Alpha Decay

Alpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (two protons and two neutrons) and thereby transforms (or 'decays') into an atom with a mass number 4 less and atomic number 2 less. For example:

238U → 234Th + α

Because an alpha particle is the same as a helium-4 nucleus, which has mass number 4 and atomic number 2, this can also be written as:

92U → 234
90Th + 4

The alpha particle also has a charge +2, but the charge is usually not written in nuclear equations, which describe nuclear reactions without considering the electrons. This convention is not meant to imply that the nuclei necessarily occur in neutral atoms.

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 energy and 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 the electromagnetic 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) of tellurium (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).

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.

Most of the helium produced on Earth (approximately 99% of it) 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.

Read more about Alpha Decay:  History, Uses, Toxicity

Other articles related to "alpha decay, alpha, decay, decays":

Alpha Decay - Toxicity
... Being relatively heavy and positively charged, alpha particles tend to have a very short mean free path, and quickly lose kinetic energy within a short distance of their ... In general, external alpha radiation is not harmful since alpha particles are effectively shielded by a few centimeters of air, a piece of paper, or the thin layer of ... Even touching an alpha source is usually not harmful, though many alpha sources also are accompanied by beta-emitting radio daughters, and alpha emission is also accompanied by gamma photon ...
... Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus ... They are generally produced in the process of alpha decay, but may be produced also in other ways ... Alpha particles are named after the first letter in the Greek alphabet, α ...
... has a half-life of 8.1 hours the longest-lived isotope existing in naturally occurring decay chains is 219At with a half-life of 56 seconds ... Alpha decay characteristics for sample astatine isotopes Mass number Mass excess Mass excess of daughter Average energy of alpha decay Half-life Probability ... internal energy), making the former likely to decay into the latter ...
Cluster Decay
... Cluster decay (also named heavy particle radioactivity or heavy ion radioactivity) is a type of nuclear decay in which a parent atomic nucleus with A nucleons and Z protons emits a cluster of Ne neutrons ... For example 223 88Ra → 14 6C + 209 82Pb This type of rare decay mode was observed in radioisotopes that decay predominantly by alpha emission, and it occurs only in a small percentage of the decays ... The branching ratio with respect to alpha decay is rather small (see the Table below) ...
Actinide - Isotopes
... Actinium-228 is a member of radioactive thorium series formed by the decay of 228Ra it is a β– emitter with a half-life of 6.15 hours ... The most important are 241Am and 243Am, which are alpha-emitters and also emit soft, but intense γ-rays both of them can be obtained in an isotopically pure form ... Its alpha radiation is rather weak (1.45×10−3% with respect to β-radiation), but is sometimes used to detect this isotope ...

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