In close collaboration with colleagues from several universities and research institutions, researchers of the Helmholtz Institute Jena have succeeded in measuring the x-ray spectrum of the heaviest two-electron ion, i.e. heliumlike uranium, with an unprecedented energy resolution of ten times higher than previously possible [1]. This has been accomplished by deploying novel metallic magnetic calorimeter (MMC) detectors at the CRYRING@ESR storage ring at GSI Helmholtzzentrum für Schwerionenforschung. In contrast to conventional x-ray detectors which measure electric charges, the MMCs detect the tiny temperature rise upon absorption of an x-ray in the detector material. Helliumlike ions are simplest multi-electron atomic systems and thus provide ideal ground for testing our present understanding of state-of-the-art atomic structure theory, i.e. bound-state quantum electrodynamics (QED).

Resolving the x-ray lines corresponding to certain transitions in helliumlike uranium for the first time provides a highly accurate test of the interaction of electrons with the quantum vacuum and with each other in the presence of the extremely strong field of the uranium nucleus. Overall, our measurement confirms the predictions of bound-state QED for the heaviest two-electron ion. Such high accuracy tests in the so far largely unexplored regime of extreme electromagnetic fields are a prerequisite for using QED predictions as a solid foundation in the search for physics beyond the Standard Model of particle physics.

­­Reference:

[1] Ph. Pfäfflein, et al.: "Quantum Electrodynamics in Strong Electromagnetic Fields: Substate Resolved K⁢𝛼 Transition Energies in Heliumlike Uranium", Phys. Rev. Lett. 134, 153001, doi:10.1103/PhysRevLett.134.153001.