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The resulting "daughter" products of that radioactive decay series also produce both alpha and beta particles.
Related rare earth such as lanthanum are often produced from the same sources, with monazite being up to 25% lanthanum.
Capitalizing on this extraordinary success, the watch makers soon captured other markets as well, most notably the emerging ladies watch market share.
Always one step ahead of the competition and eerily in tune with the needs and wants of the consumer, Rotary today features ladies, gents and also unisex watches for any taste.
This decay process means these thoriated glass lenses can gradually become more radioactive over time, as the more highly radioactive decay products build up in the glass. You would expect the radioactivity to decrease over time.
But after chemically purifying the thorium from its ore sources, the thorium is relatively free of these daughter products.
These slightly radioactive elements such as thorium or rare earth elements such as lanthanum were used to produce desired highly refractive glasses.
Another source of rare earths such as lanthanum is cerite, which contains the element cerium.Cheap glasses may have iron salts in them, often yielding the familiar green color of some iron salt contaminated glasses.Cerium is often added to glass to convert iron impurities into colorless compounds, yielding clear glass suitable for optical uses.Notes: The site still exists, but many of the pages seem to be no longer maintained and available and I have not been able to reach the original site developer, Robert Monaghan, who collected and edited the following information from use net postings.
This was restored from a copy I made in November 2003; Bob may have added to or edited this page after that. Industar 61 lens (lanthanum glass) Lens Faults Pages Photo. Maybe some of those bargains you got on EBAY are "hot" in a different sense than you might have thought!Net on Radioactive Lenses (gamma levels..) Radioactive Fed-5s (slight..) Radioactive Optics Posting (WJM) Radioactive Pentax Lens (GM tube sound file..) Romancing the Rare Earths On Enhancing Filters by Howard Ross U. Govt Surplus "Hot" Cameras and Lenses You will find two kinds of radioactive lenses - those that were contaminated and sold as government surplus, and those that are naturally "hot" or radioactive due to the radioactive elements or radioactive impurities used in their manufacture. I am assuming that heavily contaminated and radioactive government surplus items end up in government low level radioactive waste land-fills or other burial sites.