- Alpha particle
Alpha particles (named after and denoted by the first letter in the
Greek alphabet, α) consist of two protons and two neutrons bound together into a particle identical to a heliumnucleus; hence, it can be written as He2+ or 42He2+. They are a highly ionizing form of particle radiation, and have low penetration. The alpha particlemass is 6.644656×10-27 kg, which is equivalent to the energy of 3.72738 GeV. The charge of an alpha particle is equal to +2e, where e is the magnitude of charge on an electron, e=1.602176462x10-19C.
Alpha particles are emitted by
radioactivenuclei such as uranium, thorium, actinium, or radiumin a process known as alpha decay. This sometimes leaves the nucleus in an excited state, with the emission of a gamma rayremoving the excess energy. In contrast to beta decay, alpha decay is mediated by the strong nuclear force. In classical physics, alpha particles do not have enough energy to escape the potential of the nucleus. However, the quantum tunnelling effect allows them to escape.
When an alpha particle is emitted, the
atomic massof an element goes down by about 4.0015 atomic mass units, due to the loss of two neutronsand two protons. The atomic numberof the atom goes down by exactly two, as a result of the loss of two protons - the atom becomes a new element. Example of this are when uraniumbecomes thorium, or radiumbecomes radongas due to alpha decay.
The energy of alpha particles varies, with higher energy alpha particles being emitted from larger nuclei, but most alpha particles have energies of between 3.0 and 7.0
MeV(million electron-volts). This is a substantial amount of energy for a single particle, but their high mass (four a.m.u.) means alpha particles have a lower speed (with a typical kinetic energy of 5.0 MeV the speed is 15,000 km/s) than any other common type of radiation (β particles, γ rays, neutrons, etc). Because of their charge and large mass, alpha particles are easily absorbed by materials, and they can travel only a few centimeters in air. They can be absorbed by tissue paper or the outer layers of human skin (about 40 micrometres, equivalent to a few cells deep) and so are not generally dangerous to life unless the source is ingested or inhaled. Because of this high mass and strong absorption, however, if alpha radiation does enter the body (most often because radioactive material has been inhaled or ingested), it is the most destructive form of ionizing radiation. It is the most strongly ionizing, and with large enough doses can cause any or all of the symptoms of radiation poisoning. It is estimated that chromosomedamage from alpha particles is about 100 times greater than that caused by an equivalent amount of other radiation. The alpha emitter polonium-210is suspected of playing a role in lung cancerand bladder cancerrelated to tobacco smoking.cite journal
last = Radford
first = Edward P.
coauthors = Vilma R. Hunt
title = Polonium-210: A Volatile Radioelement in Cigarettes
journal = Science
volume = 143
issue = 3603
pages = 247–249
date = January 17, 1964
url = http://www.sciencemag.org/cgi/content/abstract/143/3603/247
doi = 10.1126/science.143.3603.247
accessdate = 2008-05-06
pmid = 14078362 ]
smoke detectors contain a small amount of the alpha emitter americium-241. This isotope is extremely dangerous if inhaled or ingested, but the danger is minimal if the source is kept sealed. Many municipalities have established programs to collect and dispose of old smoke detectors, to keep them out of the general waste stream.
Because alpha particles occur naturally, but can have
energyhigh enough to participate in a nuclear reaction, study of them led to much early knowledge of nuclear physics. Physicist Ernest Rutherfordused alpha particles emitted by Radium bromideto infer that J. J. Thomson's Plum pudding modelof the atom was fundamentally flawed. In Rutherford's gold foil experiment conducted by his students Hans Geigerand Ernest Marsden, a narrow beam of alpha particles was established, passing through very thin (a few hundred atoms thick) gold foil. The alpha particles were detected by a zinc sulfidescreen, which emits a flash of light upon an alpha particle collision. Rutherford hypothesisized that, assuming the "plum pudding" model of the atom was correct, the positively charged alpha particles would be only slightly deflected, if at all, by the dispersed positive charge predicted. It was found that some of the alpha particles were deflected at much larger angles than expected, and some bounced back. Although most of the alpha particles went straight through as expected, Rutherford commented that the few particles that were deflected was akin to shooting a fifteen inch shell at tissue paper only to have it bounce off, again assuming the "plum pudding" theory was correct. It was determined that the atom's positive charge was concentrated in a small area in its center, making the positive charge dense enough to deflect any positively charged alpha particles that came close to what was later termed the nucleus. Note: it was not known at the time that alpha particles were themselves nuclei nor was the existence of protons or neutrons known. Rutherford's experiment led to the Bohr model(named for Niels Bohr) and later the modern wave-mechanical model of the atom.
Rutherford's work also improved on previous measurements of the ratio of an alpha particle's mass to charge, allowing him to deduce that alpha particles were helium nuclei. [The Timetables of Science|pages=411]
In computer technology,
Dynamic random access memory(DRAM) " soft errors" were linked to alpha particles in 1978 in Intel's DRAM chips. The discovery led to strict control of radioactive elements in the packaging of semiconductor materials, and the problem was largely considered to be "solved".
list of alpha emitting materials
** γ (gamma) rays
** δ (delta) rays
** ε (epsilon) rays
*cite book | author=Tipler, Paul; Llewellyn, Ralph | title=Modern Physics (4th ed.) | publisher=W. H. Freeman | year=2002 | id=ISBN 0-7167-4345-0
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