Alpha Decay

Alpha Decay


Alpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particleand thereby transforms (or 'decays') into an atom with a mass number 4 less and atomic number2 less. For example:[1]

 [http://en.wikipedia.org/wiki/Uranium-238 238 92 U] → [http://en.wikipedia.org/wiki/Thorium-234 234 90 Th] + 4 4 2 He2+ ]

which can also be written as:

 238 U → 234 Th + α.

An alpha particle is the same as a helium-4 nucleus, which has mass number 4 and atomic number 2.

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 (in nuclear fission, a number of different pairs of daughters of approximately equal size are formed). Alpha decay is the most likely cluster decay because of the combined extremely high binding energyand relatively small mass of the helium-4 product nucleus (the alpha particle).

Alpha decay, like other cluster decays, is fundamentally a quantum tunneling process. Unlike beta decay, alpha decay is governed by the interplay between the nuclear force and theelectromagnetic force.

Alpha decay typically occurs in the heaviest nuclides. In theory it can occur only in nuclei somewhat heavier than nickel (element 28), where 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) oftellurium (element 52).

Alpha particles have a typical kinetic energy of 5 MeV (that is, ≈0.13% of their total energy, i.e. 110 TJ/kg) and a speed of 15,000 km/s. This corresponds to a speed of around 0.05 c. 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).

<p style="margin-top: 0.4em; margin-bottom: 0.5em; line-height: 19px; color: rgb(0, 0, 0); font-family: sans-serif;">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, so their forward motion is effectively stopped within a few centimeters of air.

History
Alpha particles were first described in the investigations of radioactivity by Ernest Rutherford in 1899, and by 1907 they were identified as He2+ ions. For more details of this early work, see Alpha particle#History of discovery and use. By 1928, George Gamow had solved the theory of the alpha decay via tunneling. The alpha particle is trapped in a potential well by the nucleus. Classically, it is forbidden to escape, but according to the then newly discovered principles of quantum mechanics, it has a tiny (but non-zero) probability of "tunneling" through the barrier and appearing on the other side to escape the nucleus. Gamow solved a model potential for the nucleus and derived from first principles a relationship between the half-life of the decay, and the energy of the emission, which had been previously discovered empirically, and was known as the Geiger–Nuttall law.[2]

Uses
Americium-241, an alpha emitter, is used in smoke detectors. The alpha particles ionize air between a small gap. A small current is passed through that ionized air. Smoke particles from fire that enter the air gap reduce the current flow, sounding the alarm. Alpha decay can provide a safe power source for radioisotope thermoelectric generators used for space probes and artificial heart pacemakers. Alpha decay is much more easily shielded against than other forms of radioactive decay. Plutonium-238, for example, requires only 2.5 millimetres of lead shielding to protect against unwanted radiation. Static eliminators typically use polonium-210, an alpha emitter, to ionize air, allowing the 'static cling' to more rapidly dissipate.