Thermal neutrons energy

Thermal neutron absorption cross section. Simply designated cross section, it represents the ease with which a given nuclide can absorb a thermal neutron (energy less than or equal to 0.025 eV) and become a different nuclide. The cross section is given here in units of barns (1 barn = 10 cm ). If the mode of reaction is other than ( ,y), it is so indicated. 

Table 8.2. Cross sections (Tn,r of nuclear fission by thermal neutrons (energy 0.025 cV) and mean number v of neutrons set free by fission.

Model (2) is an earlier version oi the Model (1) calculation. It is identical with Model (1) except, for the thermal constants which are calculated using i lMaxwellian thermal neutron energy spectrum rather than the Wigner-Wilkins spectrum. 

Neutron absorphon is the absorption of the neutron by the nucleus of an atom. Rather than being scattered off, the neutron is actually absorbed by the nucleus. This process may occur at any energy, although it is more common at lower energies after a neutron has been slowed down by inelastic and elastic scattering. Once the neutron has reached thermal neutron energies, it will eventually be absorbed, whether by uranium-235 to initiate another fission reaction, or by shielding materials or other structures of the reactor. 

As previously discussed, a resonance at thermal neutron energies leads to a variation in both the real and imaginary parts of the scattering length. This was calculated using (3.20) and (3.21) with the parameters taken from [19] (Table 7.2). 

Since most of the nuclear reactions In N Reactor occur at thermal or near-thermal neutron energies the most Important cross sections are those at these energies. The Vestcott formulation Is particularly useful since It defines an effective cross section for a materiel, vhlch when multiplied by the prqper flux gives the total reaction rate over the entire thermal and slowlng-down neutron spectrum. The flux employed Is nvo vhere Vq Is 2200 m/s (the velocity of neutrons corresponding to a neutron temperature of 20 c) and n Is the total neutron density (Integrated over all energies). 

To successfully employ this modality of NCT, it is essential to have a source of well-thermalized neutrons. Suitable thermal neutron energy spectra can be obtained at the NCT facilities of the Kyoto University Research Reactor Institnte (KUR), at the Japan Atomic Energy Research Institute (JRR4) and at reactors in a nnmber of other countries. A Maxwellian distribution, with good elimination of concomitant y-rays from the irradiation field, can be achieved by scattering with Bi atoms (bisthmus... 

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