Radiation exposure
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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 to be the electric charge freed by the radiation divided by the mass of the air. As a measure of radiation damage it is less useful than the analogous concept of absorbed dose, nevertheless since exposure is convenient to measure directly in gaseous ionization detectors and since it is easily converted to dose, it is commonly used in the nuclear industry.
The SI unit of exposure is the coulomb per kilogram, however the roentgen is commonly used internationally in the nuclear industry.[2] There are approximately 3876 roentgens in one coulomb per kilogram.
Exposure to dose conversion
Dose is the measure of energy per unit mass deposited by ionizing radiation. For a given radiation field, the 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 0.965 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]
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,[4]
- 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.[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 |
| cesium-137 | 3.400 |
| radium-226 | 8.25 |
References
- N. J. Carron, An Introduction to the Passage of Energetic Particles through Matter, 2007, Taylor and Francis Group
- Glenn F. Knoll, Radiation Detection and Measurement, fourth edition, 2010, John Wiley and Sons, Inc.
- Andrew Holmes-Siedle and Len Adams, Handbook of Radiation Effects, second edition, 2002, Oxford University Press
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