Alpha radiation

Alpha decay, or α-decay, is a type of radioactive decay in which an atomic nucleus emits an alpha particle and thereby transforms or 'decays' into an atom with a mass number that is reduced by 4 and an atomic number that is reduced by 2.

An alpha particle is identical to the nucleus of a helium-4 atom, which consists of two protons and two neutrons. For example, uranium-238 decays to form thorium-234:

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Both mass number and atomic number are conserved: the mass number is 238 on the left side and (234 + 4) on the right side and the atomic number is 92 on the left side and (90 + 2) on the right side. 

An atomic nucleus releasing and alpha particle.

image: Inductiveload -wikipedia

Alpha particles have a charge +2, but as a nuclear equation describes a nuclear reaction without considering the electrons, a convention that does not imply that the nuclei necessarily occur in neutral atoms, the charge is not usually shown.

Alpha decay typically occurs in the heaviest nuclides. 


What elements can produce alpha decay?

Theoretically

 it can occur only in nuclei somewhat heavier than nickel (element 28), where the overall binding energy per nucleon is no longer a minimum and the nuclides are therefore unstable toward spontaneous fission-type processes. 

In practice, 

this mode of decay has only been observed in nuclides considerably heavier than nickel, with the lightest known alpha emitter being the lightest isotopes (mass numbers 106–110) of tellurium (element 52).

How common is alpha decay?

Alpha decay is by far the most common form of cluster decay, where the parent atom ejects a defined daughter collection of nucleons, leaving another defined product behind. It is the most common form because of the combined extremely high binding energy and relatively small mass of the alpha particle. Like other cluster decays, alpha decay is fundamentally a quantum tunneling process. Unlike beta decay, it is governed by the interplay between both the nuclear force and the electromagnetic force.

How energetic is alpha decay?

Alpha particles have a typical kinetic energy of 5 MeV (or ≈ 0.13% of their total energy, 110 TJ/kg) and have a speed of about 15,000,000 m/s, or 5% of the speed of light. There is surprisingly small variation around this energy, due to the heavy dependence of the half-life of this process on the energy produced (see equations in the Geiger–Nuttall law).

A piece of uranium in a cloud chamber emits alpha radiation. source: youtube


Why alpha radiation is less dangerous than other ionising radiation

Because of their relatively large mass, +2 electric charge and relatively low velocity, alpha particles are very likely to interact with other atoms and lose their energy, and their forward motion can be stopped by a few centimeters of air.  However, if an alpha decay occurs in living tissue this can be highly damaging to nearby biological molecules like DNA.

Helium on Earth

Approximately 99% of the helium produced on Earth is the result of the alpha decay of underground deposits of minerals containing uranium or thorium. The helium is brought to the surface as a byproduct of natural gas production.


Reference: Alpha decay. (2015, April 6). In Wikipedia, The Free Encyclopedia. Retrieved 09:55, April 19, 2015, from http://en.wikipedia.org/w/index.php?title=Alpha_decay&oldid=655144789