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 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 (C/kg), however the roentgen (R) is commonly used internationally in the nuclear industry.[2] 1 R equals 0.000258 C/kg; 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]

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

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

Quantity Name Symbol Unit Year System
Activity (A) curie Ci 3.7×1010 s−1 1953 non-SI
becquerel Bq s−1 1974 SI
rutherford Rd 106s−1 1946 non-SI
Exposure (X) röntgen R esu / 0.001293g of air 1928 non-SI
Fluence (Φ) (reciprocal area) m−2 (cm−2) 1962 SI (non-SI)
Absorbed dose (D) erg·g−1 1950 non-SI
gray Gy J·kg−1 1974 SI
Dose equivalent (H) röntgen equivalent man rem 100 erg·g−1 1971 non-SI
sievert Sv J·kg−1×WR 1977 SI

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]

## 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

## Notes

1. ^ Knoll, p. 56
2. ^ Holmes-Siedle and Adams, p. 4
3. ^ Carron, p. 141
4. ^ Knoll, p. 57
5. ^ Stanford University Environmental Health and Safety, radionuclide safety data sheets
6. ^ 10 CFR 20.1004. US Nuclear Regulatory Commission. 2009.
7. ^ The Council of the European Communities (1979-12-21). "Council Directive 80/181/EEC of 20 December 1979 on the approximation of the laws of the Member States relating to Unit of measurement and on the repeal of Directive 71/354/EEC". Retrieved 19 May 2012.