beta particle

A high-speed electron or positron, especially one emitted in radioactive decay.

The name first applied in 1897 by Ernest Rutherford to one of the forms of radiation emitted by radioactive nuclei. Beta particles can occur with either negative or positive charge (denoted β⁻ or β⁺) and are now known to be either electrons or positrons, respectively. Electrons and positrons are now referred to as beta particles only if they are known to have originated from nuclear beta decay. Their observed kinetic energies range from zero up to about 5 MeV in the case of naturally occurring radioactive isotopes, but can reach values well over 10 MeV for some artificially produced isotopes. See also Radioactivity; Alpha particles; Electron; Gamma rays; Positron.

When a nucleus beta-decays, it emits two particles at the same time: One is a beta particle; the other, a neutrino or antineutrino. With this emission, the nucleus itself undergoes a transformation, changing from one element to another. In the case of isotopes that β⁺-decay, each decaying nucleus emits a positron and a neutrino, simultaneously reducing its atomic number by one unit; for those isotopes that β⁻-decay, each nucleus emits an electron and an antineutrino while increasing its atomic number by one. In both classes of decay, the energy released by the nuclear transformation is shared between the two emitted particles. Though the energy released by a particular nuclear transformation is always the same, the fraction of this energy carried away by the beta particle is different for each decaying nucleus. (The neutrino always carries away the remainder, thus conserving energy overall.) When observed collectively, the decaying nuclei of a simple radioactive source emit their beta particles with a continuous distribution of kinetic energies covering the range from zero up to the total nuclear decay energy available.

Radioactive samples often contain several radioactive isotopes. Since each isotope has its own decay energy and beta-particle energy distribution, the energy spectrum of beta particles observed from such a sample would be the sum of a number of distributions, each with a different end-point energy. Indeed, many isotopes, especially those artificially produced with accelerators, can themselves beta-decay by additional paths that also release part of the energy in the form of gamma radiation.

As a beta particle penetrates matter, it loses its energy in collisions with the constituent atoms. Two processes are involved. First, the beta particle can transfer a small fraction of its energy to the struck atom. Second, the beta particle is deflected from its original path by each collision and, since any change in the velocity of a charged particle leads to the emission of electromagnetic radiation, some of its energy is lost in the form of low-energy x-rays (bremsstrahlung). Though the energy lost by a beta particle in a single collision is very small, many collisions occur as the particle traverses matter, causing it to follow a zigzag path as it slows down. See also Bremsstrahlung.

The thickness of material that is just sufficient to stop all the beta particles of a particular energy is called the range of those particles. For the continuous energy distribution normally associated with a source of beta particles, the effective range is the one that corresponds to the highest energy in the primary spectrum. That thickness of material stops all of the beta particles from the source. The range depends strongly on the electron energy and the density of the absorbing material.

The slowing-down processes have the same effect on both β⁻ and β⁺ particles. However, as antimatter, the positron (β⁺) cannot exist for long in the presence of matter. It soon combines with an atomic electron, with which it annihilates, the masses of both particles being replaced by electromagnetic energy. Usually this annihilation occurs after the positron has come to rest and formed a positronium atom, a bound but short-lived positron-electron system. In that case, the electromagnetic energy that is emitted from the annihilation takes the form of two 511-keV gamma rays that are emitted in opposite directions to conserve momentum. See also Positronium.

Beta particles are detected through their interaction with matter. One class of detectors employs gas as the detection medium. Ionization chambers, proportional counters, and Geiger-Müller counters are of this class. In these detectors, after entering through a thin window, the beta particles produce positive ions and free electrons as they collide with atoms of the gas in the process of their slowing down. An electric field applied across the volume of gas causes these ions and electrons to drift along the field lines, causing an ionization current that is then processed in external electronic devices. See also Ionization chamber; Particle detector.

More precise energy information can be achieved with scintillation detectors. In certain substances, the ion-electron pairs produced by the passage of a charged particle result in the emission of a pulse of visible or near-ultraviolet light. If a clear plastic scintillator is used, it can be mounted on a photomultiplier tube, which converts the transmitted light into a measurable electrical current pulse whose amplitude is proportional to the energy deposited by the incident beta particle. See also Scintillation counter.

Even better energy information comes from semiconductor detectors, which are effectively solid-state ionization chambers. When a beta particle enters the detector, it causes struck electrons to be raised into the conduction band, leaving holes behind in the valence band. The electrons and holes move under the influence of an imposed electric field, causing a pulse of current to flow. Such detectors are useful mainly for low-energy beta particles. See also Junction detector.

Any one of these detectors can be combined with a magnetic spectrometer. Beta particles, like any charged particles, follow curved paths in a perpendicular magnetic field, their radius of curvature being proportional to the square of their energy. Their detected position on exiting the magnetic field can be precisely related to their energy. The best current measurement of the electron antineutrino mass comes from a spectrometer measurement of the tritium beta-decay spectrum. See also Neutrino.

(beta ray, beta radiation) particulate ionizing radiation consisting of either negative electrons (negatrons) or positive electrons (positrons) emitted from the nucleus of an unstable element. This phenomenon is called beta decay.

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.

β⁻ decay (electron emission)

An unstable atomic nuclei 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):

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)

Beta decay

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:

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 higher than that of the mother nucleus.

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

See also

• electron
• particle physics
• α (alpha) particles
• β (beta) particles
• rays:
  • γ (gamma) rays
  • n (neutron) rays
  • δ (delta) rays
  • ε (epsilon) rays

References

http://www.epa.gov/radiation/radionuclides/strontium.htm

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