Radiative stopping power (Srad) is due to radiative interactions and is mostly based on Bremsstrahlung production alone. Different from collision (ionization) stopping power, only light charged particles
(electrons and positrons) experience appreciable energy losses through
these interactions. This type of radiative stopping power results from charged particle Coulomb interaction with nuclei of the absorber. Energy spent in radiative collisions is carried away from the charged-particle track by the photons.
The radiative stopping power can be evaluated easily by the mass radiative stopping power Srad (in MeV · cm2/g), which is one type of the mass stopping powers. Srad is given by,
Na -- The number of atoms per unit mass (Na = N/m)
σrad -- The total cross section for bremsstrahlung production
Ei -- The initial total energy of the light charged particle, given by Ei = EKi +
EKi -- The initial kinetic energy of the light charged particle
Inserting σrad for non-relativistic particles Srad can by re-written by,
where Brad is a slowly varying function of Z and Ei. The mass radiative stopping power Srad is proportional to (NAZ2/A), that indicates a proportionality with the atomic number Z of the matter and the initial total energy Ei of the light
charged particle as shown in Figure 4431.
Figure 4431. Mass radiative and collision stopping powers for electrons in water (H2O), aluminum (Al)
and lead (Pb)
shown with red and green curves, respectively, against the electron kinetic energy. Adapted from NIST.