[acidburnNSA]

Just quick response on the last point: Photofission reactions happen all the time in places like nuclear reactors. There are no materials that I'm aware of that photofission explosively. The photofission reactions are generally very rare compared to all the other reactions. Heck, natural uranium in your kitchen counter spontaneously fissions all the time but that doesn't lead to an explosion because 1 million atoms is a lot less than 1e23 atoms. Am I missing something?

[raverbashing]

Agree, go boom is a bit of an exaggeration, I meant that you probably don't want materials that undergo fission while absorbing your radiation