I really doubt it would be Thorium decay. Naturally occurring Thorium has a half life of 14 billion years. As far as I know, Thorium was never an additive to optical glass, although as a heavy metal I guess it could be used as a substitute for lead in "lead crystal" to some extent.
I'm a chemistry guy, but I never got around to solid state chemistry as far as how it interacts with crystallography. I'm sure someone on this forum knows what heavy metals are used in different brands of optical glass.