mesothorium
nounEtymology From meso- + thorium. Probably so-called because these two isotopes (²²⁸Ra and ²²⁸Ac) are, in the decay chain, intermediate between normal thorium (²³²Th) and radiothorium (²²⁸Th).
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Natural sciences, Physical sciences, Physics Certain members of the decay chain of thorium, mainly isotopes of radium and actinium