Note: In ion-beam analysis, when the incident ion is far from the target nucleus, the atom in which the nucleus sits looks neutral to the ion as a result of screening. The screening reduces the scattering cross section slightly from the Rutherford cross section (about $1\mathrm{\%}$ per 50 increase in the target atomic number for $2\mathrm{MeV}$ $\mathrm{He}$). The effect becomes more pronounced and increasingly uncertain as the energy decreases and, for low-energy ion scattering spectrometry, the cross section is no longer really well-known.