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. \[S_{\ce{AB}}(\varepsilon) = xS_{\ce{A}}(\varepsilon) + yS_{\ce{B}}(\varepsilon)\] where \(S_{\ce{AB}}(\varepsilon)\) is the stopping cross section of the compound \(\ce{A}_{x}\ce{B}_{y}\), and \(S_{\ce{A}}(\varepsilon)\) and \(S_{\ce{B}}(\varepsilon)\) are the stopping cross sections of elements \(\ce{A}\) and \(\ce{B}\), respectively.