**STOPPING POWER** refers to the inelastic energy losses by an electron moving through a medium with density *r* are described by the total mass–energy stopping power $\left ( \frac{S}{\rho} \right )_{tot}$, which represents the kinetic energy $E_K$ loss by the electron per unit path length *x*, or:

$\left ( \frac{S}{\rho} \right )_{tot}$ consists of two components: the mass collision stopping power ($\left ( \frac{S}{\rho} \right )_{col}$), resulting from electron–orbital electron interactions (atomic excitations and ionizations), and the mass radiative stopping power ($\left ( \frac{S}{\rho} \right )_{rad}$), resulting from electron–nucleus interactions (bremsstrahlung production):

(S/ρ)tot = (S/ρ)col + (S/ρ)rad

(2)**LINEAR ENERGY TRANSFER** of charged particles in a medium is the quotient dE/dl, where *dE* is the average energy locally imparted to the medium by a charged particle of specified energy in traversing a distance of *dl*.