Alpha particles

Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be produced in other ways. Alpha particles are named after the first letter in the Greek alphabet, α. The symbol for the alpha particle is α or α2+. Because they are identical to helium nuclei, they are also sometimes written as He2+ or 4 2He2+ indicating a helium ion with a +2 charge (missing its two electrons). If the ion gains electrons from its environment, the alpha
particle becomes a normal (electrically neutral) helium atom4
2He.Alpha particles, like helium nuclei, have a net spin of zero. Due to the mechanism of their production in standard alpha radioactive decay, alpha particles generally have a kinetic energy of about 5 MeV and a velocity in the vicinity of 5% the speed of light. (See discussion below for the limits of these figures in alpha decay.) They are a highly ionizing form of particle radiation, and (when resulting from a radioactive alpha 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, so-called long-range alpha particles from ternary fission are three times as energetic and penetrate three times as far. As noted, the helium nuclei that form 10–12% of cosmic rays are also usually of much higher energy than those produced by nuclear decay processes and are thus capable of being highly penetrating and able to traverse the human body and also many meters of dense solid shielding, depending on their energy. To a lesser extent, this is also true of very high-energy helium nuclei produced by particle accelerators.
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. Alpha radiation is an average of about 20 times more dangerous, and in experiments with inhaled alpha emitter up to 1000 times more dangerous, than an equivalent activity of beta emitting or gamma emitting radioisotopes.

Name



 Some science authors use doubly ionized helium nuclei (He2+ ) and alpha particles as interchangeable terms. The nomenclature is not well defined, and thus not all high-velocity helium nuclei are considered by all authors to be alpha particles. As with beta and gamma particles/rays, the name used for the particle carries some mild connotations about its production process and danger, but these are not rigorously applied. 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.
 A higher energy version of alphas than
produced in alpha decay is a common product of an uncommon nuclear fission result called ternary fission. However, helium nuclei
produced by particle accelerators (cyclotrons, synchrotrons, and the like) are less likely to be referred to as "alpha particles".

alpha decay
The best-known source of alpha particles is alpha decay of heavier (> 106 u atomic weight) atoms. When an atom emits an alpha particle alpha decay, the atom's mass number decreases by four due to the loss of the four nucleons in the alpha particle. The atomic number of
the atom goes down by exactly two, as a result of the loss of two
protons – the atom becomes a new element. Examples of this sort of nuclear transmutation are when uranium becomes thorium, or radium
becomes radon gas, due to alpha decay.
Alpha particles are commonly emitted by all of the larger radioactive nuclei such as uranium, thorium, actinium, and radium, as well as the transuranic elements. Unlike other types of decay, alpha decay as a process must have a minimum-size atomic nucleus that can support it. 

The smallest nuclei that have to date been found to be capable of
alpha emission are beryllium-8 and the lightest nuclides of tellurium (element 52), with mass numbers between 106 and 110. The process of alpha decay sometimes leaves the nucleus in an excited state, wherein the emission of a gamma ray then removes the excess energy.


Applications

Some smoke detectors contain a small amount of the alpha emitter americium-241. The alpha particles ionize air
within a small gap. A small current is passed through that ionized air. Smoke particles from a fire that enter the air gap
reduce the current flow, sounding the alarm. The 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.
Alpha decay can provide a safe power source for radioisotope thermoelectric generators used for space probes and
artificial heart pacemakers. Alpha decay is much more easily shielded against than other forms of radioactive decay. 
Plutonium-238, a source of alpha particles, requires only 2.5 mm of lead ad shielding to protect against unwanted
radiation.
Static eliminators typically use polonium-210, an alpha emitter, to ionize air, allowing the "static cling" to more rapidly
dissipate.
Researchers are currently trying to use the damaging nature of alpha-emitting radionuclides inside the body by
directing small amounts towards a tumor. The alphas damage the tumor and stop its growth, while their small
penetration depth prevents radiation damage to the surrounding healthy tissue. This type of cancer therapy is called
unsealed source radiotherapy

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