Positron emission, beta plus decay, or β⁺ decay is a subtype of radioactive decay called

beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...

, in which a proton inside a radionuclide nucleus is converted into a

neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...

while releasing a positron and an

electron neutrino The electron neutrino () is an elementary particle which has zero electric charge and a spin of . Together with the electron, it forms the first generation of leptons, hence the name electron neutrino. It was first hypothesized by Wolfgang Pauli ...

(). Positron emission is mediated by the

weak force Weak may refer to: Songs * Weak (AJR song), "Weak" (AJR song), 2016 * Weak (Melanie C song), "Weak" (Melanie C song), 2011 * Weak (SWV song), "Weak" (SWV song), 1993 * Weak (Skunk Anansie song), "Weak" (Skunk Anansie song), 1995 * "Weak", a song ...

. The positron is a type of beta particle (β⁺), the other beta particle being the electron (β⁻) emitted from the β⁻ decay of a nucleus. An example of positron emission (β⁺ decay) is shown with magnesium-23 decaying into sodium-23: : → + + 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 one chemical element into an atom of an element with an

atomic number The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of every ...

that is less by one unit. Positron emission occurs only very rarely naturally on earth, when induced by a

cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...

or from one in a hundred thousand decays of

potassium-40 Potassium-40 (40K) is a radioactive isotope of potassium which has a long half-life of 1.25 billion years. It makes up about 0.012% (120 ppm) of the total amount of potassium found in nature. Potassium-40 undergoes three types of radioactive d ...

, a rare isotope, 0.012% of that element on earth. Positron emission should not be confused with

electron emission In physics, electron emission is the ejection of an electron from the surface of matter, or, in beta decay (β− decay), where a beta particle (a fast energetic electron or positron) is emitted from an atomic nucleus transforming the original nu ...

or beta minus decay (β⁻ decay), which occurs when a neutron turns into a proton and the nucleus emits an electron and an antineutrino. Positron emission is different from

proton decay In particle physics, proton decay is a hypothetical form of particle decay in which the proton decays into lighter subatomic particles, such as a neutral pion and a positron. The proton decay hypothesis was first formulated by Andrei Sakharov ...

, the hypothetical decay of protons, not necessarily those bound with neutrons, not necessarily through the emission of a positron, and not as part of nuclear physics, but rather of

particle physics Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) an ...

Discovery of positron emission

In 1934 Frédéric and

Irène Joliot-Curie Irène Joliot-Curie (; ; 12 September 1897 – 17 March 1956) was a French chemist, physicist and politician, the elder daughter of Pierre and Marie Curie, and the wife of Frédéric Joliot-Curie. Jointly with her husband, Joliot-Curie was awar ...

bombarded aluminium with

alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be pr ...

s (emitted by

polonium Polonium is a chemical element with the symbol Po and atomic number 84. Polonium is a chalcogen. A rare and highly radioactive metal with no stable isotopes, polonium is chemically similar to selenium and tellurium, though its metallic character ...

) to effect the nuclear reaction + → + , and observed that the product isotope emits a positron identical to those found in cosmic rays by

Carl David Anderson Carl David Anderson (September 3, 1905 – January 11, 1991) was an American physicist. He is best known for his discovery of the positron in 1932, an achievement for which he received the 1936 Nobel Prize in Physics, and of the muon in 1936. B ...

in 1932. This was the first example of decay (positron emission). The Curies termed the phenomenon "artificial radioactivity", because 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

Isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numb ...

s which undergo this decay and thereby emit positrons include

carbon-11 Carbon (6C) has 15 known isotopes, from to , of which and are stable. The longest-lived radioisotope is , with a half-life of years. This is also the only carbon radioisotope found in nature—trace quantities are formed cosmogenically by ...

nitrogen-13 Nitrogen-13 (13N) is a radioisotope of nitrogen used in positron emission tomography (PET). It has a half-life of a little under ten minutes, so it must be made at the PET site. A cyclotron may be used for this purpose. Nitrogen-13 is used to tag ...

, oxygen-15,

fluorine-18 Fluorine-18 (18F) is a fluorine radioisotope which is an important source of positrons. It has a mass of 18.0009380(6) u and its half-life is 109.771(20) minutes. It decays by positron emission 96% of the time and electron capture 4% of the time ...

copper-64 Copper-64 (64Cu) is a positron and beta emitting isotope of copper, with applications for molecular radiotherapy and positron emission tomography. Its unusually long half-life (12.7-hours) for a positron-emitting isotope makes it increasingly us ...

