Mean Excitation Energy

Abstract

The mean excitation energy, $\overline{I}$, appears in the logarithmic term of the soft collision stopping power. In practice, it is determined through extraction from measurements of the energy loss of a charged particle traversing a medium. However, there remains significant interest in understanding the theoretical aspects of the parameter, including deriving it ab initio and discovering that quite simplistic models can recreate the fundamental physical properties of $\overline{I}$. In this chapter, theoretical models of increasing complexity are considered in the calculation of the mean excitation energy. The chapter is concluded with a review of the mechanisms of experimental determination of $\overline{I}$ and parameterisations of the quantity. Although the latter are of limited practical use since tabulated values of $\overline{I}$ exist, it is of historical interest to consider them.