Positron emission or beta plus 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 an electron neutrino.
Positron emission is mediated by the weak force. The positron is a type of
beta particle, the other beta particle being the electron emitted from the β−
decay of a nucleus. An example of positron emission is shown
with magnesium-23 decaying into sodium-23:
23 12Mg → 23
11Na + e+ + ν 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.
Positron emission should not be confused with electron emission or beta minus
decay, which occurs when a neutron turns into a proton and the nucleus emits an
electron and an antineutrino. Electron capture is also occasionally
classified as a type of beta decay. In some ways, electron capture can be
regarded as an equivalent to positron emission, since capture of an electron
results in the same transmutation as emission of a positron. Electron capture
occurs 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. Electron capture always competes with positron emission where
the latter is seen, and in addition, occurs as the only type of beta decay in
proton-rich nuclei when there is not enough decay energy to support positron
emission. Discovery of positron emission
In 1934 Frédéric and Irène Joliot-Curie bombarded aluminium with alpha particles
to effect the nuclear reaction 4 2He + 27
13Al → 30 15P + 1
0n, and observed that the product isotope 30
15P emits a positron identical to those found in cosmic rays by Carl David
Anderson in 1932. This was the first example of β+ decay. The Curies termed
the phenomenon "artificial radioactivity," since 30
15P is a short-lived nuclide which does not exist in nature. The discovery of
artificial radioactivity would be cited when the husband and wife team won the
Nobel Prize. Positron-emitting isotopes
Isotopes which undergo this decay and thereby emit positrons include
carbon-11, potassium-40, nitrogen-13, oxygen-15, aluminium-26, sodium-22,
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: 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. Via the weak interaction, quarks can change flavor
from down to up, resulting in electron emission. 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 2mec2 = 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 2mec2, then positron emission cannot occur and
electron capture is the sole decay mode. Certain isotopes 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 2me, 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.
References External links
The LIVEChart of Nuclides - IAEA with filter on β+ decay