Precision X-Ray Spectroscopy of He-like Uranium at CRYRING@ESRHYBRID

Helium-like ions are the simplest atomic multibody systems. Their study provides a unique testing ground for the interplay of the effects of electron–electron correlation, relativity and quantum electrodynamics (QED) within a wide range of electromagnetic field strengths. Especially heavy highly charged ions are ideal for probing higher order QED terms. For the 1s state in uranium for example, their contributions are on the 1 eV level at binding energies of above 100 keV. However, for ground state transitions in ions with nuclear charge Z > 54 there is currently no data available with sufficient resolution and accuracy to challenge state-of-the-art theory [1]. This is due to the fact that the typical spectral resolution of semiconductor detectors (a few 100 eV FWHM at 100 keV photon energy) is not sufficient to resolve the various components of the Kα radiation. In this context, the development of the maXs series of detectors [2] (cryogenic microcalorimeter arrays for high resolution X-ray spectroscopy) within SPARC is of particular importance. These detectors can be tailored for different regions of the X-ray spectrum. One variant out of this family of metallic magnetic calorimeter (MMC) detectors offers a spectral resolution of a few tens of eV FWHM at 100 keV incident photon energy in combination with a broad spectral acceptance down to a few keV. These properties enable new types of precision X-ray studies [3, 4].
In spring 2021 an X-ray spectroscopy study of helium-like uranium (U90+) has been performed at the electron cooler of the low-energy storage ring CRYRING@ESR at GSI, Darmstadt using MMC detectors [5, 6]. The characteristic photon emission associated with the relaxation of excited U90+ as a result of radiative recombination between stored U91+ ions and cooler electrons has been investigated. The achieved spectral resolution of better than 90 eV at X-ray energies close to 100 keV reveals the substructure of the Kα1 and Kα2 lines [7]. The application of two detectors in forward and backward direction furthermore enabled the determination of the Doppler shift from the recorded spectra [8]. In this contribution, I will present the performed experiment, the achieved results, as well as the improvements developed for the follow up experiment scheduled for summer 2025.

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[1] P Indelicato 2019 J. Phys. B: At. Mol. Opt. Phys. 52 232001 
[2] C Pies et al 2012 J. Low Temp. Phys. 167 269–279
[3] D Hengstler et al 2015 Phys. Scr. 2015 014054
[4] S Kraft-Bermuth et al. 2018 Atoms 2018 59 [5] Ph Pfäfflein et al. 2022 Phys. Scr. 97 114005
[6] Ph Pfäfflein et al. 2022 Atoms 2023 11(1), 5
[7] Ph Pfäfflein et al. 2025 Phys. Rev. Lett. 134 153001
[8] F M Kröger et al. 2023 Atoms 2023 11(2), 22

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