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Alpha decay (a.k.a. α decay) is the radioactive process in which an alpha (α) particle (containing two neutrons and two protons) is ejected from the nucleus. An alpha particle is identical to the nucleus of a helium atom. All nuclei with an atomic number (Z) greater than 82, are considered unstable. These are considered “neutron-rich” and undergo alpha decay commonly.
Alpha decay occurs in the nuclei of heavy elements, like radium, uranium, thorium, etc. When a nucleus of Ra (radium) decays, it emits an alpha particle and becomes an Rn (radon) nucleus. In general, during alpha decay, the atomic number (Z) is reduced by two, and the mass number (A), by four. For example, alpha decay generates Rn-222 with atomic number 86 and the mass number 222 from Ra-226 with the atomic number 88 and the mass number 226.
The total amount of energy emitted in Alpha decay is shared between the recoil daughter nuclide, the alpha particle, and gamma rays. The amount of kinetic energy possessed by the Alpha particle depends upon the ratio of the mass of the Alpha particle with the mass of the recoil daughter nuclide 4. Alpha particles are very heavy (when compared with beta particles) and contain high amounts of energy (4-10 MeV).
The Alpha particle speed is ~20000 km/s and they interact with matter, causing much ionization over a very short distance. They usually pass short distances (a 5 MeV alpha particle will travel about 20 micrometers in silicon) and can be stopped by a sheet of paper. Alpha particles do not produce Bremsstrahlung radiation when slowing down.
Alpha particles are not generally dangerous unless the source is ingested or inhaled since alpha radiation is the most destructive form of ionizing radiation.
Historically, radium and radon were the principal alpha emitters of medical interest. Radium-223 dichloride is still used today in treating osseous metastases. Other alpha emitters are being researched for therapeutic approaches using radiopharmaceuticals that can target the delivery of short half-life alpha emitters into cancerous cells. Due to their very short range, alpha particles have the potential to deliver a lethal radiation dose to small metastatic cell clusters, while mostly sparing the surrounding tissue. All work with alpha emitters must be conducted under very strictly controlled conditions.
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