Induced nuclear reactions

The fission ofor Tu liberates, on average, two to three neutrons. One neutron is required to sustain the nuclear fission chain reaction. In a nuclear breeder reactor, the extra neutrons are used to induce nuclear reactions that lead to the production of Tu. The sequence begins by arranging for... 

Neutrons readily induce nuclear reactions, but they always produce nuclides on the high neutron-proton side of the belt of stability. Protons must be added to the nucleus to produce an unstable nuclide with a low neutron-proton ratio. Because protons have positive charges, this means that the bombarding particle must have a positive charge. Nuclear reactions with positively charged particles require projectile particles that possess enough kinetic energy to overcome the electrical repulsion between two positive particles. 

Neutron Activation Analysis X-Ray Fluorescence Particle-Induced X-Ray Emission Particle-Induced Nuclear Reaction Analysis Rutherford Backscattering Spectrometry Spark Source Mass Spectrometry Glow Discharge Mass Spectrometry Electron Microprobe Analysis Laser Microprobe Analysis Secondary Ion Mass Analysis Micro-PIXE... 

In terms of atomic spectrometry, NAA is a method combining excitation by nuclear reaction with delayed de-excitation of the radioactive atoms produced by emission of ionising radiation (fi, y, X-ray). Measurement of delayed particles or radiations from the decay of a radioactive product of a neutron-induced nuclear reaction is known as simple or delayed-gamma NAA, and may be purely instrumental (INAA). The y-ray energies are characteristic of specific indicator radionuclides, and their intensities are proportional to the amounts of the various target nuclides in the sample. NAA can thus... 

Charged particle activation analysis (CPAA) is based on charged particle induced nuclear reactions producing radionuclides that are identified and quantified by their characteristic decay radiation. CPAA allows trace element determination in the bulk of a solid sample as well characterization of a thin surface layer. 

In contrast to PIXE and RBS, where forces are respectively electromagnetic and electrostatic, this kind of microanalysis uses low range nuclear forces. The analysis is based on the detection of the y-rays emitted from nuclei that are in an excited state following a charged particle induced nuclear reaction. 

In CPAA, the incident charged particle induces nuclear reactions which produce radionuclides, and the characteristic decay radiation of the latter is measured. Qualitative analysis of the radionuclide is achieved by measuring its energy and/or... 

Ion induced nuclear reactions for light element analysis in general and hydrogen analysis in particular have been extensively discussed in the literature. Only a few ion-induced nuclear reactions have cross sections large enough to be useful for materials analysis, and these are listed in published compilations (Amsel et al., 1971 Mayer and Rimini, 1977). 

Extensive channeling measurements on 2H implanted into silicon have been published by Bech Nielsen (1988). These measurements also use the 3He-induced nuclear reaction in conjunction with extensive modeling using the statistical equilibrium model already described. The 2H implants were done at 30 K, and lattice location of the 2H was done as a function of annealing. 

Structural materials for fission reactors (as well as for future fusion reactors) are being exposed to intense neutron flux for many years. In the case of fusion reactor, 14-MeV neutrons, produced by the fusion reaction of d + t He + n, induce nuclear reactions of... 

Indeed, this happens every moment in the Earth s atmosphere. The upper atmosphere is bombarded with cosmic rays fast-moving subatomic particles produced by extremely energetic astrophysical processes such as nuclear fusion in the sun. When cosmic rays hit molecules in the atmosphere, they induce nuclear reactions that spit out neutrons. Some of these neutrons react with nitrogen atoms in air, converting them into a radioactive isotope of carbon carbon-14 or radiocarbon , with eight neutrons in each nucleus. This carbon reacts with oxygen to form carbon dioxide. About one in every million million carbon atoms in atmospheric carbon dioxide is C. 

Target Material subjected to particle bombardment (as in accelerator) or irradiation (as in reactor) in order to induce nuclear reaction. 

Radiocarbon Dating. This is a method of estimating Ihe age of carbon-containing materials by measuring the radioactivity of the carbon in them. The validity of this method rests upon certain observations and assumptions, of which the following statement is a brief summaiy. The cosmic rays entering the atmosphere undergo various transformations, one of which results in the formation of neutrons, which in turn, induce nuclear reactions in the nuclei of individual atoms of the adnosphere. The dominant reaction is... 

