The most stringent laboratory-based experimental limits on the existence of submegaelectronvolt sterile neutrinos are currently set by decay spectroscopy of radioactive ${}^7\mathrm{Be}$ embedded into superconducting sensors. The systematic uncertainties are dominated by the modeling of the electron shakeup and shakeoff spectra that are not based on state-of-the-art atomic theory and do not include electron correlations or relativistic effects. We have used the multiconfiguration Dirac-Fock formalism to obtain correlated wave functions ab initio and compute all single and double shake processes in the electron capture decay of atomic ${}^7\mathrm{Be}$. The simulations can explain some but not all of the observed spectral features, likely because the wave functions are modified by the Ta sensor material that the ${}^7\mathrm{Be}$ is embedded into. The new models also show that the $L/K$ electron capture ratio of ${}^7\mathrm{Be}$ in Ta has previously been slightly underestimated revising the previous value of 0.070(7) to a new value of 0.0756(20).

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• Atomic & molecular structure
• Autoionization & Auger processes
• Collective effects in atomic physics
• Electron & muon capture
• Electron correlation calculations for atoms & ions
• Electronic excitation & ionization
• Electronic transitions
• Extensions of fermion sector
• Fine & hyperfine structure
• Particle dark matter
• Relativistic & quantum electrodynamic effects in atoms, molecules,& ions
• Sterile neutrinos
• 6 ≤ A ≤ 19