Nucleus, atomic radioactive decomposition

The most important class of first-order reactions is the radioactive decomposition of atomic nuclei. Each nucleus of radium 226 or other radio-nucleide has a probability of decomposition in unit time that is independent of the concentration (in general, of the presence of other particles), and in consequence the process of radioactive decay is represented by Equations 10-1 and 10-4. 

In 1934 Fermi developed his theory of /3 decay, in order to explain the puzzling observation that some radioactive nucleides shoot out an electron in the course of radioactive decomposition, although they were supposed to be composed only of protons and neutrons. He pointed out that atoms emit photons when they change from one quantum state to another, although it is not believed that the atoms contain the photons instead, it is accepted that the photon is created at the time when it is emitted. Fermi suggested that the electrons, the (3 particles, are created when the radioactive nucleus undergoes decomposition, and that at the same time one of the neutrons inside the nucleus becomes a proton, and a neutrino (or, rather, an antineutrino) is emitted. 

For very many years, the alchemist s dream of changing base metals into gold was ridiculed even by the most reputable of scientists. Although it was known that the nuclei of certain atoms undergo alteration in the course of natural radioactive decay, researchers inability to exercise any control over the nature or rate of these spontaneous decompositions probably did much to foster the belief that the nucleus of the atom was inviolate. However, in the year 1919 the English physicist Ernest Rutherford accomplished the first transmutation of an element, and this notable discovery was quickly followed by other equally significant developments. 

Besides the generators described above there are X-ray sources based on radioactive materials to provide the excitation of the sample. The advantage of using these materials is that an isotope can be selected to provide a mono-energetic beam of radiation that is optimized for the specific application. One method consists to select a radionuclide that is transformed by internal electron capture (lEC). This mode of decomposition corresponds to the transition of one level-K electron into the nucleus of the atom. For a nuclide X, the phenomenon is summarized as follows ... 

Radioactivity is the spontaneous decomposition of a nucleus to form another nucleus and produce one or more particles. We can write a nuclear equation to represent radioactive decay, in which both A (mass number) and Z (atomic number) must be conserved. 

Most nuclei in nature are stable and remain intact indefinitely. Radionuclides, however, are unstable and spontaneously emit particles and electromagnetic radiation. Emission of radiation is one of the ways in which an unstable nucleus is transformed into a more stable one that has less energy. The emitted radiation is the carrier of the excess energy. Uranium-238, for example, is radioactive, undergoing a nuclear reaction emitting helium-4 nuclei. The helium-4 particles are known as alpha ( ) particles, and a stream of them is called alpha radiation. When a nucleus loses an alpha particle, the remaining fragment has an atomic number of 90 and a mass number of 234. The element with atomic number 90 is Th, thorium. Therefore, the products of uranium-238 decomposition are an alpha particle and a thorium-234 nucleus. We represent this reaction by the nuclear equation... 

The first two reactions are found for many of the heavy radioactive nucleides. Alpha emission has been observed also for a number of the neutron-rich nucleides in the rare-earth region. The third reaction, positron emission, occurs for most neutron-rich nucleides, many of which also decompose by electron capture (the fourth reaction). (Electron capture is classed as a spontaneous decomposition because the electrons are always available in the atom for capture it is the s electrons, principally l5, that are captured they are the only electrons with finite probability at the nucleus.) The last two reactions, proton and neutron emission, occur only rarely. 

Carbon exists in five isotopic forms. That is to say there are atoms of carbon with five different nuclear characteristics, specifically with different numbers of neutrons. These variations are denoted as carbon-11 (C-11), carbon-12 (C-12), carbon-13 (C-13), carbon-14 (C-14) and carbon-15 (C-15) in which the numbers of neutrons vary ftom 5 to 9 respectively. Of these isotopes, C-12 and C-13 are stable while the others are imstable and radioactive. In order to achieve stability, radioactive decay occurs involving the emission of particles from the nucleus. This occms at a constant rate for each isotope for C-11 the half-life, i.e. the time required for half the original number of atoms to decay is 20.3 minutes, for C-15 it is 2.5 seconds and for C-14 it is 5730 years. The long half life of the latter isotope allows it to be used as a means of estimating the age of carbon-rich substances that have been preserved through bmial in enviromnents that prohibit decomposition. The approximate age is determined by comparing the amount of radioactive carbon in the preserved sample with that in a modem sample. While the physical properties of the isotopes vary in this way, their chemical properties are the same, as reflected by their atomic munber of six. 

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