Range (particle radiation)

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In passing through matter, charged particles ionize and thus loose energy in many steps, until their energy is (almost) zero. The distance to this point is called the range of the particle. The range depends on the type of particle, on its initial energy and on the material which it passes.

The energy loss per unit distance (and hence, the density of ionization) also depends on the type and energy of the particle and on the material. Usually, the energy loss per unit distance increases while the particle slows down. The curve describing this fact is called the Bragg curve. Shortly before the end, the energy loss passes through a maximum, the Bragg Peak, and then drops to zero (see the figures in Bragg Peak and in stopping power). This fact is of great practical importance for radiation therapy.

The range of alpha particles in ambient air amounts to several centimeters, this type of radiation can therefore be stopped already by a sheet of paper. The range of beta radiation, on the other hand, frequently amounts to several hundred centimeters of air.

The mean range can be calculated by integrating the inverse Stopping power over energy.