radiation exposure

Exposure is the amount of ionizing photon present in a radiation beam. The amount of ionizing photon in a medium is determined by the number of ions generated by them. Air is an universal medium for measuring exposure.

A Roentgen X or Gamma is the amount of radiation through which 0.001293 grams of air-entrapped corpuscular emission can flow 1esu of any sign.

The SI unit of exposure is Roentgen (R). An exposure unit is the amount of photons that generate a coulomb charge in 1 kilogram of air. 

1R = 2.58 X 10-4 C / Kg of Air

The radiation monitor device is callibrate in the zone or millimeter. The output of an x-ray machine is measured by mR / mAs. For example, a 75Kv, X-ray machine with a 2mm Al filtration gives an output of 5mR / mAs at 100cm distance.

Measurement of exposure is easier with an ionization chamber filled with air. The effective atomic number of air and soft tissue is almost the same. Therefore, the exposure measured in diagnostic X-ray energy is similar to that of soft tissue. Exposure is defined only for X-ray and gamma radiation in air.