HELP on THERMAL NEUTRON CAPTURE csa (barns, 10éçë cmç)
Slow moving neutrons (thermal neutrons, those with energies of order 0.05eV) are easily captured by nuclei, which may then become neutron rich and decay by Γ (electron) emission. The thermal neutron capture cross sectional area (expressed in barns, [10éçëcmç]) is a measure of how easily neutrons are absorbed by nuclei, the larger the csa, the greater probability of absorption. The real 'shadow' area of a nucleus is of the order of 1 barn for the heavier elements, but the thermal neutron capture csa for some nuclides can be very much higher than this ( the highest being Gadolinium at 49000 barns). Part of the explanation for this much greater apparent area is that slow neutrons have a large deBroglie wavelength compared with the diameter of a nucleus and can be captured at great distance from the nucleus. Another reason is the existence of nuclear resonances: when the energy (frequency) of the approaching neutron matches an internal resonance of the nucleus, then the probability of union is greatly enhanced. The smallest thermal neutron capture csa is for oxygen at 280 microbarns. In nuclear reactors, Zirconium, with a low neutron absorption, is used to contain the fuel rods, whereas Uranium-238 absorbs neutrons and is either induced into nuclear fission, or goes on absorbing more neutrons and eventually becoming Plutonium-239 following Γ-decay; this is how many transuranic elements are made. Boron and Carbon have small capture areas and so are used as moderators in nuclear reactors where fast fission neutrons are slowed down by multiple collisions with the nuclei of the moderator. The heaviest of transuranic elements, however, can only be made by bombarding lighter elements with high-energy ions. Shown is elemental average based on natural abundance.
