(r). The international unit of quantity or dose for both X-rays and γ-rays. It is defined as the quantity of X or γ rays that will produce as a result of ionization one electrostatic unit of electricity of either sign in 1 cc (0.001293 g) of dry air as measured at 0C and standard atmospheric pressure. The use of the roentgen unit has been extended to include particle radiation such as α- and β-particles and protons and neutrons.See Rad; Curie.