22.101 Applied Nuclear Physics (Fall 2004) Lecture 14 (11/1/04) Charged-Particle Interactions: Stopping Power, Collisions and Ionization

When a swift charged particle enters a materials medium it will interact with the electrons and nuclei in the medium and begins to lose energy as it penetrates into the medium. The interaction can be generally thought of as collisions between the charged particle and either the atomic electron or the nucleus (considered separately). The energy given off will result in ionization, production of ion-electron pairs, in the medium; also it can appear in the form of electromagnetic radiation, a process known as bremsstrahlung (braking radiation). We are interested in describing the energy loss per unit distance traveled by the charged particle, and the range of the particle in various materials, the latter being defined as the distance traveled from the point of entry to the point of being essentially rest. A charged particleis called ‘heavy’ if its rest mass is large compared to the rest mass of the electron. Thus mesons, protons, α -particles, and of course fission fragments are all heavy charged particles. By the same token, electrons and positrons are ‘light’ particles. If we ignore nuclear forces and consider only the interactions arising from Coulomb forces, then we can speak of four principal types of charged-particle interactions: