Former unit of ion dose, symbol: R. The ion dose of 1 roentgen is equal to the generation of an electrostatic charge unit by gamma or x-radiation in 1 cm3 dry air under normal conditions (1.293 mg air). The new unit of the ion dose is coulomb divided by kilogram (C/kg). 1 R = 258 ยตC/kg; 1 C/kg approx. 3 876 R.