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Old December 1, 2012, 08:25 PM   #14
30Cal
Senior Member
 
Join Date: December 3, 2002
Posts: 1,093
It's pretty well documented. They were using almost 30% by weight. Plus the purified thorium mixed into the glass goes through a long decay chain; most of the daughters have relatively short half-lives. So it actually gets worse with time.

http://www.orau.org/ptp/collection/c...cameralens.htm
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