Problems with radioactive dating methods
Unfortunately, the rare earths include some chemically very similar elements as contaminants, many of which are mildly radioactive.
The chemistry of lanthanum and its associated rare earths is so closely related that it is readily possible to have radioactive contaminants end up in the desired lanthanum salts used in making optical glasses.
The amount of such contaminants could also easily vary from batch to batch, depending on the degree of contamination in the original monazite or other mineral sources being used.
I don't think that the original levels of thorium or lanthanum specified for use in these lenses [e.g., in patent filings] is the cause of their radioactivity.
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But if you have a lens using early specialty glasses (wide angles, fast lenses..) from before the 1970s, you should consider checking your lens for radioactivity.
Eventually a more highly radioactive equilibrium will be reached, as in the original radioactive ores.So it isn't the thorium or the lanthanum that causes the problem here.The radioactivity of these early lenses is caused by contaminants in the ingredients (e.g., thorium salts) used to make the early lenses.So over the years, your "hot" lenses are likely to get more radioactive rather than less. Lanthanum has two isotopes in its natural form, one of which is weakly radioactive.
Another source of rare earths such as lanthanum is cerite, which contains the element cerium.
These slightly radioactive elements such as thorium or rare earth elements such as lanthanum were used to produce desired highly refractive glasses.