- Beta particle
Beta particles are high-energy, high-speed
electrons or positrons emitted by certain types of radioactivenuclei such as potassium-40. The beta particles emitted are a form of ionizing radiationalso known as beta rays. The production of beta particles is termed beta decay. They are designated by the Greek letter beta (β). There are two forms of beta decay, β− and β+, which respectively give rise to the electronand the positron.
β− decay (electron emission)
An unstable atomic nucleus with an excess of
neutrons may undergo β− decay, where a neutron is converted into a proton, an electronand an electron-type antineutrino (the antiparticleof the neutrino):
:SubatomicParticle|neutron → SubatomicParticle|proton + SubatomicParticle|electron + SubatomicParticle|electron antineutrino
This process is mediated by the
weak interaction. The neutron turns into a proton through the emission of a virtual W− boson. At the quarklevel, W− emission turns a down-type quark into an up-type 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.
Beta decay commonly occurs among the neutron-rich fission byproducts produced in nuclear reactors. Free neutrons also decay via this process. This is the source of the copious amount of electron antineutrinos produced by fission reactors.
β+ decay (positron emission)
Unstable atomic nuclei with an excess of
protons may undergo β+ decay, also called inverse beta decay, where a proton is converted into a neutron, a positronand an electron-type neutrino:
:SubatomicParticle|proton → SubatomicParticle|neutron + SubatomicParticle|positron + SubatomicParticle|electron neutrino
Beta plus decay can only happen inside nuclei when the absolute value of the
binding energyof the daughter nucleus is higher than that of the mother nucleus.
Inverse beta decay is one of the steps in
nuclear fusionprocesses that produce energy inside stars.
Interaction with other matter
Being composed of charged particles, beta radiation is more strongly ionising than gamma radiation.
When passing through matter, a beta particle is decelerated by electromagnetic interactions and may give off
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.
Inverse beta decay of a radioactive tracer
isotopeis the source of the positrons used in positron emission tomography(PET scan).
Henri Becquerel, while experimenting with fluorescence, accidentally found out that Uraniumexposed a black paper wrapped photographicplate with some unknown radiationthat could not be turned off like X-rays. Ernest Rutherfordcontinued these experiments and discovered two different kinds of radiation:
alpha particlesthat did not show up on the Becquerel plates because they were easily absorbed by the black wrapping paper (actually just about any sheet of paper fully absorbs alpha particles)
* beta particles which are 100 times more penetrating that alpha particles.He published his results in 1899.
Beta particles are able to penetrate living matter to a certain extent (radiation intensity from a small source of radioactive material decreases as one over the distance squared) and can change the structure of struck molecules. In most cases such change can be considered as damage with results possibly as severe as cancer and death. If the struck molecule is
DNAit can show a spontaneous mutation. If this mutated DNA is in gametesthe mutation may be passed to new generations. Almost all mutations are considered genetic defects.
Beta sources can be used in
radiation therapyto kill cancer cells.
Some sources claim that betavoltaic cells will be available in the near future to supply power to laptops and mobile phones without recharging for the expected useful life of the product. Since such cells use materials undergoing decay their power output is limited to its
half-liferelated to the device's power needs. For example tritiumhas a half life of approximately 12 years, meaning after that span of time a cell powered by this isotopewould produce half the power it did when assembled assuring that at some point the betavoltaic cell would die without more tritium.Fact|date=June 2008
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