Beta particle

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Alpha radiation consists of helium nuclei and is readily stopped by a sheet of paper. Beta radiation, consisting of electrons or positrons, is stopped by thin aluminum plate, but gamma radiation requires shielding by dense material such as lead, steel, or concrete. Alfa beta gamma radiation.svg
Alpha radiation consists of helium nuclei and is readily stopped by a sheet of paper. Beta radiation, consisting of electrons or positrons, is stopped by thin aluminum plate, but gamma radiation requires shielding by dense material such as lead, steel, or concrete.

A beta particle, also called beta ray or beta radiation (symbol β), is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus during the process of beta decay. There are two forms of beta decay, β decay and β+ decay, which produce electrons and positrons respectively. [1]

Electron subatomic particle with negative electric charge

The electron is a subatomic particle, symbol
, whose electric charge is negative one elementary charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no known components or substructure. The electron has a mass that is approximately 1/1836 that of the proton. Quantum mechanical properties of the electron include an intrinsic angular momentum (spin) of a half-integer value, expressed in units of the reduced Planck constant, ħ. As it is a fermion, no two electrons can occupy the same quantum state, in accordance with the Pauli exclusion principle. Like all elementary particles, electrons exhibit properties of both particles and waves: they can collide with other particles and can be diffracted like light. The wave properties of electrons are easier to observe with experiments than those of other particles like neutrons and protons because electrons have a lower mass and hence a longer de Broglie wavelength for a given energy.

Positron subatomic particle with positive charge

The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1 e, a spin of 1/2, and has the same mass as an electron. When a positron collides with an electron, annihilation occurs. If this collision occurs at low energies, it results in the production of two or more gamma ray photons.

Radioactive decay Process by which an unstable atom emits radiation

Radioactive decay is the process by which an unstable atomic nucleus loses energy by emitting radiation, such as an alpha particle, beta particle with neutrino or only a neutrino in the case of electron capture, or a gamma ray or electron in the case of internal conversion. A material containing such unstable nuclei is considered radioactive. Certain highly excited short-lived nuclear states can decay through neutron emission, or more rarely, proton emission.


Beta particles with an energy of 0.5 MeV have a range of about one metre in air; the distance is dependent on the particle energy.

Beta particles are a type of ionizing radiation and for radiation protection purposes are regarded as being less ionising than alpha particles, but more ionising than gamma rays. The higher the ionising effect, the greater the damage to living tissue.

Ionizing radiation radiation that carries enough energy to liberate electrons from atoms or molecules

Ionizing radiation is radiation that carries enough energy to detach electrons from atoms or molecules, thereby ionizing them. Ionizing radiation is made up of energetic subatomic particles, ions or atoms moving at high speeds, and electromagnetic waves on the high-energy end of the electromagnetic spectrum.

Radiation protection, also known as radiological protection, is defined by the International Atomic Energy Agency (IAEA) as "The protection of people from harmful effects of exposure to ionizing radiation, and the means for achieving this". The IAEA also states "The accepted understanding of the term radiation protection is restricted to protection of people. Suggestions to extend the definition to include the protection of non-human species or the protection of the environment are controversial". Exposure can be from a radiation source external to the human body or due to the bodily intake of a radioactive material.

Alpha particle helium-4 nucleus; a particles consisting of two protons and two neutrons bound together

Alpha particles, also called alpha ray 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 produced in other ways. Alpha particles are named after the first letter in the Greek alphabet, α. The symbol for the alpha particle is α or α2+. Because they are identical to helium nuclei, they are also sometimes written as He2+
or 4
indicating a helium ion with a +2 charge. If the ion gains electrons from its environment, the alpha particle becomes a normal helium atom 4

Beta decay modes

β decay (electron emission)

Beta decay. A beta particle (in this case a negative electron) is shown being emitted by a nucleus. An antineutrino (not shown) is always emitted along with an electron. Insert: in the decay of a free neutron, a proton, an electron (negative beta ray), and an electron antineutrino are produced. Beta-minus Decay.svg
Beta decay. A beta particle (in this case a negative electron) is shown being emitted by a nucleus. An antineutrino (not shown) is always emitted along with an electron. Insert: in the decay of a free neutron, a proton, an electron (negative beta ray), and an electron antineutrino are produced.

