either of the two radioactive elements which are decay products of thorium. Mesothorium I is now called radium-228. Mesothorium II is now called actinium-228
mesothorium in American English
(ˌmɛsoʊˈθɔriəm; ˌmɛzoʊˈθɔriəm)
noun
1.
a radioactive isotope of radium (mesothorium 1), formed from thorium
2.
a radioactive isotope of actinium (mesothorium 2), formed from this isotope of radium