The term exposure refers to the concentration, in air, of radiation at a specific point and is the ionization produced in a specific volume of air: E = Q / m where E is exposure, Q is the quantity of charge on the ions and m is the unit mass of air.

Exposure describes the ability of X-ray photons to ionize air and cannot be used for protons, neutrons or electrons. Exposure decreases with the square of the distance from an x-ray source (inverse square law). It was historically measured in roentgens (R), however the SI unit is the coulomb per kilogram (C kg-1):

  • 1R = 2.58×10−4 C kg-1

It is typically measured with an ionization chamber.

Exposure can be converted to absorbed dose with a conversion factor called f-factor. F-factor takes account for the photon energy of the x-rays and the medium irradiated. 

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rID: 18982
Section: Physics
Tag: stub, refs
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