Exposure:A More Detailed Definition
Exposure is defined as charge to which the target material is exposed per unit mass
The unit of exposure to electromagnetic radiation is the Roentgen
The Roentgen was defined as 1 esu/cm3 air at STP
r(air)STP = 1.293 …10-3 g/cm3
1 esu = 3.34 …10-10 Coulomb
So 1R = 3.34 …10-10 C /(1 cm3 *1.293 …10-6 kg/cm3)
i.e. 1 Roentgen = 2.58…10-4 C/kg