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Stopping power is the parameter used to describe the gradual energy loss of a (heavy) ionizing (charged) particle as it penetrates into an absorbing medium. It is defined by how much energy the particle loses per unit path length in a material, while the penetration range is roughly the total distance it travels before losing all its energy. The term 'power' is historical. Absolute linear stopping power is the measure of the energy loss of the particle or electrons traversing a material expressed in units of, for example, MeV/cm and J m^{1} not J s ^{1}. The relative stopping power is the inverse ratio of the traveled length of this material to the traveled length of a reference material such as air required to produce the same energy loss. The mass stopping power is the stopping power divided by the density of the stopping material, defined as dE_{0}/d(ρs), where E_{0} is the energy of incident particles, ρ is the density of the target and s the distance traveled.
Collision (ionization) stopping power and radiative stopping power are two known types of stopping powers. The total stopping power S_{tot} for a charged particle of energy E_{0} traveling
through an absorber with atomic number Z is the sum of the radiative and
collision stopping power given by,
S_{tot} = S_{rad} + S_{col}  [4434]
Stopping powers depend on the properties of the charged particle such as its mass, charge, velocity and energy as well as on the properties of the absorbing matter such as its density and atomic number.
