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Beta particle

Ionizing radiation

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Beta particles with an energy of 0.5 MeV have a range of about one metre in the air, and can't pass through clothing or thin metal sheets; the distance is dependent on the particle's energy and the air's density and composition.

Beta particles are a type of ionizing radiation, and for radiation protection purposes, they are regarded as being more ionizing than gamma rays, but less ionizing than alpha particles. The higher the ionising effect, the greater the damage to living tissue, but also the lower the penetrating power of the radiation through matter. The ionising effect of a beta particle is caused by the particle knocking electrons from shells by either the electrostatic force or energy transfer from the beta particle to the electron by collision. They produce roughly 100 ion pairs per cm in air, this means an ion plus a released electron.

Beta decay modes

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 electron, and an electron antineutrino (the antiparticle of the neutrino):

n → p + e−
+ ν
e

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 quark into an up 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.

β− decay commonly occurs among the neutron-rich fission byproducts produced in nuclear reactors. Free neutrons also decay via this process. Both of these processes contribute to the copious quantities of beta rays and electron antineutrinos produced by fission-reactor fuel rods.

β+ decay (positron emission)

Unstable atomic nuclei with an excess of protons may undergo β+ decay, also called positron decay, where a proton is converted into a neutron, a positron, and an electron 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 greater than that of the parent nucleus, i.e., the daughter nucleus is a lower-energy state.

Beta decay schemes

The accompanying decay scheme diagram shows the beta decay of caesium-137. 137Cs is noted for a characteristic gamma peak at 661 keV, but this is actually emitted by the daughter radionuclide 137mBa. The diagram shows the type and energy of the emitted radiation, its relative abundance, and the daughter nuclides after decay.

Phosphorus-32 is a beta emitter widely used in medicine. It has a short half-life of 14.29 days and decays into sulfur-32 by beta decay as shown in this nuclear equation:

1.709 MeV of energy is released during the decay. The kinetic energy of the electron varies with an average of approximately 0.5 MeV and the remainder of the energy is carried by the nearly undetectable electron antineutrino. In comparison to other beta radiation-emitting nuclides, the electron is moderately energetic. It is blocked by around 1 m of air or 5 mm of acrylic glass.

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