A unit of quantity of radiation, devised by H. M. Parker to fill the need for a unit to be used with types of radiation other than X rays and gamma rays, since the definition of the roentgen restricted that term's use to X rays and gamma rays. The rep is defined as the quantity of radiation which, absorbed in the body, would liberate the same amount of energy as 1 roentgen of X rays or gamma rays would (the name comes from roentgen equivalent physical). The value of the rep thus depends on the amount of energy liberated in the body by X rays producing 1 esu of charge, which is determined experimentally. At first it was taken to be 32.5 electron volts per ion-pair in air, and later to be 33.5. The latter value made the rep the amount of radiation which, absorbed in the body, liberated 97 ergs, and this definition was the one generally used.

The rep was never a widely used unit. It was replaced by the SI unit, the gray. One rep is approximately 8.38 milligrays.

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