Statistical theory of pre-equilibrium emission of two particles from high-excited nuclei

A decay of a complex system is discussed for configurations intermediate between an equilibrium one and simple imput states. The intermediate configurations are discribed in terms of the Griffin exciton model. An expression for a(n,2n) reaction cross section is obtained in the case of two-particle preequillibrium emission. The excitation functions of a (n,2n) reaction calculated for 54Fe and 64Zn are compared with the frame works of the direct knock-out and statistical equillibrium models. The contribution of the preequillibrium processes to the (n,2n) cross section is sufficiently high ammounting to approximately 25 per cent for 54Fe and (> approximately) 50 per cent for 64Zn and reaching its maximum at low energies