Beta particle

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Image:Alfa beta gamma radiation.png

Beta particles are high-energy 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 1 β− decay (electron) 2 β+ decay (positron) 3 The neutrino and conservation of energy 4 See also

β⁻ decay (electron)

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):

<math>\mbox{n} \rightarrow \mbox{p} + \mbox{e}^- + \bar{\nu}_{e}</math>

A W minus Boson is emitted from the nucleus which then decays into an electron and an antineutrino. It does this by converting one of the two up quarks in the proton into a down quark, leaving two down quarks and one up quark (a Neutron) to convert an up quark into a down quark a -1 charge is carried away to observe charge conservation (the W- boson, then the electron, as an up quark carries a charge of +2/3 and the down quark has a charge of -1/3; this quark is said to have undergone a flavor change)

β⁺ decay (positron)

Unstable nuclei that are deficient in neutrons may undergo β⁺ decay, where a proton is converted into a neutron. The proton consists of two up quarks and a down quark, one of the proton up quarks decays to a down quark, doing so, it will emit a W particle, which is unstable and decays further into a positron and an electron neutrino:

<math>\mbox{p} \rightarrow \mbox{n} + \mbox{e}^+ + \nu_{e}</math>

The positron and the neutrino are emitted from the nucleus.

<math>\mbox{p(uud)} \rightarrow \mbox{n(udd)} + \mbox{W}^+ \rightarrow \mbox{e}^+ + \nu_{e}</math>

The neutrino and conservation of energy

Due to the presence of the neutrino, the atom and the beta particle do not usually recoil in opposite directions. This observation led Wolfgang Pauli to postulate the existence of neutrinos in order to prevent violation of conservation of energy and momentum laws. Beta decay is mediated by the weak nuclear force.

Beta particles may be stopped by a few millimeters of aluminium. A beta particle's flight is ten times farther than an alpha particle, as it ionizes a tenth less than an alpha particle. The beta particle can produce gamma rays when passing through lead, so lighter materials are preferred as shielding.

See also

• Alpha decay
• Alpha particle
• Beta decay
• Gamma ray
• List of particles
• Neutron
• Neutron emission
• Neutron radiation
• Nuclear physics
• Positron
• Radioactivity
• Rays: α — β — γ — δ — ε

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