Alpha decay

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Nuclear processes
Radioactive decay processes Alpha decay Beta decay Cluster decay Double beta decay Double electron capture Electron capture Gamma radiation Internal conversion Isomeric transition Neutron emission Positron emission Proton emission Spontaneous fission Nucleosynthesis Neutron Capture * The R-process * The S-process Proton capture: * The P-process * The Rp-process Spallation

Alpha decay is a form of radioactive decay in which an atomic nucleus ejects an alpha particle through the electromagnetic force and transforms into a nucleus with mass number 4 less and atomic number 2 less. For example:

[^2^_\hbox\;\to\;^2^_\hbox\;+\;^4_2\hbox^,]

although this is usually written as:

[^\hbox\;\to\;^\hbox\;+\;\alpha.]

(The second form is also preferred because, to the casual observer, the first form appears electrically unbalanced. Fundamentally, the recoiling nucleus is soon stripped of two electrons to neutralize the hungry helium anion.)

Note that an alpha particle is a helium nucleus, and that both mass number and atomic number are conserved. Alpha decay can essentially be thought of as nuclear fission where the parent nucleus splits into two daughter nuclei. Alpha decay is fundamentally a quantum tunneling process. Unlike beta decay, alpha decay is governed by the strong nuclear force.

Alpha particles with their typical kinetic energy of 5 MeV (that is ≈0.13% of their total energy, i.e. 110 TJ/kg), have a speed of 15,000 km/s.

Because of alpha decay, virtually all of the helium produced in the world comes from trapped underground deposits associated with minerals containing uranium or thorium, and brought to the surface as a by-product of natural gas production.

Alpha particles emitted by radioactive nuclei are among the most hazardous forms of radiation, if these nuclei are incorporated within a human body. As any heavy charged particle, alpha particles lose their energy within a very short distance in dense media, causing significant damage to surrounding biomolecules. On the other hand, external alpha irradiation is not harmful because alpha particles are completely absorbed by a very thin (micrometers) dead layer of skin as well as by a few centimeters of air. However, if a substance radiating alpha particles is ingested or inhaled by (or injected into) an organism it may become a risk, potentially inflicting very serious damage to the organisms' genetic makeup.

One example hazard is the gas Radon that can easily be inhaled and decay inside a human or animal lung, risking exposure of the lung tissue to alpha particles.

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