When the atom ejects an electron, the hole left behind gets filled by an electron from a neighboring atom. Then the same thing happens to the neighbor -- and so on. This is electrical current flowing. Eventually the loop closes and some hole somewhere in the universe gets filled by the original ejected electron.
The hole in one atom can get filled by an electron from a higher orbital in a neighboring atom. In that case the energy gained will be greater than the energy lost by the original electron ejection. This is the situation where you get a photon (x-ray) with a higher energy (= shorter wavelength) than the original incident photon (ultraviolet).
Of course there's no free lunch. The way this happens is that N thorium atoms eject electrons from some orbital with energy X, the electrons shuffle around, and those N holes get filled by donors from orbitals whose total energy is N*X even though some of the donors are at higher levels and some are at lower levels.
If one could make the UV source highly efficient perhaps it could be used as a battery with extremely good energy density.
Yeah I've been thinking that if we had really tiny VLSI-integrated UV lasers (which we absolutely don't, not even close) that a bunch of these 229Th atoms embedded in a silicon chip would be a device with totally fascinating properties.
We can build waveguides in silicon wafer processes pretty easily but I'm not sure we can do that at UV wavelengths. You could imagine a single, big, off-chip laser whose beam can be steered by waveguides to illuminate any of a few billion 229Th deposits. These could act like the configuration memory bits of an FPGA. They would be "almost nonvolatile" -- you'd have to refresh them every hour or so, instead of several thousand times per second (dram) or never (sram). At such a low refresh rate the steering doesn't need to be particularly fast, and having to share one laser across all the deposits would not be a problem.
Unfortunately 229Th is mildly radioactive, but so are household smoke detectors so hopefully people wouldn't freak out about this.
> Almost like an exponentially-discharging solar-powered current source (for a very specific wavelength of "solar").
If one could make the UV source highly efficient perhaps it could be used as a battery with extremely good energy density.