Other types of radioactive decay
Alpha, beta and gamma radiation are the most common types of radioactive decay but there are other ways that unstable atoms can become stable.
Fission is the process of splitting a large nucleus to form two smaller nuclei. This process also produces energy and neutrons.
Fissile materials are those which undergo fission when bombarded with slow-moving neutrons, this can then sustain the chain-reaction which is used in nuclear reactors. Common fissile materials used in nuclear fuel are Uranium-233, Uranium-235, Plutonium-239 and Plutonium-241.
When slow-moving neutrons collide with one of these nuclei, it is temporarily absorbed and changed to an extremely unstable short-lived isotope, which then splits into two fission fragments (see Figure 1). The fission fragments are not always the same; one fragment will have a mass of approximately 140, while the other fragment has a mass of about 90.
Other materials may also fission (known as fissionable material), such as the most abundant Uranium isotope, Urainum-238. However Uranium-238 requires fast neutrons for fission, the fission of Uranium-238 then in turn does not produce fast neutrons to sustain a nuclear chain reaction.
Neutrons can generally be classed as slow or fast neutrons. Slow neutrons are those with low energy and travel at slower speed, where as fast neutrons have higher energy and travel faster. Slow neutrons are used in conventional nuclear reactors as they can initiate fission in fissile material to create the nuclear chain-reaction.
Different isotopes of the same element vary only in the number of neutrons in their nucleii. This variation determines the stability of the nucleii of the isotopes; whether or not they are radioactive.
An isolated neutron is unstable and decays by emitting an electron and becoming a proton with a half-life of 13 minutes. If there are too many neutrons in the nucleus, the nucleus becomes unstable and undergoes radioactive decay by emitting a beta-particle.
Neutrons are sometimes emitted when nuclei undergo fission, or split into two main parts. Individual neutrons may interact with a nucleus by being captured, possibly making that nucleus radioactive, or by causing the nucleus to split (nuclear fission). This occurs repeatedly in a nuclear chain reaction in a nuclear reactor or nuclear (fission) weapon. Combining alpha-emitting isotopes with beryllium produces a neutron source. Accelerators are another means of producing neutron radiation. In the upper atmosphere, the interaction of cosmic radiation with air also produces neutron radiation.
Positron or beta plus (β+) emission
The anti-matter equivalent of an electron, having exactly the same mass but an equal but opposite electrical charge. Positrons are emitted from some unstable isotopes that have too few neutrons to be stable. Positrons are sometimes called beta 'plus' rays to distinguish them from more common beta-'minus' particles (electrons).
Positrons can also be produced, along with a matching electron, when gamma rays of more than 1 MeV interact with matter in a process called pair-production.
Radioisotopes that emit positrons are useful in a nuclear medicine imaging procedure called PET. A positron and electron will mutually annihilate each other if they come into close proximity with their mass disappearing and being converted into energy in the form of two gamma rays, emitted back-to-back, in opposite directions. These gamma rays are called annihilation radiation. In PET the two annihilation gamma rays are detected simultaneously allowing the position of the annihilation to be determined.
Positron emission is equivalent to the capture of an electron in electron capture. In both cases a proton is transformed into a neutron. A significant difference is that positron emission requires more energy than electron capture.
Electron capture will occur when there are too many protons in the nucleus, and there isn't enough energy to emit a positron.
In this case, one of the orbital electrons is captured by a proton in the nucleus, forming a neutron and a neutrino. Since the proton is essentially changed to a neutron, the number of neutrons increases by 1, the number of protons decreases by 1, and the atomic mass remains unchanged. By changing the number or protons, electron capture transforms the nuclide into a new element.