart from the computation of the total excitation cross sections, a detailed theoretical analysis has been performed for the magnetic sublevel population of the residual ions. To describe this population, general expressions are derived for the alignment parameters of the excited states of the ions, taking into account the relativistic and many-electron effects. Calculations are performed for the K→L and K→M excitation of helium- and lithiumlike uranium ions and for a wide range of projectile energies. It is shown that the alignment of heavy few-electron ions is sensitive to relativistic and magnetic-interaction effects and, hence, to the collision energies of the projectiles. The theoretical predictions are discussed in the context of recent measurements on the Coulomb excitation of heliumlike uranium U90+ ions which were recently performed at the GSI storage ring in Darmstadt

A vacuum double-crystal spectrometer for reference-free highly charged ions X-ray spectroscopy

We have built a vacuum double crystal spectrometer, which coupled to an electron-cyclotron resonance ion source, allows to measure low-energy x-ray transitions in highly-charged ions with accuracies of the order of a few parts per million. We describe in detail the instrument and its performances. Furthermore, we present a few spectra of transitions in Ar$^{14+}$, Ar$^{15+}$ and Ar$^{16+}$. We have developed an ab initio simulation code that allows us to obtain accurate line profiles. It can reproduce experimental spectra with unprecedented accuracy. The quality of the profiles allows the direct determination of line width.Comment: 21 pages; Version

Isotope Shift in the Dielectronic Recombination of Three-electron \u3csup\u3eA\u3c/sup\u3eNd⁵⁷⁺

Isotope shifts in dielectronic recombination spectra were studied for Li-like ANd57+ ions with A = 142 and A = 150. From the displacement of resonance positions energy shifts δE142 150(2s-2p1/2) = 40.2(3)(6) meV [(stat)(sys)] and δE142 150(2s - 2p3/2) = 42.3(12)(20)meV of 2s - 2pj transitions were deduced. An evaluation of these values within a full QED treatment yields a change in the mean-square charge radius of 142 150δ⟨ r2⟩ = -1.36(1)(3) fm2. The approach is conceptually new and combines the advantage of a simple atomic structure with high sensitivity to nuclear size

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