An unstable atomic nucleus with an excess of neutrons may undergo β decay, where a neutron is converted into a proton, an electron, and an electron antineutrino (the antiparticle of the neutrino):

Neutron nucleon (constituent of the nucleus of the atom) that has neutral electric charge (no charge); symbol n

The neutron is a subatomic particle, symbol
, with no net electric charge and a mass slightly larger than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behave similarly within the nucleus, and each has a mass of approximately one atomic mass unit, they are both referred to as nucleons. Their properties and interactions are described by nuclear physics.

Proton nucleon (constituent of the nucleus of the atom) that has positive electric charge; symbol p

A proton is a subatomic particle, symbol
, with a positive electric charge of +1e elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approximately one atomic mass unit, are collectively referred to as "nucleons".

Antiparticle concept that pairs of particles exist that have equal mass, but opposite sign for properties such as charge, lepton and baryon number

In particle physics, every type of particle has an associated antiparticle with the same mass but with opposite physical charges. For example, the antiparticle of the electron is the antielectron. While the electron has a negative electric charge, the positron has a positive electric charge, and is produced naturally in certain types of radioactive decay. The opposite is also true: the antiparticle of the positron is the electron.



This process is mediated by the weak interaction. The neutron turns into a proton through the emission of a virtual W boson. At the quark level, W emission turns a down quark into an up quark, turning a neutron (one up quark and two down quarks) into a proton (two up quarks and one down quark). The virtual W boson then decays into an electron and an antineutrino.

Weak interaction the fundamental interaction responsible for beta decay and nuclear fission

In particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is the mechanism of interaction between subatomic particles that is responsible for the radioactive decay of atoms. The weak interaction serves an essential role in nuclear fission, and the theory regarding it in terms of both its behavior and effects is sometimes called quantum flavordynamics (QFD). However, the term QFD is rarely used because the weak force is better understood in terms of electroweak theory (EWT). In addition to this, QFD is related to quantum chromodynamics (QCD), which deals with the strong interaction, and quantum electrodynamics (QED), which deals with the electromagnetic force.

In physics, a virtual particle is a transient fluctuation that exhibits some of the characteristics of an ordinary particle, while having its existence limited by the uncertainty principle. The concept of virtual particles arises in perturbation theory of quantum field theory where interactions between ordinary particles are described in terms of exchanges of virtual particles. A process involving virtual particles can be described by a schematic representation known as a Feynman diagram, in which virtual particles are represented by internal lines.

Quark elementary particle of which the proton or any other other baryon or meson is composed

A quark is a type of 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 observed or found in isolation; they can be found only within hadrons, which include baryons and mesons. For this reason, much of what is known about quarks has been drawn from observations of hadrons.

β− decay commonly occurs among the neutron-rich fission byproducts produced in nuclear reactors. Free neutrons also decay via this process. Both of these processes contribute to the copious quantities of beta rays and electron antineutrinos produced by fission-reactor fuel rods.

Nuclear fission product product of nuclear fission

Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release of heat energy, and gamma rays. The two smaller nuclei are the fission products..

Nuclear reactor device to initiate and control a sustained nuclear chain reaction

A nuclear reactor, formerly known as an atomic pile, is a device used to initiate and control a self-sustained nuclear chain reaction. Nuclear reactors are used at nuclear power plants for electricity generation and in propulsion of ships. Heat from nuclear fission is passed to a working fluid, which in turn runs through steam turbines. These either drive a ship's propellers or turn electrical generators' shafts. Nuclear generated steam in principle can be used for industrial process heat or for district heating. Some reactors are used to produce isotopes for medical and industrial use, or for production of weapons-grade plutonium. Some are run only for research. As of early 2019, the IAEA reports there are 454 nuclear power reactors and 226 nuclear research reactors in operation around the world.

β+ decay (positron emission)

Unstable atomic nuclei with an excess of protons may undergo β+ decay, also called positron decay, where a proton is converted into a neutron, a positron, and an electron neutrino:

The electron neutrino is a subatomic lepton elementary particle which has no net electric charge. Together with the electron it forms the first generation of leptons, hence the name electron neutrino. It was first hypothesized by Wolfgang Pauli in 1930, to account for missing momentum and missing energy in beta decay, and was discovered in 1956 by a team led by Clyde Cowan and Frederick Reines.



