# Thin/thick target yields

$z = \Sigma_{tot}(E_{res})L_{eff} \sqrt{\frac{\Gamma_\gamma}{\Gamma}}$
based on macroscopic cross section at the energy of the resonance $\Sigma_{tot}(E_{res})$, ‘effective length’ $L_{eff}$, radiative capture width $\Gamma_\gamma$ and total resonance width $\Gamma$. The seld-shielding factor for this resonance is then a function only of $z$ with empirical constants. The model has been extended to a group of separated resonances by taking a weighted sum of the self-shielding factors with weights
where $\Gamma_n$ is the neutron scattering width, I and J are the spins of the target and compound resonance states, respectively. FISPACT-II can use these on a group-by-group basis for all nuclides within the target material, providing the self shielding factors for all cross sections using the PRINTLIB 6 keyword.