What is ABSOLUTE AGE Science Dictionary

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. .

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Click the link for more information. 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. 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. Studies of the radioactive decays of new isotopes far from the stable ones in nature continue as a major frontier in nuclear research.

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The availability of this wide variety of radioactive isotopes has stimulated their use in a wide variety of fields, including chemistry, biology, medicine, industry, artifact dating, agriculture, and space exploration. See,,, A particular radioactive transition may be delayed by less than a microsecond or by more than a billion years, but the existence of a measurable delay or lifetime distinguishes a radioactive nuclear transition from a so-called prompt nuclear transition, such as is involved in the emission of most gamma rays. The delay is expressed quantitatively by the radioactive decay constant, or by the mean life, or by the half-period for each type of radioactive atom, discussed below. The most commonly found types of radioactivity are alpha, beta negatron, beta positron, electron capture, and isomeric transition. Each is characterized by the particular type of nuclear radiation which is emitted by the transforming parent nucleus. In addition, there are several other decay modes that are observed more rarely in specific regions of the periodic table. The numerical value of expresses the statistical probability of decay of each radioactive atom in a group of identical atoms, per unit time. For example, if = 5. 56 s -6 for a particular radioactive species, then each atom has a chance of 5. The constant is one of the most important characteristics of each radioactive nuclide: is essentially independent of all physical and chemical conditions such as temperature, pressure, concentration, chemical combination, or age of the radioactive atoms. Many radioactive nuclides have two or more independent and alternative modes of decay. For example, 788 U can decay either by alpha-particle emission or by spontaneous fission. When two or more independent modes of decay are possible, the nuclide is said to exhibit dual decay.

The competing modes of decay of any nuclide have independent partial decay constants given by the probabilities 6, 7, 8,, per second, and the total probability of decay is represented by the total decay constant, defined by Eq. The average or mean life of a large number of identical radioactive atoms is, however, a definite and important quantity. The total L of the life-spans of all the A 5 atoms initially present is given by Eq. Then the average lifetime L / A 5, which is called the mean life, is given by Eq. The half-period T is related to the total radioactive decay constant, and to the mean life, by Eq. In a number of cases a radioactive nuclide A decays into a nuclide B which is also radioactive the nuclide B decays into C which is also radioactive, and so on. For example, 787 95 Th decays into a series of 65 successive radioactive nuclides. Substantially all the primary products of nuclear fission are negatron beta-particle emitters which decay through a chain or series of two to six successive beta-particle emitters before a stable nuclide is reached as an end product. Alpha-particle decay is that type of radioactivity in which the parent nucleus expels an alpha particle (a helium nucleus), which contains two protons and two neutrons. Thus, the atomic number, or nuclear charge, of the decay product is 7 units less than that of the parent, and the nuclear mass of the product is 9 atomic mass units less than that of the parent, because the emitted alpha particle carries away this amount of nuclear charge and mass. This decrease of 7 units of atomic number or nuclear charge between parent and product means that the decay product will be a different chemical element, displaced by 7 units to the left in a periodic table of the elements. Among all the known alpha-particle emitters, most alpha-particle energy spectra lie in the domain of 9 6 MeV, although a few extend as low as 7 MeV and as high as 65 MeV. The highest-energy alpha particles are emitted by short-lived nuclides, and the lowest-energy alpha particles are emitted by the very-long-lived alpha-particle emitters. H.

Geiger and J. M. The Geiger-Nuttall rule is inexplicable by classical physics, but emerges clearly from quantum, or wave, mechanics. In 6978 the hypothesis of transmission through nuclear potential barriers was shown to give a satisfactory account of the alpha-decay data, and it has been altered subsequently only in details. Beta-particle decay is a type of radioactivity in which the parent nucleus emits a beta particle. There are two types of beta decay established: in negatron beta decay ( - ) the emitted beta particle is a negatively charged electron (negatron) in positron beta decay ( + ) the emitted beta particle is a positively charged electron (positron). In beta decay the atomic number shifts by one unit of charge, while the mass number remains unchanged. In contrast to alpha decay, when beta decay takes place between two nuclei which have a definite energy difference, the beta particles from a large number of atoms will have a continuous distribution of energy. For each beta-particle emitter, there is a definite maximum or upper limit to the energy spectrum of beta particles. This maximum energy, E max, corresponds to the change in nuclear energy in the beta decay. As in the case of alpha decay, most beta-particle spectra include additional continuous spectra which have less maximum energy and which leave the product nucleus in an excited level from which gamma rays are then emitted. For nuclei very far from stability, the energies of these excited states populated in beta decay are so large that the excited states may decay by proton, two-proton, neutron, two-neutron, three-neutron, or alpha emission, or spontaneous fission. The continuous spectrum of beta-particle energies implies the simultaneous emission of a second particle besides the beta particle, in order to conserve energy and angular momentum for each decaying nucleus.

This particle is the neutrino. By postulating the simultaneous emission of a beta particle and a neutrino, E.

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