Exposure expresses the intensity of an X- or gamma-ray beam;
The SI unit of exposure is the coulomb per kilogram (C/kg);
1 C/kg = The quantity of X- or gamma-rays such that the associated electrons emitted per kg of air at STP produce in air ions carrying 1 coulomb of electric charge;
The traditional unit of exposure is the roentgen (R);
1 R = The quantity of X- or gamma-rays such that the associated electrons emitted per kg of air at STP produce in air ions carrying 2.58 x 10-4 coulombs of electric charge;
The exposure rate is the exposure per unit time, e.g. C/kg/s;
Absorbed dose is the radiation energy absorbed per unit mass of absorbing material;
The SI unit of absorbed dose is the gray (Gy);
1 Gy = The absorption of 1 joule of radiation energy per kilogram of material;
The traditional unit of absorbed dose is the rad;
1 rad = The absorption of 10-2 joules of radiation energy per kilogram of material;
The Specific Gamma-Ray Constant expresses the exposure rate produced by the gamma-rays from a radioisotope;
The Specific Gamma-Ray Constant is expressed in SI units in C/kg/s/Bq at 1 m;
Exposure from an X- or gamma-ray source follows the Inverse Square Law and decreases with the square of the distance from the source.