Activation analysis is based on the production of radioactive nuclides by means of induced nuclear reactions on naturally occurring isotopes of the element to be determined in the sample. Although irradiations with charged particles and photons have been used in special cases, irradiation with reactor thermal neutrons or 14 MeV neutrons produced by Cockcroft-Walton type accelerators are most commonly used because of their availability and their high probability of nuclear reaction (cross section). The fundamental equation of activation analysis is given below ... 

Relative to silicon, the total elemental lithium is even 140 times less abundantin the solar photosphere than on Earth or inmeteorites, as recorded by the solar spectrum. Since the Li/Si element abundance ratio in the meteorites should also have entered the Sun when it formed, one concludes that the Sun must be destroying its initial lithium supply as it ages. This occurs at the base of the surface convection zone of the Sun. The bottom of that zone lies at a depth that is about 1/4 of the Sun s radius. Here the high temperatures (a few million degrees kelvin, MK) that solar-surface nuclei experience is hot enough to destroy lithium, especially 6Li, by nuclear interactions with protons (6Li + p —3He + 4He). Those proton-induced nuclear reactions destroy 6Li much more readily than they do 7Li because the quantum probabilities of the reaction are greater than for 7Li. As a result, to deplete elemental lithium by a factor of 140 in the Sun... 

An artificially induced nuclear reaction caused by the bombardment of a nucleus with subatomic particiles or small nucei. 

Tritium, which is radioactive (/T, tl/2 = 12.4 y), is made by the reaction 6Li(n,ct)3H in nuclear reactors. It is also formed in plasmas2 as 3H+ and by cosmic ray induced nuclear reactions in the upper atmosphere. The decay of 3H probably accounts for traces of 3He in the atmosphere. 

Scientists have learned how to make some isotopes undergo nuclear reactions. Artificial radioactivity is induced by bombardment of certain nuclei with subatomic particles (or atoms), which are produced either by other nuclear reactions or in machines called particle accelerators. For example, the first artificially induced nuclear reaction was produced by Ernest Rutherford (1871-1937) in 1919 ... 

Neutrons may lose their energy in steps by collision with other particles or they may induce nuclear reactions. In contrast to particles, photons mostly give off their energy in one step. 

K. A. Keller, J. Lange, H. Miinzel, G. Pfennig, Excitation Functions for Charged-Particle Induced Nuclear Reactions, Springer, Berlin, 1973... 

The most important photon-induced nuclear reactions are (7,n) and (7,2n) reactions, but (7,p) reactions may also be applied. The number of possible reactions increases with the energy of the photons. Photons with energies in the range between about 15 to 40MeV are preferred, and detection limits between about 1 ng and 1 pg are obtained. Photofission of heavy nuclei is achieved with photons of relatively low energy (about 5 to 10 MeV). It leads also to detection limits of about 1 ng to 1 pg. 

The classic idea of a cosmic-ray exposure (CRE) age for a meteorite is based on a simple but useful picture of meteorite evolution, the one-stage irradiation model. The precursor rock starts out on a parent body, buried under a mantle of material many meters thick that screens out cosmic rays. At a time fj, a collision excavates a precursor rock—a meteoroid. The newly liberated meteoroid, now fully exposed to cosmic rays, orbits the Sun until a time ff, when it strikes the Earth, where the overlying blanket of air (and possibly of water or ice) again shuts out almost all cosmic rays (cf. Masarik and Reedy, 1995). The quantity ff — h is called the CRE age, f. To obtain the CRE age of a meteorite, we measure the concentrations in it of one or more cosmogenic nuclides (Table 1), which are nuclides that cosmic rays produce by inducing nuclear reactions. Many shorter-lived radionuclides excluded from Table 1 such as Na (ff/2 = 2.6 yr) and °Co ty = 5.27 yr) can also furnish valuable information, but can be measured only in meteorites that feu within the last few half-Uves of those nucUdes (see, e.g., Leya et al. (2001) and references therein). 

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