Interaction of charged particles
Particle-matter interaction - Theory

In the case of heavy charged particles, i.e. from the proton onwards, with mass M very large compared to the electron mass m _e (M /m _e = 1,837), and for kinetic energies above 1 MeV, we observe particle slowdown without deviation (except in the highly improbable case of an encounter with a nucleus in the medium, which nonetheless enabled Ernest Rutherford to prove the existence of the atomic nucleus!). The conclusion is that the particle gradually loses its energy by transferring it to the electrons in the medium. This transfer of energy can take place through excitation or ionization of the atoms in the medium.