At two half-lives, it will have an intensity of 25% of a new source.
After ten half-lives, less than one-thousandth of the original activity will remain.
Thorium's radioactivity was widely acknowledged during the first decades of the 20th century.
In the second half of the century, thorium was replaced in many uses due to concerns about its radioactivity.
As more and more unstable atoms become stable atoms, less radiation is produced and eventually the material will become non-radioactive.For example, a source will have an intensity of 100% when new.At one half-life, its intensity will be cut to 50% of the original intensity.To change from an unstable atom to a completely stable atom may require several disintegration steps and radiation will be given off at each step.However, once the atom reaches a stable configuration, no more radiation is given off.