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The early work on natural radioactivity associated with uranium and thorium ores identified two distinct types of radioactivity: alpha and beta decay. The proton is the nucleus of ordinary hydrogen and is stable.
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It is rapidly captured by nuclei in matter otherwise, in free space it will undergo beta-minus decay to a proton, an electron, and an antineutrino with a half-life of 12.8 minutes. The neutron cannot long exist in the free state. Neutrons and protons are, of course, the basic building blocks of complex nuclei, having approximately unit mass on the atomic scale and having zero charge or unit positive charge, respectively. Fission fragments are themselves complex nuclei with usually between one-third and two-thirds the charge Z and mass A of the parent nucleus. In the less common forms of radioactivity, fission fragments, neutrons, or protons may be emitted.
Beta radioactivity also produces the neutrino and antineutrino, particles that have no charge and very little mass, symbolized by ν and ν, respectively. Gamma rays are electromagnetic radiations such as radio waves, light, and X-rays.
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The beta plus particle, also called the positron, is the antiparticle of the electron when brought together, two such particles will mutually annihilate each other. The beta minus particle is actually an electron created in the nucleus during beta decay without any relationship to the orbital electron cloud of the atom. Beta particles may be negatively charged (beta minus, symbol e −), or positively charged (beta plus, symbol e +). The neutral helium atom has two electrons outside its nucleus balancing these two charges.
The alpha particle is actually the nucleus of a helium-4 atom, with two positive charges 4 / 2He. The emissions of the most common forms of spontaneous radioactive decay are the alpha (α) particle, the beta (β) particle, the gamma (γ) ray, and the neutrino. The process continues until a stable nuclide has been formed. The product of a radioactive decay process-called the daughter of the parent isotope-may itself be unstable, in which case it, too, will decay. Half-lives range from more than 10 24 years for some nuclei to less than 10 −23 second ( see below Rates of radioactive transitions). The rate at which a radioactive element decays is expressed in terms of its half-life i.e., the time required for one-half of any given quantity of the isotope to decay. Radioactive decay is a property of several naturally occurring elements as well as of artificially produced isotopes of the elements. It is, in essence, an attribute of individual atomic nuclei.Īn unstable nucleus will decompose spontaneously, or decay, into a more stable configuration but will do so only in a few specific ways by emitting certain particles or certain forms of electromagnetic energy. Radioactivity, property exhibited by certain types of matter of emitting energy and subatomic particles spontaneously.
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