Particle-matter interaction - Theory

The aim of this article is to provide an overview of particle-matter interactions, so that the reader can then move on to study the various techniques for detecting radiation with energies in excess of around ten keV, such as those encountered in nuclear or particle physics. Most of the time, we're dealing with ionizing radiation that transfers its energy to the electrons in the slowing medium: we'll talk about electronic stopping power (loss of energy per unit length of medium passed through). At lower energies - an area not covered in the following pages - particles are slowed down by elastic collisions with the atoms in the medium: this is referred to as nuclear stopping power (see figure).

Here, we need to distinguish between different classes of interaction, depending on whether we're considering charged or neutral radiation, heavy or light particles.

In the case of charged particles, energy is lost through discrete energy transfers to the electrons in the medium (ionization) and, in the case of light particles (mainly electrons), through the emission of braking radiation (bremsstralung) during acceleration in the vicinity of a nucleus.

In the case of neutral radiation (gamma, neutrons...), all or part of the energy is first transferred to a charged particle in the medium (electrons, nuclei), then the charged particle is detected.

The processes described above are discrete energy transfer processes. There are macroscopic processes in which a charged particle interacts with a given medium. In this case, the particle's energy loss is negligible, but a signal is emitted by the medium indicating the particle's passage. This is the Cherenkov effect, for relativistic charged particles, and transition radiation when a charged particle passes between two media with different dielectric properties.