Lanthanum (was 90 Elmarit 2.8)

[cdunlap@rupture.ucsc.edu (Charles E. Dunlap)]

Lanthanum (La) has an atomic number of 57 and an atomic weight of 138.9055.
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.

Lanthanum is the first of the series of Rare Earth Elements that comprise
atomic numbers 57 through 71. When lens makers speak of Rare Earth glass
they are speaking of glass that has been made with the addition of elements
from this group. None of the Rare Earth elements contains any naturally
occuring isotope that is appreciably radioactive. Any that do decay have
half lives on the order of >100 billion years. These are not nearly as
radioactive as the average piece of granite.

-Charlie


>Nelson Wu wrote:
>
>> Anybody knows about the special glass type called "Lanthenam". I was told
>> that the glass was used in the 50 summicron Dual Range and 90 Elmarit 2.8.
>
>I do believe Lantenam was a big deal back in the 50s. Either it was, or
>was not radioactive, I believe was one of the things that made it stand
>out. I think not radioactive, which some optical glass was in those
>days. Hardly big news today, such as the high refractive index, low (and
>anomolus) dispersion characteristics that make some high end glass
>special now.
>
>--
>Eric Welch
>Grants Pass, OR