Mesothorium

1 definition found

From 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]