Empirical rule formulated by W.H. Bragg and R. Kleeman that states that the stopping cross section of a compound sample is equal to the sum of the products of the elemental stopping cross sections for each constituent and its atomic fraction. $$S_{\mathrm{AB}}(\epsilon )=x{S}_{\mathrm{A}}(\epsilon )+y{S}_{\mathrm{B}}(\epsilon )$$ where $S_{\mathrm{AB}}(\epsilon )$ is the stopping cross section of the compound ${\mathrm{A}}_x{\mathrm{B}}_y$, and $S_{\mathrm{A}}(\epsilon )$ and $S_{\mathrm{B}}(\epsilon )$ are the stopping cross sections of elements $\mathrm{A}$ and $\mathrm{B}$, respectively.