radiothorium
now chiefly historical
noun
An isotope of thorium (thorium-228) which occurs in small amounts in natural thorium and is formed by the beta decay of actinium-228, and which itself decays by alpha emission with a half-life of 1.9 years; (also) a mixture of radioactive isotopes of thorium, actinium, and radium, present as decay products in natural thorium.
Origin
Early 20th century; earliest use found in Journal of the Chemical Society.