radiothorium /(rā`dĭ*ŏ*thôr"ē*ŭm)/

ra·di·o·tho·ri·um

radiothorium

n.
  1. 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. (Chem.)