In a nuclear reaction two nuclei, or a nucleon and a nucleus, come together in such close contact that they interact through the strong force. The deuteron-stripping reaction of equation (7.3) is one example. A reaction which contributes to energy generation in stars is

and a nuclear reaction important in power technology is

n + 235U → fission products.

The latter two are both exothermic reactions in which the kinetic energy of the final nuclei is greater than that of the initial nuclei. In an endothermic reaction energy must be supplied before the reaction will take place, as in the reaction inverse to (8.1) above.

8.1 TheBreit-WignerformuIa

The concept of cross-section (Appendix A) is important for understanding and classifying nuclear reactions. Figure 8.1 shows the total cross-section for neutrons to interact with the 16 8 O nucleus as a function of the kinetic energy E (in the centre-of-mass system) up to E = 2.3 MeV. The principal features of the cross-section are the high but narrow resonance peaks, superposed on a slowly varying background. These peaks are due to the formation of excited states of 17O from the neutron and 16O at the resonance energies. When the energy of the incident neutron is such that the total energy of the system matches, to within the width T, one of the excited states energies of 17O, the neutron is readily accepted into the target to form that state.