Beta-plus decay can only happen inside nuclei when the absolute value of the binding energy of the daughter nucleus is greater than that of the parent nucleus, i.e., the daughter nucleus is a lower-energy state.

Beta decay schemes

Cs-137 Decay Scheme showing it initially undergoes beta decay. The 661 KeV gamma peak associated with Cs-137 is actually emitted by the daughter radionuclide. Cs-137-decay.svg
Cs-137 Decay Scheme showing it initially undergoes beta decay. The 661 KeV gamma peak associated with Cs-137 is actually emitted by the daughter radionuclide.

The accompanying decay scheme diagram shows the beta decay of Cs-137. Cs-137 is noted for a characteristic gamma peak at 661 KeV, but this is actually emitted by the daughter radionuclide Ba-137m. The diagram shows the type and energy of the emitted radiation, its relative abundance, and the daughter nuclides after decay.

Phosphorus-32 is a beta emitter widely used in medicine and has a short half-life of 14.29 days [2] and decays into sulfur-32 by beta decay as shown in this nuclear equation:


1.709 MeV of energy is released during the decay. [2] The kinetic energy of the electron varies with an average of approximately 0.5 MeV and the remainder of the energy is carried by the nearly undetectable electron antineutrino. In comparison to other beta radiation-emitting nuclides the electron is moderately energetic. It is blocked by around 1 m of air or 5 mm of acrylic glass.

Interaction with other matter

Blue Cherenkov radiation light being emitted from a TRIGA reactor pool is due to high-speed beta particles traveling faster than the speed of light (phase velocity) in water (which is 75% of the speed of light in vacuum). TrigaReactorCore.jpeg
Blue Cherenkov radiation light being emitted from a TRIGA reactor pool is due to high-speed beta particles traveling faster than the speed of light (phase velocity) in water (which is 75% of the speed of light in vacuum).

Of the three common types of radiation given off by radioactive materials, alpha, beta and gamma, beta has the medium penetrating power and the medium ionising power. Although the beta particles given off by different radioactive materials vary in energy, most beta particles can be stopped by a few millimeters of aluminium. However, this does not mean that beta-emitting isotopes can be completely shielded by such thin shields: as they decelerate in matter, beta electrons emit secondary gamma rays, which are more penetrating than betas per se. Shielding composed of materials with lower atomic weight generates gammas with lower energy, making such shields somewhat more effective per unit mass than ones made of high-Z materials such as lead.

Being composed of charged particles, beta radiation is more strongly ionizing than gamma radiation. When passing through matter, a beta particle is decelerated by electromagnetic interactions and may give off bremsstrahlung x-rays.

In water, beta radiation from many nuclear fission products typically exceeds the speed of light in that material (which is 75% that of light in vacuum), [3] and thus generates blue Cherenkov radiation when it passes through water. The intense beta radiation from the fuel rods of pool-type reactors can thus be visualized through the transparent water that covers and shields the reactor (see illustration at right).

Detection and measurement

Beta radiation detected in an isopropanol cloud chamber (after insertion of an artificial source strontium-90) Beta radiation in a cloud chamber.jpg
Beta radiation detected in an isopropanol cloud chamber (after insertion of an artificial source strontium-90)

The ionizing or excitation effects of beta particles on matter are the fundamental processes by which radiometric detection instruments detect and measure beta radiation. The ionization of gas is used in ion chambers and Geiger-Müller counters, and the excitation of scintillators is used in scintillation counters. The following table shows radiation quantities in SI and non-SI units:

