alpha decay

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

{}^2{}^{38}_{92}\hbox{U}\;\to\;{}^2{}^{34}_{90}\hbox{Th}\;+\;{}^4_2\hbox{He}^{2+},

although this is usually written as:

{}^{238}\hbox{U}\;\to\;^{234}\hbox{Th}\;+\;\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 cation.)

An alpha particle is a helium nucleus, and 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 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 velocity of around 0.05c. Even so, they are often absorbed within a few centimetres of air.

Because of alpha decay, virtually all of the helium produced on Earth 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.

[edit] History

By 1928, George Gamow had solved the theory of the alpha decay via tunnelling. 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

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, inhaled by, injected into, or introduced through some skin-penetrating object (shrapnel, corrosive chemicals) into an organism it may become a risk, potentially inflicting very serious damage to the organisms' genetic makeup.

One common 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 gas and its associated decay particles stick to the inner lining of the lung. The decay particles that remain after the air is exhaled will continue to decay over time, damaging the lung's sensitive tissue.[1] Shrapnel from depleted uranium poses another such risk of alpha-emitters.

The death of Marie Curie was caused by leukemia from exposure to alpha emitters such as radium during her years of work.

The 2006 death of Russian dissident Alexander Litvinenko is thought to be due to his being poisoned with polonium-210, an active alpha emitter.