Bleaching radiation damage

OK, pointless question time. Some older lenses with radioactive elements have a yellow to brownish cast, apparently from radiation damage creating color centers. OK, sounds reasonable. Supposedly you can bleach the color cast out by exposure to UV. OK, there's evidence for this. My question is: how? How is UV going to fix a color center? If it does anything shouldn't it cause greater damage?

I'm glad you asked. I did my Ph.D. on radiation-induced formation of colour centres (seriously). But I've forgotten most of it. A colour centre forms when radiation above the threshold energy displaces an ion from the lattice, and this vacancy can trap an electron. By exposing to UV, this electron can be removed and the displaced ion could return. But I think you are right, as the light (UV) would have to be the correct wavelength.

The light only needs to have enough energy to "lift the electron up a bit" so that it can "fall back in the right position".

UV generally doesn't have enough energy to damage crystal lattices - you need X-rays for that. But it does have enough energy to "repair" them, or at least enough of them to make a big difference.

There's a thing called "desert amethyst" which is somewhat related; it's brown glass that's turned pale purple through long exposure to UV light. But in that case the colour centers are formed by iron atoms in glass, not radiation-induced damage. The radiation colour is less stable, so it won't take a century in the desert to effect a change.

Always worth remembering that, although it may take a particular wavelength of UV to kick electrons back into place and let ions resettle, sunlight is continuum radiation and as such has *all* UV wavelengths represented, down to whatever value is blocked by the atmosphere. And of course the sun is the most powerful UV source that's convenient for most of us...

Thanks folks! Somehow I was thinking the anion would eventually diffuse too far away to recover ...

Well, it probably will, over a million year time scale -- but it's not going to go far, or fast, in solid glass unless something pretty strong is driving it (like the impact of a fast neutron or fission fragment); with lenses only 50 years old, where the index of the thorium glass hasn't changed enough to bollix up the prescription, the anions won't have moved enough to be a big problem. Wherever the anion lands after the initial event that forms it, is pretty much where it will stay. And since there are electrons throughout the glass, it's pretty easy to redistribute those a bit to match the anion up with *some* electron and then find another one elsewhere -- and so forth, net result being you reduce the overall concentration of color centers and the glass gets clearer again.