Radiation related quantities view    talk    edit
QuantityUnitSymbolDerivationYear SI equivalence
Activity (A) curie Ci3.7 × 1010 s−119533.7×1010 Bq
becquerel Bqs−11974s−1
rutherford Rd106 s−119461,000,000 Bq
Exposure (X) röntgen R esu / 0.001293 g of air19282.58 × 10−4 C/kg
Fluence (Φ)(reciprocal area)m−21962m−2
Absorbed dose (D) erg erg⋅g−119501.0 × 10−4 Gy
rad rad100 erg⋅g−119530.010 Gy
gray Gy J⋅kg−11974 J⋅kg−1
Dose equivalent (H) röntgen equivalent man rem100 erg⋅g−119710.010 Sv
sievert SvJ⋅kg−1 × WR 1977SI
  • The gray (Gy), is the SI unit of absorbed dose, which is the amount of radiation energy deposited in the irradiated material. For beta radiation this is numerically equal to the equivalent dose measured by the sievert, which indicates the stochastic biological effect of low levels of radiation on human tissue. The radiation weighting conversion factor from absorbed dose to equivalent dose is 1 for beta, whereas alpha particles have a factor of 20, reflecting their greater ionising effect on tissue.
  • The rad is the deprecated CGS unit for absorbed dose and the rem is the deprecated CGS unit of equivalent dose, used mainly in the USA.


Beta particles can be used to treat health conditions such as eye and bone cancer and are also used as tracers. Strontium-90 is the material most commonly used to produce beta particles.

Beta particles are also used in quality control to test the thickness of an item, such as paper, coming through a system of rollers. Some of the beta radiation is absorbed while passing through the product. If the product is made too thick or thin, a correspondingly different amount of radiation will be absorbed. A computer program monitoring the quality of the manufactured paper will then move the rollers to change the thickness of the final product.

An illumination device called a betalight contains tritium and a phosphor. As tritium decays, it emits beta particles; these strike the phosphor, causing the phosphor to give off photons, much like the cathode ray tube in a television. The illumination requires no external power, and will continue as long as the tritium exists (and the phosphors do not themselves chemically change); the amount of light produced will drop to half its original value in 12.32 years, the half-life of tritium.

Beta-plus (or positron) decay of a radioactive tracer isotope is the source of the positrons used in positron emission tomography (PET scan).


Henri Becquerel, while experimenting with fluorescence, accidentally found out that uranium exposed a photographic plate, wrapped with black paper, with some unknown radiation that could not be turned off like X-rays.

Ernest Rutherford continued these experiments and discovered two different kinds of radiation:

He published his results in 1899. [4]

In 1900, Becquerel measured the mass-to-charge ratio (m/e) for beta particles by the method of J. J. Thomson used to study cathode rays and identify the electron. He found that e/m for a beta particle is the same as for Thomson's electron, and therefore suggested that the beta particle is in fact an electron.


Beta particles are moderately penetrating in living tissue, and can cause spontaneous mutation in DNA.

Beta sources can be used in radiation therapy to kill cancer cells.

See also

Related Research Articles

Alpha decay emission of alpha particles by a decaying radioactive atom

Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle and thereby transforms or 'decays' into a different atomic nucleus, with a mass number that is reduced by four and an atomic number that is reduced by two. An alpha particle is identical to the nucleus of a helium-4 atom, which consists of two protons and two neutrons. It has a charge of +2 e and a mass of 4 u. For example, uranium-238 decays to form thorium-234. Alpha particles have a charge +2 e, but as a nuclear equation describes a nuclear reaction without considering the electrons – a convention that does not imply that the nuclei necessarily occur in neutral atoms – the charge is not usually shown.

Beta decay decay where electrons (β-, beta minus) or positrons (β+, positron emission) are emitted

In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta ray is emitted from an atomic nucleus. For example, beta decay of a neutron transforms it into a proton by the emission of an electron accompanied by an antineutrino, or conversely a proton is converted into a neutron by the emission of a positron with a neutrino, thus changing the nuclide type. Neither the beta particle nor its associated (anti-)neutrino exist within the nucleus prior to beta decay, but are created in the decay process. By this process, unstable atoms obtain a more stable ratio of protons to neutrons. The probability of a nuclide decaying due to beta and other forms of decay is determined by its nuclear binding energy. The binding energies of all existing nuclides form what is called the nuclear band or valley of stability. For either electron or positron emission to be energetically possible, the energy release or Q value must be positive.

Particle radiation is the radiation of energy by means of fast-moving subatomic particles. Particle radiation is referred to as a particle beam if the particles are all moving in the same direction, similar to a light beam.

Electron capture process in which a proton-rich nuclide absorbs an inner atomic electron

Electron 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 shell. This process thereby changes a nuclear proton to a neutron and simultaneously causes the emission of an electron neutrino.

