Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 84

Alpha decay (α-decay) is the radioactive emission of an α-particle which is the nucleus of helium-4 (⁴He), consisting of two protons and two neutrons which is a stable nucleus as it is doubly magic. The daughter nucleus has two protons and four nucleons fewer than the parent nucleus. In general, alpha decay leads to the ground state of the daughter nucleus so that the emitted particle carries away as much energy as possible.

By way of definition, a nucleon is either a proton or a neutron considered in its role as a component of an atomic nucleus. The number of nucleons in a nucleus defines the mass number (the nucleon number) of an isotope. In fact, protons and neutrons are best known in their role as nucleons, i.e., as the components of atomic nuclei, but they also exist as free particles. Free neutrons are unstable, with a half-life of approximately 13 minutes, but they have important applications. Protons not bound to other nucleons are the nuclei of hydrogen atoms when bound with an electron or – if not bound to anything – are ions or cosmic rays. Both the proton and the neutron are composite particles insofar as each is composed of smaller parts and, thus, neither the proton nor the neutron is an elementary particle.

A pertinent example from the area of nuclear energy is the decay of uranium-238 (²³⁸U) to thorium-234 (²³⁴Th). An alpha particle (also called alpha rays or alpha radiation) has a charge of +2e, but, as a nuclear equation, describes a nuclear reaction without considering the electrons and consists of two protons and two neutrons bound together into a particle identical to a helium-4 (⁴He) nucleus and are typically the product of the alpha decay process. The symbol for the alpha particle is α or α²⁺, and because alpha particles are identical to helium nuclei, they are also sometimes written as He²⁺ or 4₂He²⁺ which indicates a helium ion with a +2 charge (missing two electrons). Once the ion gains electrons from its environment, the alpha particle becomes a normal (electrically neutral) helium atom ⁴₂He.

Alpha decay typically occurs in the heaviest nuclides, and, theoretically, it can occur only in nuclei somewhat heavier (higher atomic number) than nickel atomic (number = element 28), where the overall binding energy per nucleon is no longer a minimum and the nuclides are therefore unstable toward spontaneous fission-type processes. In practice, this mode of decay has only been observed in nuclides considerably heavier than nickel, with the lightest known alpha emitters being the lightest isotopes (mass numbers 104 to 109) of tellurium (atomic number = element 52).

When an atom emits an alpha particle in alpha decay, the mass number of the atom decreases by four due to the loss of the four nucleons in the alpha particle. The atomic number of the atom decreases by two, as a result of the loss of two protons, and the atom becomes a new element. Also, unlike other types of decay, alpha decay as a process must have a minimum-size atomic nucleus that can support it. The process may leave the nucleus in an excited state after which the emission of a gamma ray then removes the excess energy.

See also: Beta Decay, Gamma Decay, Nuclear Energy.