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 ]

n. (Chem.) an earlier name for the thorium isotope thorium-228, given by its discoverer Otto Hahn. It is a radioactive substance formed as one of series of products in the chain of radioactive decay of thorium. Its immediate predecessor in the chain is Actinium-228, and it decays by alpha emission to radium-224 with a half-life of 1.91 years. The name radiothorium was given prior to the full understanding of the nature of isotopes of elements. [ PJC ]

n. [ NL. See Thorite. ] (Chem.) A metallic element found in certain rare minerals, as thorite, pyrochlore, monazite, etc., and isolated as an infusible gray metallic powder which burns in the air and forms thoria; -- formerly called also thorinum. Symbol Th. Atomic weight 232.0. [ 1913 Webster ]