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Positron emission is a byproduct of a type of radioactive decay known as beta plus decay. In the process of beta plus decay, an unstable balance of neutrons and protons in the nucleus of an atom triggers the conversion of an excess proton into a neutron. During the conversion process, several additional particles, including a positron, are emitted. The positron is a special type of particle known as a beta particle because it is a byproduct of beta decay.
This process of beta plus decay occurs at random all the time in elements with the potential to experience this type of radioactive decay and the energy to transform a proton into a heavier neutron. In addition to producing a neutron, beta plus decay results in the production of a neutrino and a positron. The positron is the antimatter counterpart to the electron, which means that when positrons and electrons collide, they annihilate, generating gamma rays. This property is important for researchers who harness positron emission in their work.
Radioactive decay causes the properties of an atom to change, because the balance of protons and neutrons in the nucleus shifts. This process explains why an element can exist in multiple forms known as isotopes, with each isotope having a different balance of protons and neutrons. Many isotopes are unstable, experiencing rapid decay and emitting radioactive particles in the process. This process also explains the uneven distribution of elements on Earth, as unstable elements decay into more stable forms over time, leading to a higher concentration of stable elements.
The medical community utilizes positron emission for a type of medical imaging study known as positron emission tomography (PET). In this study, isotopes known to produce positron emissions are introduced to the body and followed as they move through the body and produce gamma rays. Isotopes with short half lives which will not cause damage to the body are selected so that the PET scan will not be dangerous, and the imaging study may be combined with other imaging techniques such as magnetic resonance imaging to get a complete picture of what is going on inside a patient's body.
PET scans allow doctors to image functions of the body, perhaps most notably in the brain. The scan is not invasive, providing an appealing alternative to surgery to see the inside of the body, and it can provide a great deal of useful information. Such scans are used in medical diagnosis and in medical research, with positron emission tomography scans of the brain being especially popular for researchers in the field of neurology who are interested in the functions of the brain.