Positron emission

Positron emission or beta plus decay (β⁺ decay) is a particular type of radioactive decay, called beta decay, in which a proton is converted to a neutron, and it releases a positron and a neutrino.

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:

11 6C

→

11 5B

+

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, 7
Be) 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 by less than 2m_e, do not spontaneously decay by positron emission.

The short-lived positron emitting isotopes ¹¹C, ¹³N, ¹⁵O and ¹⁸F used for positron emission tomography are typicaly produced by proton irradiation of natural or enriched targets.^[1][2]

References

External links

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