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Beta particle
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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, known as beta 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's density and composition.
Beta particles are a type of ionizing radiation, and for radiation protection purposes, they are regarded as being more ionizing than gamma rays, but less ionizing than alpha particles. The higher the ionising effect, the greater the damage to living tissue, but also the lower the penetrating power of the radiation through matter.
Beta decay modes
[edit]β− decay (electron emission)
[edit]
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):
- n → p + e−
+ ν
e
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.
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.
β+ decay (positron emission)
[edit]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:
- p → n + e+
+ ν
e
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
[edit]
The accompanying decay scheme diagram shows the beta decay of caesium-137. 137Cs is noted for a characteristic gamma peak at 661 keV, but this is actually emitted by the daughter radionuclide 137mBa. 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 by beta decay as shown in this nuclear equation:
1.709 MeV of energy is released during the decay.[3] 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
[edit]
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 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 off bremsstrahlung X-rays.
In water, beta radiation from many nuclear fission products typically exceeds the speed of light in that material (which is about 75% that of light in vacuum),[4] and thus generates blue Cherenkov radiation when it passes through water. The intense beta radiation from the fuel rods of swimming pool reactors can thus be visualized through the transparent water that covers and shields the reactor (see illustration at right).
Detection and measurement
[edit]
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:
| Quantity | Unit | Symbol | Derivation | Year | SI equivalent |
|---|---|---|---|---|---|
| Activity (A) | becquerel | Bq | s−1 | 1974 | SI unit |
| curie | Ci | 3.7×1010 s−1 | 1953 | 3.7×1010 Bq | |
| rutherford | Rd | 106 s−1 | 1946 | 1000000 Bq | |
| Exposure (X) | coulomb per kilogram | C/kg | C⋅kg−1 of air | 1974 | SI unit |
| röntgen | R | esu / 0.001293 g of air | 1928 | 2.58×10−4 C/kg | |
| Absorbed dose (D) | gray | Gy | J⋅kg−1 | 1974 | SI unit |
| erg per gram | erg/g | erg⋅g−1 | 1950 | 1.0×10−4 Gy | |
| rad | rad | 100 erg⋅g−1 | 1953 | 0.010 Gy | |
| Equivalent dose (H) | sievert | Sv | J⋅kg−1 × WR | 1977 | SI unit |
| röntgen equivalent man | rem | 100 erg⋅g−1 × WR | 1971 | 0.010 Sv | |
| Effective dose (E) | sievert | Sv | J⋅kg−1 × WR × WT | 1977 | SI unit |
| röntgen equivalent man | rem | 100 erg⋅g−1 × WR × WT | 1971 | 0.010 Sv |
- 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 spectroscopy
[edit]The energy contained within individual beta particles is measured via beta spectrometry; the study of the obtained distribution of energies as a spectrum is beta spectroscopy. Determination of this energy is done by measuring the amount of deflection of the electron's path under a magnetic field.[5]
Applications
[edit]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).
History
[edit]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:
- alpha particles that did not show up on the Becquerel plates because they were easily absorbed by the black wrapping paper
- beta particles which are 100 times more penetrating than alpha particles.
He published his results in 1899.[6]
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.
Health
[edit]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
[edit]References
[edit]- ^ "Radiation Basics". United States Nuclear Regulatory Com. 2017-10-02.
- ^ Lawrence Berkeley National Laboratory (9 August 2000). "Beta Decay". Nuclear Wall Chart. United States Department of Energy. Archived from the original on 3 March 2016. Retrieved 17 January 2016.
- ^ a b "Phosphorus-32" (PDF). nucleide.org. Laboratoire Nationale Henri Bequerel. Archived (PDF) from the original on 2022-10-09. Retrieved 28 June 2022.
- ^ The macroscopic speed of light in water is 75% of the speed of light in vacuum (called c). The beta particle is moving faster than 0.75 c, but not faster than c.
- ^ Boeglin, Werner. "4. Beta Spectroscopy — Modern Lab Experiments documentation". wanda.fiu.edu.
