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The Collaborative International Dictionary of English v.0.48 (gcide)

Mesothorium \Mes`o*tho"ri*um\, n. [NL.; meso- thorium.]
(Chem.)
a radioactive isotope of radium (radium-228) with a half-life
of 5.8 years. Also called {mesothorium-1} or {mesothorium I}
to distinguish it from a subsequent decay product,
mesothorium II (actinium-228). It was discovered in 1907 by
Otto Hahn as a decay product of thorium (produced by decay of
thorium-232). Mesothorium-1 (radium-228) in turn produces
actinium-228 (mesothorium-2) as the first product of its
radioactive decay, and the actinium-228 in turn decays
quickly (half-life of 6 hours) to thorium-228 (which is also
called {radiothorium}; the thorium-228 has a half-life of
1.91 years, shorter than that of the radium-228). It was
discovered and named before full recognition of the nature of
isotopes of the elements, and was distinguished from other
variants of radium by its half-life and mode of production
and decay. It was also cheaper to prepare than other
short-lived radium isotopes, and was thus sold commercially,
for use, e.g. in making watch dials readable in the dark by
painting the hands and hour marks with a self-luminous paint
containing the radioactive substance; it is therefore often
referred to (e.g. in regulatory legislation) as though
distinct from radium. It was one of the isotopes believed
responsible for radiation-induced diseases observed in
industrial workers who painted radium on watch dials in the
late 1950's and early 1960's. The primary isotope of radium
(radium-226) has a half-life of 1620 years, and these
isotopes with shorter half-lives proved difficult to isolate
and study for the purpose of finding the cause of such
diseases.
[Webster 1913 Suppl. PJC]

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