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Alpha radiation consists of helium nuclei and is readily stopped by a sheet of paper. Beta radiation, consisting of electrons, is halted by an aluminum plate. Gamma radiation is eventually absorbed as it penetrates a dense material.

Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also 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 electron and the positron.

Contents

β decay (electron emission)

Beta decay

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-type antineutrino (the antiparticle of the neutrino):

np + e + νe

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-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 positron and an electron-type neutrino:

pn + e+ + νe

Beta plus decay can only happen inside nuclei when the absolute value of the binding energy of the daughter nucleus is higher than that of the mother nucleus.

Inverse beta decay is one of the steps in nuclear fusion processes that produce energy inside stars.

Interaction with other matter

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 aluminum. 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 bremsstrahlung x-rays.

Uses

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 isotope is the source of the positrons used in positron emission tomography (PET scan).

History

Henri Becquerel, while experimenting with fluorescence, accidentally found out that Uranium exposed a black paper wrapped photographic plate 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 1897.

Health

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 DNA it can show a spontaneous mutation.

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

Future use

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-life related to the device's power needs. For example tritium has a half life of approximately 12 years, meaning after that span of time a cell powered by this isotope would produce half the power it did when assembled assuring that at some point the betavoltaic cell would die without more tritium.[citation needed]

See also

References

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