Alpha decay - Revision history

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[[File:Alpha Decay.svg|thumb|240px|right|Visual representation of alpha decay]]
{{Nuclear physics}}
'''Alpha decay''', or α-decay, is a type of [[radioactivity|radioactive decay]] in which an [[atomic nucleus]] emits an [[alpha particle]] and thereby transforms (or 'decays') into an atom with a [[mass number]] 4 less and [[atomic number]] 2 less. For example, [[uranium-238]] decaying through α-particle emission to form [[thorium-234]] can be expressed as:<ref name="suchocki">Suchocki, John. ''Conceptual Chemistry'', 2007. Page 119.</ref>

:<math>\mathrm{~^{238}_{92}U}\rightarrow\mathrm{~^{234}_{90}Th} + {\alpha }</math>

Because an [[alpha particle]] is the same as the nucleus of a [[helium-4]] atom - consisting of two [[proton]]s and two [[neutrons]] and thus having [[mass number]] 4 and [[atomic number]] 2 - this can also be written as:

:<math>\mathrm{~^{238}_{92}U}\rightarrow\mathrm{~^{234}_{90}Th} + \mathrm{~^{4}_{2}He}</math>

Notice how, on either side of the nuclear equation, both the mass number and the atomic number are conserved: the mass number is 238 on the right side and (234 + 4) on the left side, and the atomic number is 92 on the right side and (90 + 2) on the left side.

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 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 [[isotope]]s (mass numbers 106–110) of [[tellurium]] (element 52).

Alpha decay is by far the most common form of [[cluster decay]] where the parent [[atom]] ejects a defined [[decay product|daughter]] collection of [[nucleon]]s, 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 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 ''[[Speed of light|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 [[mineral]]s containing [[uranium]] or [[thorium]]. The helium is brought to the surface as a byproduct of [[natural gas]] production.

==History==
[[Image:Alphaspektroskopie.JPG|thumb|right|Alpha source beneath a radiation detector]]
Alpha particles were first described in the investigations of radioactivity by [[Ernest Rutherford]] in 1899, and by 1907 they were identified as He<sup>2+</sup> 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 "[[quantum tunnelling|tunneling]]" through the [[potential barrier|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]].<ref>[http://www.phy.uct.ac.za/courses/phy300w/np/ch1/node38.html For Gamow's derivation of this law, see]</ref>

==Uses==
[[Americium-241]], an [[alpha emitter]], is used in [[smoke detector]]s. The alpha particles [[Ionization|ionize]] air between a small gap. A small [[Electric current|current]] is passed through that ionized air. Smoke particles from fire that enter the air gap reduce the current, sounding the alarm.

Alpha decay can provide a safe power source for [[radioisotope thermoelectric generator]]s used for [[space probe]]s<ref>{{cite web |url=http://solarsystem.nasa.gov/rps/rtg.cfm |title=Radioisotope Thermoelectric Generator |work=Solar System Exploration |publisher=[[NASA]] |accessdate=25 March 2013}}</ref> and [[Artificial pacemaker|artificial heart pacemakers]].<ref>{{cite web |url=http://osrp.lanl.gov/pacemakers.shtml |title=Nuclear-Powered Cardiac Pacemakers |work=Off-Site Source Recovery Project |publisher=[[Los Alamos National Laboratory|LANL]] |accessdate=25 March 2013}}</ref> 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 eliminator]]s typically use [[polonium-210]], an alpha emitter, to ionize air, allowing the 'static cling' to more rapidly dissipate.

==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 source. This results in several [[MeV]] being deposited in a relatively small volume of material. This increases the chance of cellular damage in cases of internal contamination. 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 dead skin cells that make up the [[epidermis (skin)|epidermis]]. Even touching an alpha source is usually not harmful, though many alpha sources also are accompanied by [[beta decay|beta-emitting]] radio daughters, and alpha emission is also accompanied by gamma photon emission. If substances emitting alpha particles are ingested, inhaled, injected or introduced through the skin, then it could result in a measurable [[Equivalent dose|dose]].

