Common symbols
X
SI unitC/kg
Other units
röntgen
In SI base unitsAs/kg

Radiation exposure is a measure of the ionization of air due to ionizing radiation from photons; that is, gamma rays and X-rays. It is defined as the electric charge freed by such radiation in a specified volume of air divided by the mass of that air.

The SI unit of exposure is the coulomb per kilogram (C/kg), which has largely replaced the roentgen (R). One roentgen equals 0.000258 C/kg; an exposure of one coulomb per kilogram is equivalent to 3876 roentgens.

As a measure of radiation damage exposure has been superseded by the concept of absorbed dose which takes into account the absorption characteristic of the target material.

## Exposure conversion to absorbed dose

Dose is the measure of energy per unit mass deposited by ionizing radiation. For a given radiation field, the absorbed dose will depend on the type of matter which absorbs the radiation. For instance, for an exposure of 1 roentgen by gamma rays with an energy of 1 MeV, the dose in air will be 0.877 rad, the dose in water will be 0.975 rad, the dose in silicon will be 0.877 rad, and the dose in averaged human tissue will be 1 rad. A table giving the exposure to dose conversion for these four materials for a variety of gamma ray energies can be found in the reference.

## Exposure rate constant

The gamma ray field can be characterized by the exposure rate (in units of, for instance, roentgen per hour). For a point source, the exposure rate will be linearly proportional to the source's radioactivity and inversely proportional to the square of the distance,

F = Γ×α / r2

where F is the exposure rate, r is the distance, α is the source activity, and Γ is the exposure rate constant, which is dependent on the particular radionuclide used as the gamma ray source.

Below is a table of exposure rate constants for various radionuclides. They give the exposure rate in roentgens per hour for a given activity in millicuries at a distance in centimeters.

Exposure rate constants for various radionuclides R•cm2 / hr•mCi
cobalt-60 12.838
molybdenum-99 1.03
technetium-99m (6 hour) 0.720
silver-110m (250 day) 14.9
caesium-137 3.400
iodine-125 (unfiltered) 1.46
iridium-192 (unfiltered) 4.69

The following table shows radiation quantities in SI and non-SI units:

Quantity Unit Symbol Derivation Year SI equivalence
Activity (A) becquerel Bq s−1 1974 SI unit
curie Ci 3.7 × 1010 s−1 1953 3.7×1010 Bq
rutherford Rd 106 s−1 1946 1,000,000 Bq
Exposure (X) coulomb per kilogram C/kg C⋅kg−1 of air 1974 SI unit
röntgen R esu / 0.001293 g of air 1928 2.58 × 10−4 C/kg
Absorbed dose (D) gray Gy J⋅kg−1 1974 SI unit
erg per gram erg/g erg⋅g−1 1950 1.0 × 10−4 Gy