Abeta particle, also calledbeta ray orbeta radiation (symbolβ), is a high-energy, high-speedelectron orpositron emitted by theradioactive decay of anatomic nucleus, known asbeta decay. There are two forms of beta decay, β− decay and β+ decay, which produce electrons and positrons, respectively.[2]
Beta particles with an energy of 0.5 MeV have a range of about one metre in the air; the distance is dependent on the particle's energy and the air'sdensity and composition.
Beta particles are a type ofionizing radiation, and forradiation protection purposes, they are regarded as being more ionising thangamma rays, but less ionising thanalpha particles. The higher the ionising effect, the greater the damage to living tissue, but also the lower thepenetrating power of the radiation through matter.
Beta decay. A beta particle (in this case a negative electron) is shown being emitted by anucleus. 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 anelectron antineutrino are produced.
This process is mediated by theweak interaction. The neutron turns into a proton through the emission of avirtualW− boson. At thequark 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.
β− decay commonly occurs among the neutron-richfission byproducts produced innuclear 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.
Unstable atomic nuclei with an excess of protons may undergo β+ decay, also called positron decay, where a proton is converted into a neutron, apositron, and anelectron neutrino:
p → n + e+ + ν e
Beta-plus decay can only happen inside nuclei when the absolute value of thebinding energy of the daughter nucleus is greater than that of the parent nucleus, i.e., the daughter nucleus is a lower-energy state.
Caesium-137 decay scheme, showing it initially undergoes beta decay. The 661 keV gamma peak associated with137Cs is actually emitted by the daughter radionuclide.
The accompanying decay scheme diagram shows the beta decay ofcaesium-137.137Cs is noted for a characteristic gamma peak at 661 keV, but this is actually emitted by the daughter radionuclide137mBa. 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. It has a short half-life of 14.29 days[3] and decays into sulfur-32 bybeta decay as shown in this nuclear equation:
1.709 MeV of energy is released during the decay.[3] The kinetic energy of theelectron varies with an average of approximately 0.5 MeV and the remainder of the energy is carried by the nearly undetectableelectron 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 ofacrylic glass.
BlueCherenkov radiation light being emitted from aTRIGA 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 andgamma, 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 ofaluminium. 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 larger atoms 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 offbremsstrahlungX-rays.
In water, beta radiation from manynuclear fission products typically exceeds the speed of light in that material (which is about 75% that of light in vacuum),[4] and thus generates blueCherenkov radiation when it passes through water. The intense beta radiation from the fuel rods ofswimming pool reactors can thus be visualized through the transparent water that covers and shields the reactor (see illustration at right).
Beta radiation detected in an isopropanolcloud chamber (after insertion of an artificial sourcestrontium-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 inion chambers andGeiger–Müller counters, and the excitation ofscintillators is used inscintillation counters.The following table shows radiation quantities in SI and non-SI units:
Thegray (Gy) is the SI unit ofabsorbed dose, which is the amount of radiation energy deposited in the irradiated material. For beta radiation this is numerically equal to theequivalent dose measured by thesievert, 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.
Therad is the deprecatedCGS unit for absorbed dose and therem is the deprecatedCGS unit of equivalent dose, used mainly in the USA.
The energy contained within individual beta particles is measured viabeta spectrometry; the study of the obtained distribution of energies as aspectrum isbeta spectroscopy. Determination of this energy is done by measuring the amount of deflection of the electron's path under a magnetic field.[5]
Beta particles can be used to treat health conditions such aseye andbone 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 aspaper, 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 abetalight containstritium and aphosphor. As tritiumdecays, it emits beta particles; these strike the phosphor, causing the phosphor to give offphotons, much like thecathode-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); theamount of light produced will drop to half its original value in 12.32 years, thehalf-life of tritium.
In 1900, Becquerel measured themass-to-charge ratio (m/e) for beta particles by the method ofJ. J. Thomson used to study cathode rays and identify the electron. He found thate/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.
^ab"Phosphorus-32"(PDF).nucleide.org. Laboratoire Nationale Henri Bequerel.Archived(PDF) from the original on 2022-10-09. Retrieved28 June 2022.
^The macroscopic speed of light in water is 75% of the speed of light in vacuum (calledc). The beta particle is moving faster than 0.75 c, but not faster than c.
