Radiation exposure is a measure of the ionization of air due to ionizing radiation from photons; that is, gamma rays and X-rays.^{[1]} 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).^{[2]} One roentgen equals ; 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As a measure of radiation damage exposure has been superseded by the concept of absorbed dose which takes into account the absorption characteristic of the target material.
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.^{[3]}
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,^{[4]}
where F is the exposure rate, r is the distance, ? is the source activity, and ? 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.^{[5]}
Radionuclide | Exposure rate constant |
---|---|
cobalt-60 | 12.838 |
molybdenum-99 | 1.03 |
technetium-99m (6 hour) | 0.720 |
silver-110m (250 day) | 14.9 |
caesium-137 | 3.400 |
radium-226 | 8.25 |
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 × 10^{10} s^{-1} | 1953 | 3.7×10^{10} Bq | |
rutherford | Rd | 10^{6} 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 | |
rad | rad | 100 erg?g^{-1} | 1953 | 0.010 Gy | |
Dose equivalent (H) | sievert | Sv | J?kg^{-1} × W_{R} | 1977 | SI unit |
röntgen equivalent man | rem | 100 erg?g^{-1} | 1971 | 0.010 Sv |
Although the United States Nuclear Regulatory Commission permits the use of the units curie, rad, and rem alongside SI units,^{[6]} the European Union European units of measurement directives required that their use for "public health ... purposes" be phased out by 31 December 1985.^{[7]}