, gallium-68, bromine-78,

rubidium-82 Rubidium-82 (82Rb) is a radioactive isotope of rubidium. 82Rb is widely used in myocardial perfusion imaging. This isotope undergoes rapid uptake by myocardiocytes, which makes it a valuable tool for identifying myocardial ischemia in Positron E ...

, yttrium-86, zirconium-89,

sodium-22 There are 22 isotopes of sodium (11Na), ranging from to , and two isomers ( and ). is the only stable (and the only primordial) isotope. It is considered a monoisotopic element and it has a standard atomic weight of . Sodium has two radioact ...

aluminium-26 Aluminium-26 (26Al, Al-26) is a radioactive isotope of the chemical element aluminium, decaying by either positron emission or electron capture to stable magnesium-26. The half-life of 26Al is 7.17 (717,000) years. This is far too short for the i ...

, strontium-83, and

iodine-124 There are 37 known isotopes of iodine (53I) from 108I to 144I; all undergo radioactive decay except 127I, which is stable. Iodine is thus a monoisotopic element. Its longest-lived radioactive isotope, 129I, has a half-life of 15.7 million year ...

. As an example, the following equation describes the beta plus decay of carbon-11 to boron-11, emitting a positron and a

neutrino A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...

: :

Emission mechanism

Inside protons and neutrons, there are

fundamental particle In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be elementary include electrons, the fundamental fermions (quarks, leptons, antiqu ...

s called quarks. The two most common types of quarks are ''

up quark The up quark or u quark (symbol: u) is the lightest of all quarks, a type of elementary particle, and a significant constituent of matter. It, along with the down quark, forms the neutrons (one up quark, two down quarks) and protons (two up quark ...

s'', which have a charge of +, and ''

down quark The down quark or d quark (symbol: d) is the second-lightest of all quarks, a type of elementary particle, and a major constituent of matter. Together with the up quark, it forms the neutrons (one up quark, two down quarks) and protons (two up ...

s'', with a − charge. Quarks arrange themselves in sets of three such that they make protons and

s. In a proton, whose charge is +1, there are two ''up'' quarks and one ''down'' quark ( + − = 1). Neutrons, with no charge, have one ''up'' quark and two ''down'' quarks ( − − = 0). Via the

weak interaction In nuclear physics and particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interaction ...

, quarks can change flavor from ''down'' to ''up'', resulting in

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...

emission. Positron emission happens when an ''up'' quark changes into a ''down'' quark, effectively converting a proton to a neutron. Nuclei which decay by positron emission may also decay by

electron capture Electron capture (K-electron capture, also K-capture, or L-electron capture, L-capture) is a process in which the proton-rich nucleus of an electrically neutral atom absorbs an inner atomic electron, usually from the K or L electron shells. Thi ...

. For low-energy decays, electron capture is energetically favored by 2''m''_e''c''² = , 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 fraction In particle physics and nuclear physics, the branching fraction (or branching ratio) for a decay is the fraction of particles which decay by an individual decay mode or with respect to the total number of particles which decay. It applies to eithe ...

of positron emission. However, if the energy difference is less than 2''m''_e''c''², then positron emission cannot occur and electron capture is the sole decay mode. Certain otherwise electron-capturing isotopes (for instance, ) are stable in

galactic cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...

s, because the electrons are stripped away and the decay energy is too small for positron emission.

Energy conservation

A positron is ejected from the parent nucleus, and the daughter (Z−1) atom must shed an orbital electron to balance charge. The overall result is that the mass of two electrons is ejected from the atom (one for the positron and one for the electron), and the β⁺ decay is energetically possible

if and only if In logic and related fields such as mathematics and philosophy, "if and only if" (shortened as "iff") is a biconditional logical connective between statements, where either both statements are true or both are false. The connective is b ...

the mass of the parent atom exceeds the mass of the daughter atom by at least two electron masses (2''m''_e; 1.022 MeV). Isotopes which increase in mass under the conversion of a proton to a neutron, or which decrease in mass by less than 2''m''_e, cannot spontaneously decay by positron emission.

Application

These isotopes are used in positron emission tomography, a technique used for medical imaging. The energy emitted depends on the isotope that is decaying; the figure of applies only to the decay of

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

References

External links

* '
Live Chart of Nuclides: nuclear structure and decay data
'' (main decay modes) - IAEA {{Authority control Radioactivity Electron Antimatter