Cross sections in the resolved-resonance region: S-matrix vs R-matrix approaches

The statistical model of compound nucleus reactions is widely used in nuclear reaction calculations. Statistical model calculations produce average cross sections that depend smoothly on energy. However, in the resolved-resonance region (RRR), experimental cross sections show distinct resonances that are usually modeled by R-matrix theory. Moreover, R-matrix fits in the RRR are used to test the assumptions of the statistical model. Recently, R-matrix fits to experimental data for neutron scattering off Pt isotopes showed a violation of the expected Porter-Thomas distribution (PTD) for the reduced neutron widths. I will discuss a statistical model calculation based on the Gaussian orthogonal ensemble of random-matrix theory that yields cross sections in the RRR with distinct resonances. Using this approach, I studied the distribution of neutron resonance widths obtained from an R-matrix fit of calculated elastic cross sections for s-wave neutron scattering off 194Pt. I find that, if the pure s-wave potential scattering is accounted for in the R-matrix approach, the R-matrix fit yields the PTD for the neutron widths.