\quad\int_{4\pi}d\Omega^\prime\int^{\infty}_{0}dE^\prime\,\Sigma_s(\mathbf{r},E^\prime\rightarrow  E,\mathbf{\hat{\Omega}}^\prime\rightarrow  \mathbf{\hat{\Omega}},t)\varphi(\mathbf{r},E^\prime,\mathbf{\hat{\Omega}^\prime},t)+s(\mathbf{r},E,\mathbf{\hat{\Omega}},t)