Exposure is based on the ability of the radiation to produce ionization in air. It is defined only for radiation producing interactions in air. The exposure is the absolute value of the total charge of the ions of one sign produced in air when all the electrons liberated by the photons per unit mass of air are completely stopped, i.e. X = dQ/dm, where X is the exposure, dQ the total charge and dm the mass of air. The SI unit of exposure is Coulomb per kilogram (C/kg). The older unit of exposure was the Roentgen (R), where 1R = 2.58e-4 C/kg.
The absorbed dose, D, is the energy absorbed per unit mass. The quantity is defined for all ionizing radiation and for any material. The SI unit of absorbed dose is the Gray (Gy), where 1 Gy = 1 J/kg. The older unit of absorbed dose was the rad, where 1 Gy = 100 rad.
It is possible to calculate the absorbed dose in a material if the exposure is known i.e.
D(Gy) = f · X(C/kg)
where f is a conversion coefficient depending on the medium. The absorbed energy in a quantity of air exposed to 1 C/kg of X rays is 0.869 Gy. Hence for air, f = 0.869. The f value for water (similar to tissue) is 0.95 for 100 keV photons. Numerically therefore the dose/dose rate in tissue is similar to the exposure/exposure rate in air.
- IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology, L 2: Radiation units and dose quantities.