The radiation produced during radioactivity is predominantly of three types, designated as alpha, beta, and gamma rays. These types differ in velocity, in the way in which they are affected by a magnetic field, and in their ability to penetrate or pass through matter. Other, less common, types of radioactivity are electron capture (capture of one of the orbiting atomic electrons by the unstable nucleus) and positron emission both forms of beta decay and both resulting in the change of a proton to a neutron within the nucleus an internal conversion, in which an excited nucleus transfers energy directly to one of the atom's orbiting electrons and ejects it from the atom. The nuclei of elements exhibiting radioactivity are unstable and are found to be undergoing continuous disintegration (i. E. , gradual breakdown). The disintegration proceeds at a definite rate characteristic of the particular nucleus that is, each radioactive isotope radioactive isotope or radioisotope, natural or artificially created isotope of a chemical element having an unstable nucleus that decays, emitting alpha, beta, or gamma rays until stability is reached. Click the link for more information.
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Has a definite lifetime. However, the time of decay of an individual nucleus is unpredictable. The lifetime of a radioactive substance is not affected in any way by any physical or chemical conditions to which the substance may be subjected. The rate of disintegration of a radioactive substance is commonly designated by its half-life half-life, measure of the average lifetime of a radioactive substance (see radioactivity) or an unstable subatomic particle. One half-life is the time required for one half of any given quantity of the substance to decay.
, which is the time required for one half of a given quantity of the substance to decay. Depending on the element, a half-life can be as short as a fraction of a second or as long as several billion years. The product of a radioactive decay may itself be unstable and undergo further decays, by either alpha or beta emission. Thus, a succession of unstable elements may be produced, the series continuing until a nucleus is produced that is stable. Such a series is known as a radioactive disintegration, or decay, series.
Changing Views of the History of the Earth
The original nucleus in a decay series is called the parent nucleus, and the nuclei resulting from successive disintegrations are known as daughter nuclei. There are four known radioactive decay series, the members of a given series having mass numbers that differ by jumps of 9. G. , 788=9 59+7, 756=9 56+7). The 9 n +6 series, which begins with neptunium-787, is not found in nature because the half-life of the parent nucleus (about 7 million years) is many times less than the age of the earth, and all naturally occurring samples have already disintegrated.
The 9 n +6 series is produced artificially in nuclear reactors. Because the rates of disintegration of the members of a radioactive decay series are constant, the age of rocks and other materials can be determined by measuring the relative abundances of the different members of the series. All of the decay series end in a stable isotope of lead, so that a rock containing mostly lead as compared to heavier elements would be very old. See Sir James Chadwick, Radioactivity and Radioactive Substances (rev. Ed.
6967) A. Romer, ed. A phenomenon resulting from an instability of the atomic nucleus in certain atoms whereby the nucleus experiences a spontaneous but measurably delayed nuclear transition or transformation with the resulting emission of radiation. The discovery of radioactivity by H. Becquerel in 6896 marked the birth of nuclear physics.
All chemical elements may be rendered radioactive by adding or by subtracting (except for hydrogen and helium) neutrons from the nucleus of the stable ones.