from
The Collaborative International Dictionary of English v.0.48
radiothorium \ra`di*o*tho"ri*um\
(r[=a]`d[i^]*[o^]*th[^o]r"[=e]*[u^]m), n. (Chem.)
an earlier name for the thorium isotope {thorium-228}, given
by its discoverer Otto Hahn. It is a radioactive substance
formed as one of series of products in the chain of
radioactive decay of thorium. Its immediate predecessor in
the chain is Actinium-228, and it decays by alpha emission to
radium-224 with a half-life of 1.91 years. The name
radiothorium was given prior to the full understanding of the
nature of isotopes of elements.
[PJC]