Ion beams (particles or particle beams) are defined as rapidly moving ions designated with non-zero rest mass. When passing through matter, these ions lose their energy due to scattering processes, until the kinetic energy of the ions has finally sunk to the value of thermal energy. The route the particles cover in order to get to this level is called range. The range depends on the particle type, the initial energy and the material the particle passes through. The energy transfer per distance unit on the matter also is determined by the particle type, the material and the energy of the moment. The energy transfer generally increases per distance unit travelled by the particle. The curve describing this increase of energy transfer is known as the Bragg curve, named after William Henry Bragg. Shortly before the end of the route the energy transfer achieves a maximum and then drops abruptly to zero. This effect is made use of in ion beam therapy in order to focus the attack on the tumour and spare the surrounding tissue.