Natural radioactivity refers to the natural products of radioactive decay. There are three natural series of radioactive decay.
- Uranium series (starts with 238U)
- Actinium series (starts with 235U)
- Thorium series (starts with 232Th)
Each of these series gives risk to a number of daughter products, most of which are also radioactive. The final product is a stable isotope of lead for all series.
These radioactive isotopes and their daughter products are able to be accumulated (enriched) from other isotopes of the same element through a variety of methods (known as isotope separation)
- Diffusion (allowing smaller isotopes to pass through a membrane)
- Centrifusion (heavier isotopes will tend to accumulate on the outer edge of a spinning device)
- Electromagnetic (deflection by a magnetic field, with heavier isotopes suffering less deflection)
- Laser (ionises atoms with a particular isotope only)
- Exploiting different reaction rates
The most commonly used naturally radioactive isotope in medicine was radium. This is a member of the uranium series. It is found in minute concentrations in uranium ore, from which it can be separated by chemical means.
Artificial radionuclides are produced by bombarding particles (sometimes already radioactive) with other particles, leading to creation of new products.
- Bombardment with alpha particles from a radioactive source (eg. radium) can lead to formation of new radioactive elements.
- Bombardment with protons
- Bombardment with deuterons
- Bombardment with neutrons. This is often the easiest method of creating an artificial radionuclide as the neutron has no charge and can penetrate the Coulomb field surrounding the nucleus (unlike protons or other ions).
- Photodisintegration occurs when high energy photons (over 10 MeV) interact with a nucleus, releasing a neutron or other particle. This interaction is far less common than other photon interactions, even at energies over 10 MeV.
- Fission occurs when a heavy element is bombarded with neutrons, causing its nucleus to break apart into a number of fission products. Fission products may be used in brachytherapy, eg: 90Sr, 131I.
- Fusion involves the combination of small nuclei into a larger nuclei. This requires incredible energy to occur and is not used to produce radionuclides.