Localized disease that causes decay and cavities in teeth. It begins at the tooth's surface and may penetrate the dentin and the pulp cavity. Microorganisms in the mouth are believed to consume sugars and produce acids that eat away at tooth enamel. The dentin's protein structure is then destroyed by enzymes. Diet, general health, structural tooth defects, and heredity affect the risk of having caries. Prevention involves avoiding excessive sweets, brushing and flossing the teeth, and having regular dental care. Treatment includes restoration of teeth with cavities. Fluoridation of water can reduce the occurrence of caries by as much as 65percnt.
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Property exhibited by certain types of matter of emitting radiation spontaneously. The phenomenon was first reported in 1896 by Henri Becquerel for a uranium salt, and it was soon found that all uranium compounds are radioactive due to the uranium's radioactivity. In 1898 Marie Curie and her husband discovered two other naturally occurring, strongly radioactive elements, radium and polonium. The radiation is emitted by unstable atomic nuclei (see nucleus) as they attempt to become more stable. The main processes of radioactivity are alpha decay, beta decay, and gamma decay. In 1934 it was discovered that radioactivity could be induced in ordinary matter by artificial transmutation.
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Type of radioactivity in the most common form of which an unstable atomic nucleus dissipates energy by gamma emission, producing gamma rays. Gamma decay also includes two other processes, internal conversion and internal pair production. In internal conversion, excess energy in a nucleus is transferred to one of its own orbiting electrons and the electron is ejected from the atom. In internal pair production, excess energy is converted into an electron and a positron, which are emitted together. Typical half-lives (see half-life) for gamma emission range from about 10−9 to 10−14 second.
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Any of three processes of radioactive disintegration in which a beta particle is spontaneously emitted by an unstable atomic nucleus in order to dissipate excess energy. Beta particles are either electrons or positrons. The three beta-decay processes are electron emission, positron emission, and electron capture. The process of beta decay increases or decreases the positive charge of the original nucleus by one unit without changing the mass number. Though beta decay is in general a slower process than gamma or alpha decay, beta particles can penetrate hundreds of times farther than alpha particles. Beta decay half-lives are a few milliseconds or more. Seealso radioactivity.
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Type of radioactive disintegration (see radioactivity) in which some unstable atomic nuclei dissipate excess energy by spontaneously ejecting an alpha particle. Alpha particles have two positive charges and a mass of four atomic mass units; they are identical to helium nuclei. Though they are emitted at speeds about one-tenth that of light, they are not very penetrating and have ranges in air of about 1–4 in. (2.5–10 cm). Alpha decay commonly occurs in elements with atomic numbers greater than 83 (bismuth), but can occur in some rare-earth elements in the atomic-number range of 60 (neodymium) to 71 (lutetium). Alpha decay half-lives range from about a microsecond (10−6 second) to billions of years (1017 seconds).
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