ABSTRACT

The probability of a nuclear reaction occurring can be conveniently expressed in terms of a cross section. It is useful to refer to the probability of a nuclear reaction with a single target nucleus, without account of the effects of neighbouring nuclei. Niels Bohr was the first person to have pointed out that one can consider a nuclear reaction to consist of two stages: formation of a compound system C (the compound nucleus), and its decay into reaction products. A compound nucleus possesses a set of quasi-stationary states with finite lifetimes which are due to the possibility of nuclear decay into a particle, or a group of particles, and a final nucleus. If the energy of a particle in the centre-of-mass system is close to the energy of one of the nuclear compound levels then the probability of producing the compound nucleus becomes particularly large, and the nuclear reaction cross section greatly increases forming a resonance maximum.