Conversion: Exposure to Absorbed Dose
Dose is defined as the amount of energy deposited by ionizing radiation in a substance. For a given radiation field, the absorbed dose will depend on the type of matter which absorbs the radiation. Although a large number of possible interactions are known, there are three key interaction mechanisms of gamma rays with matter.
For instance, for an exposure of 1 roentgen by gamma rays with an energy of 1 MeV, the dose in air will be 0.876 rad. This can be determined using the ionization energy of dry air at 20 °C and 101.325 kPa of pressure, which is 33.97 J/C. Therefore, an exposure of 2.58×10−4 C/kg (1 roentgen) would deposit an absorbed dose of 8.76×10−3 J/kg (0.876 rad) in dry air at those conditions. A table giving the exposure to dose conversion for various materials for a variety of gamma ray energies can be found in literature.
What is Exposure
Radiation exposure is a measure of the ionization of air due to ionizing radiation from high-energy photons (i.e. X-rays and gamma rays). Radiation exposure is defined as the sum of electrical charges (∆q) on all the ions of one sign produced in air when all the electrons, liberated by photons in a volume of air whose mass is ∆m, are completely stopped in air.
Radiation exposure is given the symbol X. The SI unit of radiation exposure is the coulomb per kilogram (C/kg), but in practice, the roentgen is used.
What is Absorbed Dose
Absorbed dose is defined as the amount of energy deposited by ionizing radiation in a substance. Absorbed dose is given the symbol D. The absorbed dose is usually measured in a unit called the gray (Gy), which is derived from the SI system. The non-SI unit rad is sometimes also used, predominantly in the USA.
Units of absorbed dose:
- Gray. A dose of one gray is equivalent to a unit of energy (joule) deposited in a kilogram of a substance.
- RAD. A dose of one rad is equivalent to the deposition of one hundred ergs of energy in one gram of any material.