The [[relative biological effectiveness]] (RBE) of alpha radiation is higher than that of beta or gamma radiation. RBE quantifies the ability of radiation to cause certain biological effects, notably either [[cancer]] or [[necrosis|cell-death]], for equivalent radiation exposure. The higher value for alpha radiation is generally attributable to the high [[linear energy transfer]] (LET) coefficient, which is about one ionization of a chemical bond for every [[angstrom]] of travel by the alpha particle. The RBE has been set at the value of 20 for alpha radiation by various government regulations. The RBE is set at 10 for [[neutron]] irradiation, and at 1 for [[Beta decay|beta radiation]] and ionizing photons.

However, another component of alpha radiation is the [[recoil]] of the parent nucleus, termed alpha recoil. Due to the [[conservation of momentum]] requiring the parent nucleus to recoil, the effect acts much like the 'kick' of a rifle butt when a bullet goes in the opposite direction. This gives a significant amount of energy to the recoiling nucleus, which also causes ionization damage (see [[ionizing radiation]]). The total energy of the recoil nucleus is readily calculable, and is roughly the weight of the alpha (4 [[Atomic mass unit|u]]) divided by the weight of the parent (typically about 200 u) times the total energy of the alpha. By some estimates, this might account for most of the internal radiation damage, as the recoil nuclei are typically [[heavy metal (chemistry)|heavy metals]] which preferentially collect on the [[chromosome]]s. In some studies,<ref>
{{cite journal
|author=Winters TH, Franza JR
|year=1982
|title=Radioactivity in Cigarette Smoke
|journal=[[New England Journal of Medicine]]
|volume=306 |issue=6 |pages=364–365
|doi=10.1056/NEJM198202113060613
}}</ref> this has resulted in a RBE approaching 1,000 instead of the value used in governmental regulations.

The largest natural contributor to public radiation dose is [[radon]], a naturally occurring, radioactive gas found in soil and rock.<ref>[http://www.ans.org/pi/resources/dosechart/ ANS : Public Information : Resources : Radiation Dose Chart]</ref> If the gas is inhaled, some of the radon particles may attach to the inner lining of the lung. These particles continue to decay, emitting alpha particles which can damage cells in the lung tissue.<ref>EPA Radiation Information: Radon. October 6, 2006, [http://www.epa.gov/radiation/radionuclides/radon.htm], Accessed December 6, 2006</ref> The death of [[Marie Curie]] at age 66 from [[leukemia]] was probably caused by prolonged exposure to high doses of ionizing radiation, but it is not clear if this was due to alpha radiation or X-rays. Curie worked extensively with radium, which decays into radon,<ref>Health Physics Society, "Did Marie Curie die of a radiation overexposure?" [http://www.hps.org/publicinformation/ate/q535.html]</ref> along with other radioactive materials that emit [[beta decay|beta]] and [[gamma ray]]s. However, Curie also worked with unshielded X-ray tubes during World War I, and analysis of her skeleton during a reburial showed a relatively low level of radioisotope burden.

Russian dissident [[Alexander Litvinenko]]'s 2006 murder by [[radiation poisoning]] is thought to have been carried out with [[polonium-210]], an alpha emitter.

== See also ==
*[[Alpha particle]]
*[[Beta decay]]

==References==
{{reflist}}
*[http://www.ct.infn.it/~rivel/Didat/SilDet.pdf Alpha emitters by increasing energy (Appendix 1)]

== External links ==
* [[Image:Ndslivechart.png]] '''[http://www-nds.iaea.org/livechart The LIVEChart of Nuclides - IAEA ]''' with filter on alpha decay
* [http://nagysandor.eu/AsimovTeka/AlphaExamples/index_en.html Alpha decay with 3 animated examples] showing the recoil of daughter

{{Nuclear processes}}

{{DEFAULTSORT:Alpha Decay}}
[[Category:Nuclear physics]]
[[Category:Radioactivity]]

{{Link FA|hr}}