http://en.wikipedia.org/wiki/Decay_chain
Or, if you like pictures:
http://www.chemsoc.org/networks/lear...ve_decay_1.htm
Various thorium isotopes crop up in the decay chains of other radioactive elements (you can see them in the Radium and Actinium chains at Wikipedia), but the thorium in lenses is the long-lived, and hence naturally ocurring isotope Th-232.
I have a 7" Aero Ektar. I would happily use it, if I had a need. I think I once worked out that when focussed at infinity it would irradiate me at the (conservative) level allowed for occupational radiation workers here in Sweden. It is actually more of a hazard when packed away in a rucksack against my back. That said, I keep it in a far corner of the basement, and don't let my kids play with it, despite it being an excellent fire-starter.
One thing that would be interesting would be to see if the optical performance changed on bleaching the colour centres. It should do, but whether the amount is measureable I don't know.
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