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(Science: radiobiology, unit) The special unit of exposure, based on a quantity of ionisation (charge) produced by the absorption of X or gamma radiation energy in a specified mass of air under standard conditions.

1R = 2.58 x 10-4 C kg-1 or air.

For radiation protection purposes, an exposure to 1 roentgen of x or gamma rays (air kerma of ~10-2 J kg-1) is generally assumed to produce an absorbed dose of 1 rad in water or soft tissue.

Abbreviation: R