CPB Ljubljana 2018 course book

Kerma K

Let us consider a volume element dV filled by a material of mass dm at a point in photon radiation field. The quantity kerma refers to the kinetic energy of charged particles such as electrons and positrons that have been liberated by uncharged particles (photons). Kerma does not include the energy that has been expended against the binding energies of these charged particles even if this is usually a relatively small component. The name kerma for this quantity is derived from the Kinetic Energy Released per unit Mass. tr is the sum of the initial kinetic energies of all charged particles liberated by uncharged particles within a volume element dV of a material containing a mass dm=ρ.dV of that material, then the kerma K (J.kg -1 ) is given by If dE

dE

dE

1

tr

tr

K





.

air

dm

dV



The name of the SI unit for kerma is gray (Gy): 1 Gy = 1 J.kg -1 . Considering dE tr it has to be mentioned that this includes also the kinetic energy of Auger electrons. dm=ρ.dV