Predominant Nuclear Excitation by Electron Capture Driven by Beam-Induced Return Currents
Predicted nearly five decades ago, nuclear excitation by electron capture (NEEC) remains experimentally elusive because its weak resonant signal is obscured by competing electron-driven excitation channels. Here we show that beam-induced surface return currents naturally overcome this limitation by creating a self-organized resonant electron source for the 8.36-eV nuclear transition in solid-density 229Th. A relativistic electron beam drives localized return currents along the solid surface, which simultaneously generate capture vacancies through impact ionization and provide resonant electrons for NEEC via a drifted Fermi distribution. Their spatial separation from the driving beam and the Pauli exclusion principle strongly suppress competing nuclear excitation by inelastic electron scattering. For experimentally available parameters, we predict 2.40*10^5 NEEC events with a NEEC fraction of 97.68%, and show that the excitation yield can be tuned over several orders of magnitude while preserving NEEC dominance. These results establish beam-induced return currents as a controllable route to resonant nuclear excitation in solids and open a new avenue for studying electron-driven nuclear processes.