mesothorium

Etymology
. Probably so-called because these two isotopes ($228$Ra and $228$Ac) are, in the decay chain, intermediate between normal thorium ($232$Th) and radiothorium ($228$Th).

Noun

 * 1)  Certain members of the decay chain of thorium, mainly isotopes of radium and actinium

Usage notes
Mesothorium I is radium-228. Mesothorium II is actinium-228.

Translations

 * Italian: mesotorio