The detection of elementary particles is based on their interactions with matter. Swiftly moving charged particles produce ionization and it is this ionization that is the basis for most techniques of particle detection. During the 1930s cosmic rays were studied primarily with cloud chambers, in which droplets form along the trails of ions left by the cosmic rays. If the cloud chamber is in a region of magnetic field, the tracks show curvature. According to the Lorentz force law, the component of the momentum in the plane perpendicular to the magnetic field is given by p(MeV/c) = 0.300×10−3B(gauss)r (cm) or p(GeV/c) = 0.300× B(T)r (m), where r is the radius of curvature. By measuring the track of a particle in a cloud chamber it is possible to deduce the momentum of the particle.

The energy of a charged particle can be deduced by measuring the distance it travels before stopping in some medium. The charged particles other than electrons slow primarily because they lose energy through the ionization of atoms in the medium, unless they collide with a nucleus. The range a particle of a given energy will have in a medium is a function of the mass density of the material and of the density of electrons.