CPB Ljubljana 2018 course book

Exposure X

Let us consider a volume element dV filled by air of mass dm at a point in a photon radiation field. If dQ is the absolute value of the total charge of the ions of one sign produced in air when all the electrons and positrons liberated or created by photons in the air filled volume element dV are completed stopped in this volume , then the exposure X (C.kg -1 ) is defined as

dm dQ

dV dQ

. 1   

X

air

a

dm=ρ

.dV

α

where ρ α is the density of air in volume dV. It has to be mentioned that according to this definition ionisation produced by Auger electrons are included in dQ where ionisation produced by radiative processes such as bremsstrahlung and fluorescence photons is not included in dQ . The SI unit of Exposure is 1 C.kg -1 .