Alpha decay
From Wikipedia, the free encyclopedia
Alpha decay is a type of radioactive decay in which an atom emits an alpha particle and transforms (or decays) into an atom with a mass number 4 less and atomic number 2 less. For example:
although this is typically written as:
(The second form is preferred because the first form appears electrically unbalanced. Fundamentally, the recoiling nucleus is very quickly stripped of two electrons to neutralize the ionized helium cation.)
An alpha particle is identical to a helium nucleus, and both mass number and atomic number are the same. Alpha decay is a form of nuclear fission where the parent atom splits into two daughter products. Alpha decay is fundamentally a quantum tunneling process. Unlike beta decay, alpha decay is governed by the strong nuclear force.
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 0.05c. Because of their relatively large mass, +2 charge and relatively low velocity, they are very likely to interact with other atoms and loose their energy, so they are effectively absorbed within a few centimetres of air.
Most of the helium produced on Earth comes from the alpha decay of underground deposits of minerals containing uranium or thorium. The helium is brought to the surface as a by-product of natural gas production.
[edit] History
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.
[edit] Toxicity
As any heavy charged particle, alpha particles lose their energy within a very short distance in dense media, causing significant damage to surrounding biomolecules. Generally, external alpha irradiation is not harmful because alpha particles are completely absorbed by the thin layer of dead skin cells in the outermost layer of the skin as well as by a few centimeters of air. However, if a substance radiating alpha particles is ingested, inhaled by, injected into, or introduced through the skin (shrapnel, corrosive chemicals) into an organism it may become a risk, potentially inflicting very serious cellular damage.
A common household source of alpha radiation is radon, a naturally occurring, radioactive gas found in soil, rock and sometimes groundwater. When radon gas is inhaled, some of the radon particles stick to the inner lining of the lung. The particles that remain continue to decay over time, emitting alpha particles which may damage cells in the lung tissue.[1] Shrapnel deposited in the body from depleted uranium poses another such internal risk of alpha particle radiation dose.
The death of Marie Curie at age 66 from leukemia, was likely caused from radiation exposure, such as alpha particles from radium, during her years of important research on the subject.
The 2006 assassination of Russian dissident Alexander Litvinenko is thought to have been caused by poisoning with polonium-210, an active alpha emitter.
