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Precise energy determination of Ni Kα and Fe Kβ in high ionization states using a microcalorimeter

Marc Botz1,2, Luciano Gottardi3, Martin de Wit3, Chintan Shah4,1,5, Liyi Gu3,6, François Pajot7, Thomas Pfeifer1, and José R. Crespo López-Urrutia1

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Phys. Rev. A 114, 052808 – Published 9 November, 2026

DOI: https://doi.org/10.1103/ynpq-nhq6

Abstract

Accurate laboratory transition energies for the Ni Kα−Fe Kβ blend are urgently needed both for atomic-structure benchmarking and interpretation of astrophysical x-ray spectra observed with space telescopes. Crucially, inferred abundances of these elements, which are essential for the diagnostics of hot plasmas, depend on the treatment of those overlapping transitions. By combining a transition-edge-sensor x-ray microcalorimeter array and a superconducting electron beam ion trap, we distinguish with line widths between 3.5eV and 5.5eV the key transitions from Ni xx to Ni xxvii, including previously unmeasured ones, from the Kβ lines of Fe xx to Fe xxv. Collisional-radiative calculations using the Flexible Atomic Code agree well with our results. By calibrating all sensors individually and correcting for their temporal drifts we achieve an average uncertainty below 20km/s for their transition energies, easing the interpretation of astrophysical spectra. Furthermore, our charge-state-resolved results benchmark theory for complex open-shell ions in the intermediate coupling regime, for which we compare the results from pure configuration interaction and calculations including many-body perturbation theory (MBPT) and find a significant improvement when including MBPT.

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Phys. Rev. A 114, 052808– Published 9 November, 2026
  • Received 19 June 2026
  • Accepted 21 October 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Open access publication funded by Max Planck Society.

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