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Radioactivity is the process by which unstable atomic nuclei decay. This process normally produces ionizing radiation with a relatively large amount of energy. This energy can be harnessed in the form of nuclear power, or it can be very dangerous if released by radioactive contamination in the environment.
Atomic nuclei are bound together by the strong nuclear force, which is much stronger than the forces encountered in everyday life, electromagnetism and gravity. It is, however, a very short-range force. As a result, atomic nuclei are held together very strongly in spite of the very strong electromagnetic repulsion of the protons for one another. If an atomic nucleus changes state in some way, a large amount of energy will be released. That is, the energy released will be much greater than that released in a chemical reaction involving a single molecule; chemical reactions rely on the electromagnetic force.
Unstable atomic nuclei can undergo a number of different reactions:
Beta decay: In simple beta decay, a neutron becomes a proton, emitting a beta particle (a high-speed electron). This converts the nucleus to another element, raising its atomic number by one. Alternatively, a proton can become a neutron, emitting a beta plus particle (a high-speed positron). This reaction decreases the atomic number by one. This second reaction is very rare among naturally occurring isotopeIsotopes are atoms of a chemical element whose nuclei have the same atomic number, Z but different atomic weights, A''. The word isotope meaning at the same place comes from the fact that isotopes are located at the same place on the periodic table. The as, but happens for many artificial isotopes. Both reactions normally leave the nucleus in an excited state.
Alpha emission: In alpha emission, a nucleus emits an alpha particleAlpha particles or alpha rays are a form of particle radiation which are highly ionizing and have low penetration. They consist of two protons and two neutrons bound together into a particle that is identical to a helium nucleus, and can be written as He2 (a heliumHelium is a colorless, odorless, tasteless chemical element, one of the noble gases of the periodic table of elements. Its boiling and melting points are the lowest among the elements; except in extreme conditions, it exists only as a gas. The second most-4 nucleus). This changes both the atomic number and the number of neutrons in the nucleus, and it normally leaves the nucleus in an excited state.
Gamma emission: a nucleus transitions from an excited stateAn excited state of an atom, molecule or nucleus possesses more energy than the ground state. The lifetime of an atom, molecule or nucleus in an excited state is usually short. Molecules return to the ground state from an excited state by releasing energy to a lower-energy state, emitting a gamma rayThis article is about electromagnatic radiation. For the power metal band, see Gamma Ray (band Gamma rays (often denoted by the Greek letter gamma, γ) are an energetic form of electromagnetic radiation (see Electromagnetic spectrum) produced by radi (a high-energy photonFor the Japanese anime video, see Photon (anime). In physics, the photon (from Greek φοτος, meaning light is a quantum of excitation of the quantised electromagnetic field and is one of the elementary particles studied by qu).
Internal conversionInternal conversion is the act of returning from an excited state by an atom or molecule. This is an alternative to gamma-ray emission, which is the usual method for an excited nucleus to return to the ground state. In internal conversion, the excited nuc: an excited nucleus transfers its energy to a tightly bound electron that is ejected from the atom. Characteristic X-Rays are subsequently emitted as electrons from outer orbits fill the vacant spaces.
Fission: a nucleus breaks into two smaller nuclei, and possibly some fast neutrons, beta particles, alpha particles, and gamma rays. Fission is normally quite rare unless the nucleus has absorbed a neutron. Since fission releases neutrons itself, this allows (under suitable conditions) a nuclear chain reaction which leads to a very large number of fission events in a short period. This is the process used in nuclear weapons and nuclear reactors.
Many of the atomic nuclei that arise from these processes are themselves unstable, so in large samples of radioactive material, many decay chains are going on simultaneously.
An unstable nucleus waits a random amount of time before it decays; this is a quantum mechanical process. This time is not, however, completely unpredictable. The decay time follows an exponential distribution, and the most useful way to characterize the process is to give the half-life of a nucleus: the time after which there is a 50% chance that it will have decayed. When dealing with a macroscopic amount of radioactive material, the half-life is the time after which half of the material remains in its original form. Half-lives vary from effectively infinite, for stable elements, to hundreds of thousands of years for uranium, to microseconds and less for artificial isotopes.
In the natural world, radioactivity comes from naturally-occurring radioactive isotopes. Like all heavy elements, these come originally from the interiors of stars. Some, such as uranium, were formed directly in stars, and are still present only because their half-lives are so long that they have not yet completely decayed. Radiogenic isotopes, such as carbon-14, are present because they are formed by the decay of longer-lived elements (this is how all the helium currently available was formed: although it is not radioactive, it escapes from the Earth easily, so helium is obtained from underground reservoirs).
Human technology has found uses for many radioactive isotopes. These uses have ranged from nuclear weapons to nuclear power to glowing watch dials to smoke detectors. Most of these isotopes are manufactured in nuclear reactors, which allow the creation of unstable or unusual isotopes through the exposure of stable isotopes to neutrons.
In Unicode, the sign for radioactivity is U+2622 (☢).
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