Beta particles

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Beta particles

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 Alpha particles, Electron, Gamma rays, Positron, Radioactivity

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 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 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 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 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 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 Neutrino

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