One of the earliest observed properties of x-rays was their ability to ionize air, and in 1928 the International Congress of Radiology specified this property as a means of measuring the amount of x-radiation, and defined a unit which was named the roentgen , (R).
The roentgen is defined as that amount of x- or gamma radiation which will
produce by ionisation one electrostatic unit of
charge in 1 cubic centimeter of dry air at STP. The SI unit of
exposure is the coulomb per kilogram
( 
).
The concept of exposure is now seldom used in
radiation
protection but is usually reserved for expressing
the output of radioactive sources of radiation.