alpha particle

noun Physics.
a positively charged particle consisting of two protons and two neutrons, emitted in radioactive decay or nuclear fission; the nucleus of a helium atom.

Origin:
1900–05

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Collins
World English Dictionary
alpha particle
 
n
a helium-4 nucleus, containing two neutrons and two protons, emitted during some radioactive transformations

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Medical Dictionary

alpha particle n.
A positively charged nuclear particle, indistinguishable from a helium atom nucleus and consisting of two protons and two neutrons.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
alpha particle  
A positively charged particle that consists of two protons and two neutrons bound together. It is emitted by an atomic nucleus undergoing radioactive decay and is identical to the nucleus of a helium atom. Because of their relatively large mass, alpha particles are the slowest and least penetrating forms of nuclear radiation. They can be stopped by a piece of paper. See more at radioactive decay.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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FOLDOC
Computing Dictionary

alpha particle definition


bit rot

The Free On-line Dictionary of Computing, © Denis Howe 2010 http://foldoc.org
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Encyclopedia Britannica
Encyclopedia

alpha particle

positively charged particle, identical to the nucleus of the helium-4 atom, spontaneously emitted by some radioactive substances, consisting of two protons and two neutrons bound together, thus having a mass of four units and a positive charge of two. Discovered and named (1899) by Ernest Rutherford, alpha particles were used by him and coworkers in experiments to probe the structure of atoms in thin metallic foils. This work resulted in the first concept of the atom as a tiny planetary system with negatively charged particles (electrons) orbiting around a positively charged nucleus (1909-11). Later, Rutherford bombarded nitrogen with alpha particles, changing it to oxygen, in the first artificially produced nuclear transmutation (1919). Today, alpha particles are produced for use as projectiles in nuclear research by ionization-i.e., by stripping both electrons from helium atoms-and then accelerating the now positively charged particle to high energies

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
The beryllium absorbs the alpha particle and emits a neutron.
The main reason that an alpha particle is less penetrating is that it is much
  larger than a beta particle.
Make an alpha particle with two neutrons and two protons.
The alpha particle is identical to the nucleus of a helium atom.
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