Archived posting to the Leica Users Group, 1996/04/04

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Subject: Radioactive danger in old optical glasses
From: (Charles E. Dunlap)
Date: Thu, 4 Apr 1996 13:43:41 -0800

I remember that someone mentioned Thorium as an additive to optical glass.
100% of natural Th is unstable. Although it also has a half life over a
billion years, all of it is decaying, greatly increasing the radioactivity
of a sample. Perhaps this was the scare back then.


>Charles E. Dunlap wrote:
>> La has two naturally occuring isotopes: La-139 is stable and has a relative
>> abundance in nature of 99.91% while La-138 has a half life of 105 billion
>> years (>20 times the age of the Earth) and an abundance of 0.09%. La in
>> optical glass or even in pure elemental form would not be considered
>> radioactive in the lay use of the term.
>Interesting. Then what I was reading must have been info on the fact
>that LA type rare earth glasses were NOT radioactive. There was some
>sort of glass in those days that was stirring up a controversy because
>of the radioactivity of the elements. Of course, back in the 50s (right
>up 'til today) there was a lot of hysteria with anything radioactive.
>Even Ansel Adams (latest victim of Bill Gate's ever expanding
>empire) left the Sierra Club over the issue of atomic energy.
>Eric Welch
>Grants Pass, OR