Decay
An unstable (radioactive) nucleus can become stable by emitting particles and energy - a process called "decay". These particles or energy (in the form of electromagnetic waves) are collectively called radiation. The radiation emitted can either be alpha particles, beta particles, gamma rays or neutrons.
During the decay process of a radioactive material, the total number of that nuclei decreases with time. The time it takes for half of its original amount to decay is called the half-life. Each radionuclide has a characteristic half-life. The half-lives of radionuclides may vary from millionths of a second to millions of years.
| Radionuclide | Half-life |
|---|---|
| Radon-219 | 4 seconds |
| Potassium-38 | 7.6 minutes |
| Selenium-73 | 7.2 hours |
| Iodine-131 | 8 days |
| Cobalt-60 | 5.26 years |
| Caesium-137 | 30 years |
| Carbon-14 | 5,730 years |
| Iodine-129 | 15,700,000 years |
| Uranium-235 | 703,800,000 years |
| Potassium-40 | 1,277,000,000 years |
After one half-life, the level of radioactivity of a radioactive material is halved, after two half-lives, it is reduced to one quarter and so on.