^{93m}Mo Isomer Depletion via Beam-Based Nuclear Excitation by Electron Capture

A recent nuclear physics experiment [C. J. Chiara et al., Nature (London) 554, 216 (2018)NATUAS0028-083610.1038/nature25483] reports the first direct observation of nuclear excitation by electron capture (NEEC) in the depletion of the ^{93m}Mo isomer. The experiment used a beam-based setup in which Mo highly charged ions with nuclei in the isomeric state ^{93m}Mo at 2.4 MeV excitation energy were slowed down in a solid-state target. In this process, nuclear excitation to a higher triggering level led to isomer depletion. The reported excitation probability P_{exc}=0.01 was solely attributed to the so-far unobserved process of NEEC in lack of a different known channel of comparable efficiency. In this work, we investigate theoretically the beam-based setup and calculate excitation rates via NEEC using state-of-the-art atomic structure and ion stopping-power models. For all scenarios, our results disagree with the experimental data by approximately 9 orders of magnitude. This stands in conflict with the conclusion that NEEC was the excitation mechanism behind the observed depletion rate.