mesothorium
/ˌmɛsə(ʊ)ˈθɔːrɪəm/Chemistry
noun
Either of two radioactive nuclides in the thorium series: mesothorium I (symbol MsThI), the isotope of radium with mass number 228, produced by the alpha decay of thorium 232, or mesothorium II (symbol MsThII), the isotope of actinium with mass number 228, produced by the beta decay of mesothorium I.
Origin
Early 20th century; earliest use found in Journal of the Chemical Society. From scientific Latin Mesothorium from Meso- + -thorium.