Neutron-induced reactions capture

Using the shorthand notation, examples of photon capture, electron knockout and neutron-induced reactions are, respectively ... 

Lutetium-177 produced by neutron capture reactions Iodine-125 produced by neutron capture followed by Tungsten-188 produced by double neutron capture Tin-117m produced by neutron inelastic scattering Copper-67 produced by fast neutron-induced reactions... 

One other neutron-induced reaction, which is of importance in certain circumstances, is the (n, 2n) reaction, where the capture of the original neutron leads to the emission of two neutrons from the compound nucleus. The most important example is that of the light element beryllium... 

Additional interactions of neutrons with nuclei include die release of charged particles by neutron-induced nuclear disintegration, Commonly known reactions are n-p. n — d. and n—ct. In these cases, the incident neutrons may contribute part of their kinetic energy to the target nucleus to effect the disintegration. Hence, more than mere neutron capture is involved, Then, there is usually a lower threshold for the neutron energy below which the reaction fails to occur, Another important reaction involving neutrons is fission, which may occur under different conditions for eidier slow or fast neutrons with appropriate fissionable material. 

Specific nuclear reactions capable of producing noticeable quantities of noble gas daughters in the Earth ( He and Ne in particular) are initiated by alpha and fission activities of the natural radioelements. Helium-3 is produced through a neutron capture reaction involving Li (HUl, 1941), whereas Ne production occurs through a number of a-induced reactions (Wetherill, 1954). In the case of helium, the He/ He ratio produced is of the order 10 and primarily reflects the lithium abundance at the site of production (Mamyrin and Tolstikhin, 1984). Eor neon, the only conspicuous isotope produced is Ne due to its low natural abundance. The present-day Ne/ He production ratio in the mantle has been calculated at 4.5 X 10 (Yatsevich and Honda, 1997) (see Ballentine and Bumard, 2002 for discussion regarding calculation of this parameter). 

As the temperature decreases further, the QSE clusters fragment more and more into smaller clusters until total breakdown of the QSE approximation, at which point the abundances of all nuclides have to be calculated from a full nuclear reaction network. In the relevant a-particfe-rich environment, the reaction flows are dominated by (a, 7) and (a, n) reactions with the addition of radiative neutron captures. Nuclei as heavy as Fe or even beyond may result. For a low enough temperature, all charged-particle-induced reactions freeze-out, only neutron captures being still possible. This freeze-out is made even more efficient if the temperature decrease is accompanied with a drop of the density p, which is especialiy efficient in bringing the operation of the p2-dependent a - a + n reaction to an end. 

Fission. The splitting of an atomic nucleus into two fragments that usually releases neutrons and y rays. Eission may occur spontaneously or may be induced by capture of bombarding particles. Primary fission products usually decay by particle emission to radioactive daughter products. The chain reaction that may result in controlled burning of nuclear fuel or in an uncontrolled nuclear weapons explosion results from the release of 2 or 3 neutrons/fission. Neutrons cause additional fissile nuclei in the vicinity to fission, producing still more neutrons, in turn producing still... 

There are two kinds of methods for production of transuranium elements as indicated in the previous O Sect. 18.1.1 neutron capture reactions in nuclear reactors and charged-particle-induced reactions at accelerators. 

In the following, neutron capture reactions that lead to a buildup of heavier nuclides (actinides) will be discussed. As has been mentioned before, inside a reactor, in addition to neutron-induced (n,f)-fission reactions, neutron-capture (n,y)-reactions take place. The products arising primarily from neutron capture in and Pu ( TJ, and Pu,... 

Fission-produced neutrons can be absorbed by any of the materials present in the reactor, with relative probabilities proportional to the neutron-absorption cross sections of the nuclei in the materials. Even for the fissile nuchdes, neutron-capture reactions compete favorably with neutron-induced-fission reactions. For example, low-energy neutron irradiation of a sample of results in the production of about half as often as it results in fission. Fuel containing the fissile actinides always contains some of the even-mass actinide nuclides as well, which are... 

The decay heat power comes mainly from five sources (1) unstable fission products, which decay via a, p-, p+, and y ray emission to stable isotopes (2) unstable actinides that are formed by successive neutron capture reactions in the uranium and plutonium isotopes present in the fuel (3) fissions induced by delayed neutrons (4) reactions induced by spontaneous fission neutrons (5) structural and cladding materials in the reactor that may have become radioactive. Heat production due to delayed neutron-induced fission or spontaneous fission is usually neglected. Activation of light elements in structural materials plays a role only in special cases. 

Despite this favorable record, the further development of nuclear power is greatly handicapped in many countries because of public concern over the radioactive products arising in the course of plant operation and the consequences of their possible release to the environment. Energy generation from the neutron-induced fission of heavy atoms is inevitably accompanied by the formation of radioactive nuclides. This is, first of all, the direct consequence of nuclear fission, which leads initially to fission products that are unstable due to an excess of neutrons in the newly formed nuclei. These products are transformed by a sequence of p decays (mainly with associated y emission) to stable end products. Moreover, neutron capture in the heavy atoms of the fuel results in the buildup of nuclei which are heavier than those of the starting element (uranium, plutonium) and which mostly decay — in part, with very long halflives — by a emission. Finally, from elements present in structural and cladding materials, as well as in the coolant, its additives and impurities, additional radionuclides are formed, induced by neutron capture reactions which take place in the intense neutron field inside the reactor pressure vessel. 

Another secondary effect is the production of radionuclides which themselves are not fission products but which are generated by neutron capture in long-lived or stable fission product nuclides. Examples of the products of such reactions are Cs and Cs, which are separated from the true members of the isobaric chains by the stable nuclides Xe and Xe, and which are formed by neutron capture in the fission products Cs and Cs, respectively. Because of the two-fold neutron-induced nuclear reaction which is necessary for their production, their concentration in the irradiated fuel depends approximately on the square of the local neutron fluence. 

When a target nucleus, zX, is irradiated by thermal neutrons, the reaction induced, known as radiative capture, is represented by ... 

Energetic particles react with solid matter in a variety of ways. Low-energy particles in the solar wind ( 1 KeV/nucleon) are implanted into solids to depths of 50 nm. Energetic heavy particles penetrate more deeply and disrupt the crystal lattice, leaving behind tracks that can be imaged by or chemically etched and observed in an optical microscope. Particles with energies of several MeV or more may induce a nuclear reaction. The two main modes of production of cosmogenic nuclides are spallation reactions and neutron capture. 

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