All nuclei with the atomic number Z>82, are considered unstable. These are considered “neutron-rich” and undergo the decay process by emitting a particle containing two neutrons and two protons.
Alpha decay is the process in which an alpha particle (containing two neutrons and two protons) is ejected from the nucleus. An alpha particle is identical to the nucleus of a helium atom.
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 a Rn (radon) nucleus, as described in the (Fig. 1: Alpha decay). 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 the atomic number 86 and the mass number 222 from Ra-226 with the atomic number 88 and the mass number 226, as shown in Fig. 1.
Alpha particles are very heavy and contain high amounts of energy (4-10 MeV). Their speed is ~20000km/s and they interact with matter, causing much ionisation 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 ionising radiation.
Historically, radium and radon were the principal alpha emitters of medical interest. These are no longer used in medicine. 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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