Scintillation counter

A scintillation counter is an instrument for detecting and measuring ionizing radiation by using the excitation effect of incident radiation on a scintillating material, and detecting the resultant light pulses.

Annihilation quantum field theoretic process of particle–antiparticle annihilation

In particle physics, annihilation is the process that occurs when a subatomic particle collides with its respective antiparticle to produce other particles, such as an electron colliding with a positron to produce two photons. The total energy and momentum of the initial pair are conserved in the process and distributed among a set of other particles in the final state. Antiparticles have exactly opposite additive quantum numbers from particles, so the sums of all quantum numbers of such an original pair are zero. Hence, any set of particles may be produced whose total quantum numbers are also zero as long as conservation of energy and conservation of momentum are obeyed.

Positron emission radioactive decay in which a proton is converted into a neutron while releasing a positron and an electron neutrino

Positron emission or beta plus decay is a subtype of radioactive decay called beta decay, in which a proton inside a radionuclide nucleus is converted into a neutron while releasing a positron and an electron neutrino (νe). Positron emission is mediated by the weak force. 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.

A radioactive tracer, radiotracer, or radioactive label, is a chemical compound in which one or more atoms have been replaced by a radionuclide so by virtue of its radioactive decay it can be used to explore the mechanism of chemical reactions by tracing the path that the radioisotope follows from reactants to products. Radiolabeling or radiotracing is thus the radioactive form of isotopic labeling.

The Cowan–Reines neutrino experiment was conducted by Clyde L. Cowan and Frederick Reines in 1956. The experiment confirmed the existence of neutrinos. Neutrinos, subatomic particles with no electric charge and very small mass, had been conjectured to be an essential particle in beta decay processes in the 1930s. With neither mass nor charge, such particles appeared to be impossible to detect. The experiment exploited a huge flux of (hypothetical) electron antineutrinos emanating from a nearby nuclear reactor and a detector consisting of large tanks of water. Neutrino interactions with the protons of the water were observed, verifying the existence and basic properties of this particle for the first time.


Radioluminescence is the phenomenon by which light is produced in a material by bombardment with ionizing radiation such as alpha particles, beta particles, or gamma rays. Radioluminescence is used as a low level light source for night illumination of instruments or signage. Radioluminescent paint used to be used for clock hands and instrument dials, enabling them to be read in the dark. Radioluminescence is also sometimes seen around high-power radiation sources, such as nuclear reactors and radioisotopes.

Double electron capture is a decay mode of atomic nucleus. For a nuclide with number of nucleons A and atomic number Z, double electron capture is only possible if the mass of the nuclide of is lower.

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In radiobiology, the relative biological effectiveness is the ratio of biological effectiveness of one type of ionizing radiation relative to another, given the same amount of absorbed energy. The RBE is an empirical value that varies depending on the particles, energies involved, and which biological effects are deemed relevant.

Gamma ray electromagnetic radiation of high frequency and therefore high energy

A gamma ray or gamma radiation, is a penetrating electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves and so imparts the highest photon energy. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900 while studying radiation emitted by radium. In 1903, Ernest Rutherford named this radiation gamma rays based on their relatively strong penetration of matter; he had previously discovered two less penetrating types of decay radiation, which he named alpha rays and beta rays in ascending order of penetrating power.

Free neutron decay

Outside the nucleus, free neutrons are unstable and have a mean lifetime of 881.5±1.5 s. Therefore, the half-life for this process is 611±1 s. The beta decay of the neutron, described above, can be denoted as follows:


  1. Lawrence Berkeley National Laboratory (9 August 2000). "Beta Decay". Nuclear Wall Chart. United States Department of Energy . Retrieved 17 January 2016.
  2. 1 2 Archived 2006-07-05 at the Wayback Machine
  3. The macroscopic speed of light in water is 75% of the speed of light in a vacuum (called "c"). The beta particle is moving faster than 0.75 c, but not faster than c.
  4. E. Rutherford (8 May 2009) [Paper published by Rutherford in 1899]. "Uranium radiation and the electrical conduction produced by it". Philosophical Magazine. 47 (284): 109–163. doi:10.1080/14786449908621245.

Further reading