What is "mesothorium"

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.

n. (context physics English) Any product of the radioactive decay of thorium, mainly isotopes of radium and actinium