Positron emission
Encyclopedia
Positron emission or beta plus decay (β+
decay) is a type of beta decay
in which a proton
is converted, via the weak force, to a neutron
, releasing a positron
(the antimatter
counterpart of an electron
) and a neutrino
.
Isotope
s 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
:
|- style="height:2em;"
| ||→ || ||+ || ||+ || ||+ ||0.96 MeV
|}
Inside protons and neutrons, there are fundamental particles called quark
s. 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 proton
s and neutron
s. 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.
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 2me, do not spontaneously decay by positron emission.
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 (for instance) are stable in galactic cosmic rays, because the electrons are stripped away and the decay energy is too small for positron emission.
Beta particle
Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay...
decay) is a type of beta decay
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
in which a proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
is converted, via the weak force, to a neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
, releasing a positron
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...
(the antimatter
Antimatter
In particle physics, antimatter is the extension of the concept of the antiparticle to matter, where antimatter is composed of antiparticles in the same way that normal matter is composed of particles...
counterpart of an electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
) and a neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
.
Isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
s which undergo this decay and thereby emit positrons include carbon-11, potassium-40
Isotopes of potassium
Potassium has 25 known isotopes from 32K to 56K. Three isotopes occur naturally: stable 39K and 41K , and the long-lived radioisotope 40K . The standard atomic mass is 39.0983 u. Naturally occurring 40K decays to stable 40Ar by electron capture or positron emission...
, nitrogen-13
Nitrogen-13
Nitrogen-13 is a radioisotope of nitrogen used in positron emission tomography . It has a half life of a little under ten minutes, so it must be made at the PET site...
, oxygen-15
Isotopes of oxygen
There are three stable isotopes of oxygen that lead to oxygen having a standard atomic mass of 15.9994 u. 17 radioactive isotopes have also been characterized, with mass numbers from 12O to 28O, all short-lived, with the longest-lived being 15O with a half-life of 122.24 seconds...
, fluorine-18
Fluorine-18
Fluorine-18 is a fluorine radioisotope which is an important source of positrons. It has a mass of 18.0009380 u and its half-life is 109.771 minutes....
, and iodine-121
Isotopes of iodine
There are 37 known isotopes of iodine and only one, 127I, is stable. Iodine is thus a monoisotopic element.Its longest-lived radioactive isotope, 129I, has a half-life of 15.7 million years, which is far too short for it to exist as a primordial nuclide...
. As an example, the following equation describes the beta plus decay of carbon-11 to boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
-11, emitting a positron and a neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
:
- {| border="0"
|- style="height:2em;"
| ||→ || ||+ || ||+ || ||+ ||0.96 MeV
|}
Inside protons and neutrons, there are fundamental particles called quark
Quark
A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never directly...
s. 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 proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s and neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s. 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.
These isotopes are used in positron emission tomography
Positron emission tomography
Positron emission tomography is nuclear medicine imaging technique that produces a three-dimensional image or picture of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide , which is introduced into the body on a...
, 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 2me, do not spontaneously decay by positron emission.
Nuclei which decay by positron emission may also decay by electron capture
Electron capture
Electron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron and simultaneously emits a neutrino...
. 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
Branching ratio
In particle physics and nuclear physics, the branching fraction for a decay is the fraction of particles which decay by an individual decay mode with respect to the total number of particles which decay. It is equal to the ratio of the partial decay constant to the overall decay constant...
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 (for instance) are stable in galactic cosmic rays, because the electrons are stripped away and the decay energy is too small for positron emission.
External links
-
The LIVEChart of Nuclides - IAEA with filter on β+ decay, in Java or HTML