- ^ 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
[edit]- Radioactivity and alpha, beta, gamma and Xrays
- Rays and Particles University of Virginia Lecture
- History of Radiation Archived 2017-05-06 at the Wayback Machine at Idaho State University
- Basic Nuclear Science Information Archived 2006-12-05 at the Wayback Machine at the Lawrence Berkeley National Laboratory
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Beta particle
View on Grokipediafrom Grokipedia
Fundamentals
Definition and Types
A beta particle is a high-energy, charged particle emitted from the nucleus of a radioactive atom during beta decay, serving as a form of ionizing radiation that differs from alpha particles (helium nuclei) and gamma rays (high-energy photons). These particles originate from instabilities in the atomic nucleus, where an imbalance in the proton-to-neutron ratio prompts emission to achieve greater stability.[10][1][2] There are two main types of beta particles, both mediated by the weak nuclear force: beta-minus (β⁻) particles, which are electrons released when a neutron decays into a proton, and beta-plus (β⁺) particles, which are positrons emitted when a proton decays into a neutron. This process allows the nucleus to adjust its composition without altering the total number of nucleons.[11][10][3] Beta particles possess a rest mass of approximately 0.511 MeV/c², equivalent for both electrons and positrons, and their velocities can range from near zero up to about 99% of the speed of light, rendering them relativistic in high-energy decays.[12]Physical Properties
Beta particles, whether electrons (β⁻) or positrons (β⁺), carry an elementary charge of magnitude C, with β⁻ having a charge of -e and β⁺ a charge of +e.[13] This charge results in strong electromagnetic interactions with matter, primarily through Coulomb forces that lead to ionization and scattering as the particles traverse atomic fields.[14] The rest mass of a beta particle is kg, corresponding to a rest energy of MeV.[15] In beta decay, beta particles acquire kinetic energies typically ranging from a few keV to several MeV, depending on the decaying nuclide, with the energy spectrum continuous and the maximum value determined by the Q-value of the decay.[14] For example, beta particles from phosphorus-32 have a maximum kinetic energy of 1.71 MeV.[14] Due to their low rest mass, beta particles quickly become relativistic even at modest kinetic energies, often approaching speeds near the speed of light . Their relativistic nature is quantified by the Lorentz factor , which exceeds 2 for kinetic energies above about 1 MeV and leads to effects such as time dilation and length contraction in high-energy scenarios.[14] These properties influence the particles' trajectories and energy loss in materials. As fundamental leptons, beta particles possess an intrinsic spin angular momentum of , where is the reduced Planck's constant, consistent with their classification as spin-1/2 fermions in the Dirac equation framework.[16] Upon slowing to rest, β⁺ particles (positrons) annihilate with ambient electrons, converting their combined rest masses into two gamma rays each of energy 0.511 MeV, emitted in opposite directions.[17]Beta Decay Processes
Negative Beta Decay
Negative beta decay, or β⁻ decay, occurs in atomic nuclei that have an excess of neutrons relative to protons, leading to instability. In this process, a neutron in the nucleus transforms into a proton, resulting in the emission of an electron (the β⁻ particle) and an electron antineutrino (ν̄_e). This transformation increases the atomic number by one while preserving the mass number, effectively converting one element into the next in the periodic table. The basic reaction is represented as: This decay conserves key quantum numbers: baryon number remains 1 (neutron and proton each have baryon number 1), electric charge balances as 0 = +1 -1 + 0, and electron lepton number is conserved as 0 = 0 +1 -1 (where the antineutrino carries lepton number -1).[1][3] The energy released in negative beta decay, known as the Q-value, determines the total kinetic energy available to the products and is calculated using nuclear masses as: where denotes nuclear mass and is the electron rest mass. Equivalently, using atomic masses (which include electron bindings), the Q-value simplifies to , accounting for the emitted electron. This energy is shared primarily between the electron and antineutrino as kinetic energy, with the daughter nucleus receiving negligible recoil in most cases; the electron's kinetic energy ranges from near zero up to a maximum of approximately Q (when the antineutrino carries minimal energy)./07%3A_Radioactive_Decay_Part_II/7.02%3A_Beta_Decay)[18] At a fundamental level, negative beta decay is governed by the weak nuclear force, mediated by the exchange of a charged W⁻ boson. In this interaction, a down quark in the neutron (udd) emits a W⁻, transforming into an up quark (uud, forming a proton), while the W⁻ subsequently decays into the electron and antineutrino. This process violates parity conservation but occurs at an introductory level without delving into detailed quark dynamics.[11][19] Prominent examples include the decay of carbon-14, used in radiocarbon dating: , with a maximum electron kinetic energy of 0.156 MeV and a half-life of 5730 years. Another is tritium (hydrogen-3) decay: , featuring a low maximum electron energy of 18.6 keV and a half-life of 12.32 years, making it useful in fusion research and neutrino mass experiments. The emission of beta particles was first observed by Henri Becquerel in 1896 during his studies of uranium salts, marking the discovery of radioactivity; the underlying mechanism, including the antineutrino's role, was clarified in the 1930s through theoretical and experimental advancements.[20][21][22][6]Positive Beta Decay
Positive beta decay, also known as β⁺ decay or positron emission, occurs in proton-rich atomic nuclei where a proton transforms into a neutron, emitting a positron (e⁺) and an electron neutrino (ν_e) to maintain conservation laws.[1] This process was theoretically predicted in 1928 by Paul Dirac through his relativistic quantum equation for the electron, which implied the existence of a positively charged counterpart to the electron.[23] The positron was experimentally discovered in 1932 by Carl Anderson during cosmic ray studies, confirming Dirac's prediction.[23] The fundamental reaction in positive beta decay is represented by the transformation of a proton within the nucleus:This weak interaction process shares conceptual similarities with negative beta decay in terms of the Q-value, defined as the atomic mass difference between parent and daughter nuclides converted to energy.[24] However, for β⁺ decay to be energetically feasible, the Q-value must exceed 1.022 MeV, equivalent to twice the rest mass energy of an electron (2m_e c²), accounting for the creation of the positron-electron pair during subsequent annihilation.[25] The available decay energy is partitioned between the positron's kinetic energy and the neutrino's energy, with the neutrino carrying away variable amounts to ensure three-body kinematics.[1] Representative examples include the decay of fluorine-18 (¹⁸F), a proton-rich isotope used in medical imaging:
This decay proceeds with 96.7% branching ratio via positron emission, transforming the nucleus from Z=9 to Z=8 while conserving mass number A=18.[26] Another example is sodium-22 (²²Na), which decays primarily by positron emission (89.6% branching ratio):
followed by excited state transitions in the daughter neon nucleus.[27] Unlike electron capture, where a proton absorbs an inner-shell orbital electron to form a neutron and neutrino without emitting a charged particle, positive beta decay produces a detectable positron that can be observed directly in experiments./Nuclear_Chemistry/Radioactivity/Nuclear_Decay_Pathways) This distinction arises because β⁺ decay requires sufficient nuclear energy release to create the positron, whereas electron capture has no such charged particle emission.[24]