Mechanisms of Nuclear Reactions

Most low-energy nuclear reactions proceed via formation of a compound nucleus (eq. (8.5)). In the compound nucleus model that was proposed in 1936 by Bohr it is assumed that the energy of the incident particle and its binding energy are distributed evenly or nearly evenly to all nucleons of the target nucleus. The excitation energy of the compound nucleus is 

The fate of a compound nucleus depends on its composition and its excitation energy, not on the way it is formed. It may be formed and decay in different ways  

In the resonance region, where the excitation functions exhibit sharp resonances, the cross sections of the individual reactions can be calculated from the line widths of the resonance lines by application of the Breit-Wigner formulas derived in 1936. Emission of neutrons from compound nuclei is preferred over emission of protons and relatively high cross sections are expected for (p, n) and (a, n) reactions if the energy of the incident particles is 1 MeV. 

If the excited states of compound nuclei overlap, statistical methods are applied which also allow prediction of the emission of particles by a compound nucleus and of the cross section of a certain nuclear reaction. 

Direct interactions proceed faster than compound nucleus reactions. The time of direct interactions is given by the time needed by the projectile to enter the nucleus and the time needed to leave the nucleus after collision. Reactions at the surface of the nucleus proceed within 10 s and reactions within the nucleus in about 10 - s. 

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