2.9.2 Units related to Exposure
Exposure refers to the ability of x-ray or gamma radiation to ionize the matter. As per IAEA Safety Glossary (2007), exposure is the sum of the electrical charges of all of the ions of one sign produced in air by X rays or gamma radiation when all electrons liberated by photons in a suitably small element of volume of air are completely stopped in air, divided by the mass of the air in the volume element. Therefore, Exposure is expressed in units of charge created after the ionization of a unit of mass (Coulomb per Kilogram), and can be expressed in absolute or rate units (see Table below). Historically, exposure has been expressed in non-SI unit Röntgen (R), being the conversion 1 Röntgen = 2.58×10−4 C/kg.
| Quantity | Symbol | Unit | Dimension | Use/Conversion of older units |
|---|---|---|---|---|
| Exposure | X | coulomb per kilogram (C/kg) | kg-1sA | ionizing effect of X and gamma rays in air |
| Exposure rate | X' | ampere per kilogram (A/kg) | A kg-1 | exposure per unit time, gamma radiation field. 1μR/h=7.17x10-14A/kg |
