Cross neutron capture reactions

The cross sections of the thermal neutron capture reaction of 186W and 187W are... 

The amount of boron required for BNCT can be estimated using the neutron capture cross sections, which are atomic properties, and thus pertain to the number, and not the mass, of the atoms present. Conservative estimates for successful therapy result in boron concentrations of around 20 ppm in tumor tissue, to at least match the dose liberated by neutron capture reactions in the other elements of biological tissue. This would correspond to around 109 boron-10 atoms per cell, assuming that one cell corresponds to 10-9 g. 

Cross sections for neutron capture reactions in units of barns (10 cm ) or millibarns (mb). Proton, alpha production and fission reactions are designated by Cp, a . Op respectively. Separate values are listed for isomeric production. 

TABLE 6.1. Thermal Neutron Capture Cross Sections and Neutron Capture Reaction Types of Selected Stable and Radioactive Nuclides"... 

Nuclide Thermal Neutron Capture Cross Section cTni[b] 1 barn = 10 cm Neutron Capture Reaction Nuclide Thermal Neutron Capture Cross Section crti,[b] 1 bam = 10 24 Neutron Capture Reaction... 

There are only three levels of below = 10 MeV. One of these levels occurs at 7.654 MeV, just above the Q-value for the Be(a,y) reaction (also a resonance reaction). This level was predicted by F. Hoyle before it was observed, based on the fact that C (and life) exists. It is the resonance with this state that magnifies the 3a cross section and provides the gateway to synthesis of heavier elements. More detailed discussions of resonance and neutron-capture reactions in astrophysics can be found in (Rolfs and Rodney 1988). 

Successive neutron capture reactions (see Fig. 18.3) depend on the P-decay constant, 2, and the neutron capture cross section, cr, of each nuclide, as well as on the neutron flux 0. In other words, the competition between neutron capture and P -decay processes should be considered. The production rate of the fth nuclide with the number of atoms Ni is expressed as... 

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... 

You should be aware that cadmium has a high cross-section for the absorption of thermal neutrons. During this absorption, or thermal neutron capture reaction, gamma radiation is emitted, the most noticeable of which is at an energy of 558 keV. If a detector system is to be used in a neutron field, then cadmium in the graded shield should be avoided. Fortunately, tin has a much lower thermal neutron cross-section and can be used instead. Difficulties due to neutron capture might be expected, and indeed have been observed, when operating a detector close to a nuclear reactor but the problem can also occur in environmental measurements (see Chapter 13, Section 13.3.4.2.)... 

The confinement region in which nuclear fusion proceeds is surrounded by a blanket in which the neutrons produced by the fusion reaction are captured to produce tritium. Because of its favorable cross section for neutron capture, lithium is the favored blanket material. Various lithium blanket... 

Dr. Flinn Antimony would be exciting to many chemists. Antimony-121 is the Mossbauer isotope of antimony. The first work was done at Wayne State University, and recently there has been a good deal of work by Ruby and others at Argonne which should be appearing shortly. It seems that antimony is similar to tin in its relationship between isomer shift and the various compounds. It is better than tin in that the isomer shift is about five times larger so that precise measurements are possible. Thanks to Ruby s work, the changes with chemical environment are well understood. The AR/R situation is clear cut, but there are some difficulties in preparing a satisfactory source. The parent is tin-121 which is m de by neutron capture by tin-120. The reaction has one of the smallest cross-sections in existence—one can place the tin in a reactor for a year and not produce much even then. However, when a source is obtained, you are in business for a while. Its half-life is 25 years. 

The most sensitive method for determining trace amounts of technetium is the neutron activation . The Tc sample is irradiated by slow neutrons. The radioactive isotope Tc with a half-life of 15.8 s is formed by the reaction Tcfn, y) Tc, the neutron capture cross section of which is comparatively large (20 bams), so that it is possible to determine amounts < 2x 10 " g of Tc. However, the method is not widely used since the half-life of Tc is very short. Moreover, this method is only convenient when a reactor or a neutron source is available. 

One of the most promising applications of polyboron hydride chemistry is boron neutron capture therapy (BNCT) for the treatment of cancers (253). Boron-10 is unique among the light elements in that it possesses an unusually high neutron capture nuclear cross section (3.8 x 10-25 m2,0.02—0.05 eV neutron). The nuclear reaction between 10B and low energy thermal neutrons yields alpha particles and recoiling lithium-7 nuclei ... 

The shape resonances have been described by Feshbach in elastic scattering cross-section for the processes of neutron capture and nuclear fission [7] in the cloudy crystal ball model of nuclear reactions. These scattering theory is dealing with configuration interaction in multi-channel processes involving states with different spatial locations. Therefore these resonances can be called also Feshbach shape resonances. These resonances are a clear well established manifestation of the non locality of quantum mechanics and appear in many fields of physics and chemistry [8,192] such as the molecular association and dissociation processes. 

The cross sections for (n,y) reactions common in reactor thermal neutron activation generally decrease with increasing neutron energy with the exception of resonance-capture cross section peaks at specific energies. This reaction is, therefore, not important in most 14 MeV activation determinations. However, some thermalization of the 14 MeV flux may always be expected due to the presence of low Z elements in the construction materials of the pneumatic tubes, sample supports, sample vial, or the sample itself (particularly when the sample is present in aqueous solution). The elements Al, Mn, V, Sn, Dy, In, Gd, and Co, in particular, have high thermal neutron capture cross sections and thermal capture products have been observed in the 14 MeV neutron irradiation of these elements in spite of care taken to reduce the amount of low Z moderating materials in the region of the sample irradiation position 25>. 

From reaction yields and cross-sections, measured under varying irradiation conditions and neutron energies, it was found that fast neutrons were effective in processes 1 and 2, an indication that the target nucleus was 238U, but that the yield was greater when thermal neutrons were used, which is typical of neutron capture. 

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