Positron emission

Positron emission or beta plus decay (β⁺ decay) is a particular type of radioactive decay (and a subtype of beta decay), in which a proton inside a radionuclide nucleus is converted into a neutron while releasing a positron and a neutrino (specifically an electron neutrino, ν_e). It is mediated by the Weak Force.

Because positron emission decreases proton number relative to neutron number, positron decay happens typically in large "proton-rich" radionuclides. Positron decay results in nuclear transmutation, changing an atom of a chemical element into an atom of an element with an atomic number that is less by one unit.

Note: Electron emission or beta minus decay (β^- decay) occurs when a neutron emits an electron and an antineutrino. Electron capture (sometimes called inverse beta decay) is also occasionally classified as a type of beta decay. (In some ways, electron capture can be regarded as the "opposite" of positron emission. It can only occur when electrons are available and requires less energy difference between parent and daughter, so occurs much more often in smaller atoms than positron emission does.)

Positron-emitting isotopes

Isotopes which undergo this decay and thereby emit positrons include carbon-11, potassium-40, nitrogen-13, oxygen-15, fluorine-18, and iodine-121. As an example, the following equation describes the beta plus decay of carbon-11 to boron-11, emitting a positron and a neutrino:

→

+

e+

+

ν e

+

0.96 MeV

Emission mechanism

Inside protons and neutrons, there are fundamental particles called quarks. The two most common types of quarks are up quarks, which have a charge of +²/₃ and down quarks, with a −¹/₃ charge. Quarks arrange themselves in sets of three such that they make protons and neutrons. In a proton, whose charge is +1, there are two up quarks and one down quark. Neutrons, with no charge, have one up quark and two down quarks. Quarks are able to change from up quarks to down quarks. It is this that causes beta radiation. Positron emission happens when an up quark changes into a down quark.

Nuclei which decay by positron emission may also decay by electron capture. For low-energy decays, electron capture is energetically favored by 2m_ec² = 1.022 MeV, since the final state has an electron removed rather than a positron added. As the energy of the decay goes up, so does the branching ratio towards positron emission. However, if the energy difference is less than 2m_ec², then positron emission cannot occur and electron capture is the sole decay mode. Certain isotopes (for instance, ) are stable in galactic cosmic rays, because the electrons are stripped away and the decay energy is too small for positron emission.

Application

These isotopes are used in positron emission tomography, a technique used for medical imaging. Note that the energy emitted depends on the isotope that is decaying; the figure of 0.96 MeV applies only to the decay of carbon-11. Isotopes which increase in mass under the conversion of a proton to a neutron, or which decrease in mass by less than 2m_e, cannot spontaneously decay by positron emission.

The short-lived positron emitting isotopes ¹¹C, ¹³N, ¹⁵O and ¹⁸F used for positron emission tomography are typically produced by proton irradiation of natural or enriched targets.

External links

• The LIVEChart of Nuclides - IAEA with filter on β⁺ decay