From AMS Glossary
(Symbol Tn.) A radioactive gas, atomic number 86, atomic weight 220; an inert gaseous element with radioactive decay, accompanied by emission of alpha particles, responsible for a portion of the ionization observed in the lower atmosphere.
It is a member of the thorium family of radioactive elements and is the immediate descendant of the radium isotope of atomic number 88 and atomic weight 224 (called thorium X at one time), which decays by alpha emission to thoron in a half-life of 3.6 days. Thoron, in turn, decays by alpha emission in a half-life of only 54.5 seconds, yielding an isotope of polonium. Thoron, like each of the other two radioactive gases (radon and actinon) that are its isotopes, enters the atmosphere by the process of exhalation after its formation by radioactive disintegration within soil or rocks. The very short half-life of thoron allows it little time to be carried from the earth's surface to higher levels, so its contribution to atmospheric ionization is made largely in the lowest few meters of the atmosphere.