Radiative neutron capture

In this group the most commonly used reaction is that of radiative neutron capture. Also here are to be found (n, 2 n) and (y, n) reactions, although very few studies have been done with these reactions, and isomeric transitions (although these may often be more profitably discussed along with electron capture reactions). 

The aforementioned requirements on neutron concentration and temperature suffice to fix qualitatively several of the main features of the nuclear flow associated with the r-process and to identify the involved nuclear physics. Figure 22 depicts the situation very schematically. In the course of the transformation of a given seed into more neutron-rich isotopes by a series of (n,y) reactions, (y,n) photodisintegrations have a rate increasing with the neutron excess or, equivalently, with the associated decrease of the neutron separation energy Sn. For low enough Sn, the (y,n) reactions counteract efficiently the radiative neutron captures. At this point, the nuclear flow may proceed to... 

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. 

The total cross sections for interactions with neutrons below the lowest energy resonance (<1 eV) varies. In this region the scattering cross section is essentially constant, and the absorption and activation cross sections vary as 1/V. The neutrons in this energy region provide the principal activation process known as the (n, ) reaction. This particular reaction is usually called the thermal neutron (0.026 eV) reaction or radiative neutron capture because a gamma ray is emitted from the compound nucleus shortly after it has been formed. 

Given the importance of the photon heating contribution in plutonium burning fast reactor cores, a comprehensive validation work is under way for ECCO/ERANOS. Within this framework, PSI has contributed with both an experimental and an analytical effort. On the experimental side, thermoluminescent detector (TLD) measurements were performed in some of the aforementioned CIRANO configurations. With regard to methods/calculational aspects, an important effort was necessary to produce consistent neutron kerma factors and photon spectra data (from photon production due to radiative neutron capture, fission, and both elastic and inelastic neutron scattering). 

It has already been proposed by [29] that the r-process results from the availability of neutron concentrations that are so high that neutron captures (especially of the radiative type) are faster than /3-decays, at least for a substantial... 

Excitation of bound states by radiative capture of slow neutrons. Radiative capture is mainly important for thermal neutrons for which there is the technical advantage that very large intensities are available from reactors. The energy spectmm of thermal capture radiations has been observed by Kinsey and his collaborators [37] for a large number of nuclei. In this work the neutron capturing sample was placed in a high flux near the core of a nuclear reactor, and collimated... 

Radiative capture Is any such process in which the capture results in an excited state that decays by emission of photons. A common example is neutron capture to yield an excited nucleus, which decays by emission of a gamma ray. 

Table 38.1 lists some examples of reactor-produced radionuclides important to nuclear medicine. There are three general reaction types used neutron radiative capture (n,y) neutron capture followed by particle emission, e.g., (n,n ), (n,p), and (n,a) and fission (n, f). The radionuclides in Table 38.1 are arranged in order of mass numbers. 

The two competing nonproductive neutron-capture processes are(l) radiative capture and (2) inelastic scattering. The radiative-capture reaction can be symbolized by the equation... 

Neutrons may collide with nuclei causing one of the following reactions inelastic scattering, elastic scattering, radiative capture, or fission. 

Radiative capture (n, y) takes place when a neutron is absorbed to produce an excited nucleus. The excited nucleus regains stability by emitting a gamma ray. 

Radiative capture, 4(n, y)A + 1. As discussed earlier, this cross section shows a 1/v energy dependence, and this process is important for low-energy neutrons. 

The measurement of neutron fluxes by foil activation is more complicated because the neutrons are not monoenergetic and the monitor cross sections are energy dependent. The simplest case is monitoring slow neutron fluxes. Radiative capture (ivy) reactions have their largest cross sections at thermal energies and are thus used in slow neutron monitors. Typical slow neutron activation detectors are Mn, Co, Cu, Ag, In, Dy, and Au. Each of these elements has one or more odd A isotopes with a large thermal (n,y) cross section, 1-2000 barns. The (n,y)... 

VOI83] J. Voignier, S. Joly, and G. Grenier, "Mesure De La Section Efficace De Capture Radiative Du Lanthane, Du Bismuth, Du Cuivre Naturel et De Ses Isotopes Pour Des Neutrons D Energie Comprise Entre 0,5 et 3 MeV," Proc. Int. Conf. Nuclear Data for Science and Technology, Antwerp, Belgium (1983), p. 759. 

It is not our purpose to make a detailed study of nuclear reactions. Such reactions indeed are only induced to an appreciable extent by very high energy radiation (5>>M.e.v.) and by thermal neutrons these two cases are not considered here. However, gamma radiation consecutive to radiative capture as well as beta, alpha, and other radiations resulting from other nuclear reactions will be studied here. Moreover, allowance has to be made... 

Recently, Abia et al. (2001 - see also Abia Wallerstein, 1998) measured the Rb, Sr, Y, and Zr abundances in a sample of N-type carbon stars. Carbon enrichement is attributed to salting of the atmosphere with 12C from the He-shell of an AGB star. The authors conclude that most of the carbon stars are of low mass, experiencing s-process nucleosynthesis phenomena dominated by the neutron source provided by a-capture s on 13C in radiative intershell layers. It is pleasing that the neutron density inferred from carbon stars is consistent with that from MS and S stars because the sequence of third dredge-ups is predicted to raise the low C/O ratio of MS and S stars to a value C/O in excess of unity which defines a cool carbon star. A similarity in mass is then expected. 

In fission reactors the transmutation reactions of principal importance involving neutrons are capture and fission. All nuclides (except He) take part in the radiative capture reaction ( , y), an example of which is... 

For epithermal neutrons (1 10 eV) large radiative capture and fission... 

For fast neutrons ( 0.1 MeV) the cross sections are relatively small, 1 b. Fission dominates over radiative capture. Of particular importance is that becomes fissionable at a neutron energy of 0.6 MeV its fission cross section increases with neutron energy above the threshold to a constant value of 0.5 b at 2 MeV. 

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