Hello Folks,
Just thought I'd spice up the talk and point out that although Thorium is only an alpha emitter, its "daughter" products are beta emitters, which require a bit more shielding than paper.
So, when 232 Thorium decays, releases an alpha, and becomes 228 Radium (HL 5.7 years), which throws off an electron (-beta particle) to 228 Actinium. That lasts just over 6 hours and throws another -beta particle to a lighter version Thorium--228 Thorium (HL 1.9 yr). Next, you loose another alpha and create 224 Radium in 3.5 days, which you create something a bit nasty, 220 Radon (HL 55 seconds) then another alpha particle and 216 Polonium (HL .14 sec). Losing another alpha, you come to 212 Lead (HL 10.6 hr). Throwing another -b particle, you come to 212 Bismuth (HL 60 min), where the decay takes two different paths. 64% of the time--it throws a -b particle and you get 212 Polonium (HL 3 * 10^-7 seconds) and immediately heads to 208 Lead. 36% Lose an a particle and head to Thallium for ~3 minutes and then throw off another -b particle becoming 208 Lead, which is stable. This liberates 42 MeV per completed decay chain for one atom of Thorium. This is happening constantly.
It's admittedly not a monster amount of MeV, however, it doesn't happen one at a time. The main problem is likely the discoloration of the Thorium doped lens element. I just don't like the idea that this wasn't revealed to us till later. I've seen stories of newer lenses still being doped with Thorium (something I'm planning on working on) well past the early 70's.
Take this info for what you will. Just remember Thorium is not just an alpha emitter.
