Alpha particle

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4.001506179127(62) u
3.727379109(93) GeV/c2[1]

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 helium nucleus, which is produced in the process of alpha decay. The alpha particle can be written as He2+
, 4
or 4
(as it is possible that the ion gains electrons from the environment. Also, electrons are not important in nuclear chemistry). Not all helium nuclei are always considered by all authors as alpha particles. As with beta particles and gamma rays, the name used for the particle carries some mild connotations about its production process and energy.[3]

Some science authors may use doubly-ionized helium nuclei (He2+) and alpha particles as interchangeable terms. Thus, alpha particles may be loosely used as a term when referring to stellar helium nuclei reactions (for example the alpha processes), and even when they occur as components of cosmic rays. However, helium nuclei produced by particle accelerators (cyclotrons, synchrotrons, and the like) are less likely to be referred to as "alpha particles" because the high energies produced by these sources highlights the striking difference in behavior of their particles from the classical alpha particles produced (and originally defined by) the process of radioactive alpha decay.

Alpha particles, like helium nuclei, have a net spin of zero, and (due to the mechanism of their production in nuclear decay) classically a total energy of about 5 MeV. They are a highly ionizing form of particle radiation, and (when resulting from radioactive alpha decay) have low penetration depth. They are able to be stopped by a few centimeters of air, or by the skin. However, as noted, the helium nuclei which form 10-12% of cosmic rays are usually of much higher energy than those produced by radioactive decay, and are thus capable of being highly penetrating, able to traverse the human body and also many meters of dense solid shielding, depending on their energy.

When alpha particle emitting isotopes are ingested, they are far more dangerous than their half life or decay rate would suggest, due to the high relative biological effectiveness of alpha radiation to cause biological damage, after alpha emitting radioisotopes enter living cells. Ingested alpha emitter radioisotopes (such as transuranics or actinides) are an average of about 20 times more dangerous, and in some experiments up to 1000 times more dangerous, than an equivalent activity of beta emitting or gamma emitting radioisotopes.

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