In 1976, physicists predicted that an electron could do more than just fly past a nucleus: it could slam into it, transferring energy and making the nucleus 'shudder.' They called this effect NEEC — nuclear excitation by electron capture. But for nearly fifty years, no one has managed to observe it in an experiment. The signal is too weak, drowned out by other electrons that simply scatter off the nucleus without being captured.
The solution came from an unexpected direction. When a relativistic electron beam (a stream of electrons accelerated to near-light speeds) strikes a solid target, reverse currents spontaneously run along its surface, like ripples spreading from a stone thrown into water. These currents create two necessary conditions at once: they knock electrons out of atoms, freeing up spots for capture, and they supply resonant electrons with just the right energy. Here, nature acts like a tuner who not only plucks the string but also dampens unwanted overtones: the Pauli principle prevents electrons from scattering off the nucleus if all lower states are occupied.
The authors calculated a concrete scenario: an electron beam with an energy of 4.5 mega-electron-volts and a current of 10 kiloamperes strikes solid thorium. In a single pulse, about 240,000 NEEC events are born, and 97.68% of all nuclear excitations would be due to capture, not parasitic scattering. By tuning the beam parameters, the yield can be controlled over a wide range while maintaining signal purity.
Thus, reverse currents turn from a nuisance into a tool. Like a mountain echo that doesn't just repeat your shout but returns it amplified and cleansed of noise.
The thorium-229 nuclear transition at 8.36 eV is so low that it can be excited by an ultraviolet laser: it's the only known nucleus that 'sees' ordinary light rather than gamma radiation.