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 Mes·o·tho·ri·um n.  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.
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