ARPANSA - Australian Radiation Protection and Nuclear Safety Agency

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

Fission is the process of splitting a large nucleus to form two smaller, more stable nuclei.

Uranium-235 is the only naturally occuring isotope that will spontaneously undergo fission. It is more common for fission to be initiated when nuclei are bombarded by slow-moving neutrons. Three isotopes are fissionable under these circumstances - uranium-235, uranium-233 and plutonium-239.

When slow moving neutrons collide with one of these nuclei, it is temporarily absorbed, to for an extremely unstable short-lived isotope which 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.

The fission of one atom releases an amount of energy around 200MeV. This is about 20 millions times the energy released by burning one atom of carbon in a coal burning power plant.

Neutrons

Neutrons are one of the basic buidling blocks of the nucleus. They are neutral, having no electrical charge and have a mass similar to the combined mass of one proton plus one electron.

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

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.

 

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