Estimate of the cross section for thermal neutrons

Cross section of the thermal neutrons in the framework of statistical approach to the complicated nuclei is considered. We calculate probability distribution $\mathcal P(z)$ to have given cross section $\sigma$ (determined by fluctuations of resonance positions and widths). $z$ is the ratio of $\sigma$ to $\sigma^*$ where $\sigma^*$ is the cross section for the model of equidistant resonances with same width. The last quantity can be presented in terms of neutron strength function for given nuclei. Probability distribution $\mathcal P(z)$ is universal for all nuclei.