Nonresonance Shake Mechanism in Neutrinoless Double Electron Capture

It is generally accepted that neutrinoless double electron capture is a resonance process. The probability for the shake followed by electron-shell ionization occurring in the course of transformation of $${}^{152}$$ Gd nuclei is calculated. Among all known nuclei, this nuclide is characterized by the lowest resonance defect and is therefore thought to be one of the main candidates for use in performing searches for the neutrinoless decay mode. As a result, the contribution of the new mechanism turns out to be smaller than that of the traditional resonance mechanism, thereby yielding a correction to the decay probability. Nevertheless, this correction is quite large, amounting to 23 $$\%$$ in the case of $${}^{152}$$ Gd. However, the correction in question grows fast with increasing resonance defect, so that, for other nuclei, we expect it to exceed the resonance-mechanism contribution and to become a dominant mechanism of neutrinoless double electron capture. It therefore appears that by no means does neutrinoless double electron capture proceed as a resonance process.