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a subsidiary unit of the exposure dose of gamma radiation and X-radiation that is determined by the ionizing effect of gamma radiation and X-radiation on the air. This unit was named in honor of W. K. Roentgen. The international abbreviation for roentgen is R.
The quanta of gamma radiation and X-radiation effect the ionization of molecules in the air, which results in the formation of pairs of charged particles, including electrons of significant kinetic energy. These electrons in turn ionize the air. An exposure dose of gamma radiation or X-radiation is equal to 1 R when the corresponding corpuscular radiation (that is, electrons) produces ions whose total charge is equal to one electrostatic unit of the electric charge of each sign in 0.001293 g of air, or in 1 cm3 of air under normal conditions. It is understood here that the charged particles formed in 1 cm3 of air expend all the energy received on ionization.
By definition, the roentgen can be used only for radiation having quanta of energy no greater than 3 million electron volts (MeV). A dose of 1 R corresponds to the formation of 2.08 × 109 pairs of ions in 1 cm3 of air or 1.61 × 1012 pairs of ions in 1 g of air. In the International System of Units, the unit of exposure dose is 1 coulomb (C) per kg. According to GOST 8848–63, 1 R = 2.57976 × 10–4 C/kg. The average ionization energy of the molecules in the air is 34 electron volts, 1 R is equivalent to 88 ergs/g. This quantity is called the roentgen equivalent physical (rep).