And you can do it in one-group theory or multi-group theory. One-group theory assumes that the neutronenergy and everything is going on at one energy and we have to make some adjustments.
Neutron shielding factors of the foil are calculated for each group in intermediate energy range with detail consideration of cadmium filter and scattering by the foil itself.
The spacing of Ir is probably somewhat over estimated since the values of the energy and absorption coefficient of the resonance neutrongroup, used in the estimation, are not very accurate.