alpha particle

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al·pha par·ti·cle (α),

a particle consisting of two neutrons and two protons, with a positive charge (2e+); emitted energetically from the nuclei of unstable isotopes of high atomic number (elements of mass number from 82 up); identical to the helium nucleus.
Synonym(s): alpha ray
Farlex Partner Medical Dictionary © Farlex 2012

alpha particle

A radioactive decay product, 4He nucleus, composed of two protons and two neutrons (the same as the nucleus of a helium-4 atom) with marked ionising capacity (3–9 million electron-volts) but a short range (3–9 cm in air, 25–40 µm in water/soft tissue), derived from alpha decay, which are created by the decay of a radioactive material or from nuclear bombardment. APs arising from radon, uranium and plutonium “daughters” are implicated in inhalation-induced neoplasia of the respiratory tract.

While alpha particles are highly tissue-destructive, they travel only short distances and are blocked by a thick piece of paper or skin; an AP is essentially a helium atom nucleus and generally carries more energy than gamma or beta radiation, depositing that energy very quickly while passing through tissue. Alpha particles cannot penetrate the outer, dead layer of skin; they therefore do not cause damage to living tissue when outside the body. When inhaled or ingested, however, APs are especially damaging because they transfer relatively large amounts of ionising energy to living cells.
Segen's Medical Dictionary. © 2012 Farlex, Inc. All rights reserved.

alpha particle

a type of subatomic particle found in the atomic nucleus.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005

al·pha par·ti·cle

(alfă pahrti-kĕl)
A particle consisting of two neutrons and two protons, with a positive charge; emitted energetically from the nuclei of unstable isotopes of mass number 82 and up.
Synonym(s): alpha ray.
Medical Dictionary for the Dental Professions © Farlex 2012
References in periodicals archive ?
Thus an alpha particle (a helium nucleus) had four times the mass of a proton but a positive charge only twice that of a proton.
The helium nucleus breaks and produces one hydrogen nucleus (a proton) and an atom of hydrogen's radioactive isotope, tritium.
Physicists had not yet straightened out the matter in detail, but in 1915 the American chemist William Draper Harkins (1875-1951) noted that the helium nucleus was not quite four times as massive as the hydrogen nucleus.
However, in one of every 10 billion collisions, the deuterons fused to form a helium nucleus and a neutral pion.
Ramayya and his coworkers have also found that californium-252 sometimes breaks into three fragments--two of moderate size and one as small as a helium nucleus (or alpha particle).
The researchers meticulously recorded the emission of four alpha-particles, each a helium nucleus with two protons and two neutrons.
They studied transitions from one energy level to another in a helium atom in which one electron has been excited so that it tends to remain much farther from the helium nucleus than it would in its ground state.
Luckily, the resulting polonium-214 nucleus decays almost immediately by emitting a telltale alpha particle (helium nucleus).