Radon-220 is a radioisotope of radon. Radon-220 is commonly referred to as thoron, since it is a natural decay product of the most stable thorium isotope (thorium-232). Radon-220 is a member of thorium series.
All isotopes of radon are radioactive, but the two radon isotopes radon-222 and radon-220 are very important from radiation protection point of view.
Radon-222. The radon-222 isotope is a natural decay product of the most stable uranium isotope (uranium-238), thus it is a member of uranium series.
- Radon-220. The radon-220 isotope, commonly referred to as thoron, is a natural decay product of the most stable thorium isotope (thorium-232), thus it is a member of thorium series.
It is important to note that radon is a noble gas, whereas all its decay products are metals. The main mechanism for the entry of radon into the atmosphere is diffusion through the soil. As a gas, radon diffuses through rocks and the soil. When radon disintegrates, the daughter metallic isotopes are ions that will be attached to other molecules like water and to aerosol particles in the air. Therefore all discussions of radon concentrations in the environment refer to radon-222. While the average rate of production of radon-220 (thoron) is about the same as that of radon-222, the amount of radon-220 in the environment is much less than that of radon-222 because of significantly shorter half-life (it has less time to diffuse) of radon-222 (55 seconds, versus 3.8 days respectively). Simply radon-220 has lower chance to escape from bedrock.
See also: Radon – Health Effects