Handling of external levels in neutron resonance fitting

Nuclear resonance cross section data must be parametrized for applications by fitting resonance-theoretic (R-matrix) curves to the data points. The curves depend on the 'internal' levels inside the fitted energy range but also on all the 'external' levels outside.Without due account of the external levels one gets troublesome edge effects and problems with potential scattering between resonances. As the external levels are mostly unknown, various ad hoc devices for their simulation are being used, for example energy-dependent nuclear radii or representation by 'picket fence' resonances, although more convenient and theoretically sound techniques are available that describe external levels statistically in terns of familiar parameters (strength function, effective radius etc.). A novel, especially convenient approximation accounts accounts for the external levels by just one fictitious pair of very broad resonances. Similarly, fine-tuning to accurately known thermal cross sections is often done by laborious adjustment of a number of bound levels, although again a simple analytical recipe involving just one bound level has been available for decades