radon-220

Radon-220 (thoron; 51.5-second half-life) was first observed in 1899 by American scientist Robert B. Owens and British scientist Ernest Rutherford, who noticed that some of the radioactivity of thorium compounds could be blown away by breezes in the laboratory. Radon-219 (actinon; 3.92-second half-life), which is

…other two natural isotopes, called thoron and actinon, because they originate in the thorium and the actinium series, respectively.