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Law Dictionary Home Dictionary Definition mesothorium

a radioactive isotope of radium radium 228 with a half life of 58 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 191 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 eg 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 eg 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 1950s and early 1960s 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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