High-resolution X-ray spectroscopy of fast-moving ions: towards intrinsic Doppler correction

This work presents a new experimental approach for precision x-ray spectroscopy of highly-charged heavy ions in a storage ring. lt deploys novel metallic magnetic microcalorimeters (MMCs) at 0° and 180° on either side of the electron cooler of the recently commissioned CRYRING@ESR at the FAIR/GSI facility. These detectors of the maXs-type combine the broad spectral coverage of semiconductor detectors with an energy resolution approaching that of crystal spectrometers. The new approach represents a breakthrough in two key aspects. First, it enables the elimination of hard to-control systematic uncertainties that are imposed on the Doppler-shift of measured x-rays, and were always hampering heavy ion storage ring experiments. Second, it achieves an energy resolution that is improved by an order of magnitude compared to semiconductor detectors, which are typically used for x-ray spectroscopy over a broad spectral range. The measurement presented in this work was conducted in 2021, and investigated N, M-+L and L-+K transitions of U90+ ions, following radiative recombination of cooler electrons into U91+ ions decelerated at 10.225 MeV/u. lt was the first successful coincidence measurement using MMC detectors at a storage ring, and achieved an energy resolution below 100 eV FWHM for photon energies ranging from a few keV to over 100 keV. Thus, the new experimental approach enabled, for the first time, an intrinsic Doppler correction in a heavy ion storage ring experiment, decoupling the correction from the hard-to-control systematic uncertainties, which are not expected to improve in the foreseeable future. In contrast, the systematic uncertainties of the new spectroscopy technique show potential of being reduced to below 1 eV with further investigation and refinement of the approach. Despite the low photon counts in the presented measurement -owing to it being the first time uranium was injected in the CRYRING, and to it being a first that U91+ was decelerated to 10.225 MeV/u and stored in a storage ring - the ion beam velocity, on which the Doppler correction depends, determined through the intrinsic method yields a statistical uncertainty that is of similar magnitude than the systematic uncertainty of the conventional method (deducing the term from the electron cooler voltage). Moreover, the new experimental approach provided the first theory-supported experimental insight into relative populations of excited states, as well as the branching ratio El/M2 of the ls2p 3P2 state in U90+.