Radioactive Decay and Half-Life

As mentioned previously, radioactive decay is the disintegration of an unstable atom with an accompanying emission of radiation. As a radioisotope atom decays to a more stable atom, it emits radiation only once. 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. For this reason, radioactive sources become weaker with time. As more and more unstable atoms become stable atoms, less radiation is produced and eventually the material will become non-radioactive.

The decay of radioactive elements occurs at a fixed rate. The half-life of a radioisotope is the time required for one half of the amount of unstable material to degrade into a more stable material. 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. 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. Although the half-life pattern is the same for every radioisotope, the length of a half-life is different. For example, Co-60 has a half-life of about 5 years while Ir-192 has a half-life of about 74 days.