An ionization chamber calibrated to measure air kerma will display units of:

• A. Milliampere-seconds (mAs)
• B. Gray (Gy)
• C. Sieverts (Sv)
• D. Coulombs per kilogram (C/kg)

Answer: (B)

An ionization chamber that is calibrated to read air kerma translates the ionization charge it collects into an energy-per-mass quantity. Air kerma is the amount of energy transferred from the radiation to air per unit mass, and for photon beams this energy transfer relates directly to the absorbed dose in air under typical measurement conditions. Because of that calibration, the readout is given in gray, the SI unit for absorbed dose (J/kg). This makes gray the most appropriate display unit because it directly expresses the energy deposited per mass, which is precisely what the chamber’s signal has been converted to.

Milliampere-seconds describes the x-ray tube current-time product, not the detector’s dose-related readout. Sieverts would be used for dose equivalent, which includes biological weighting factors and is not the raw measurement from an ionization chamber. Coulombs per kilogram is an older unit for exposure in air; while related, the calibrated readout for air kerma is expressed in gray to reflect energy deposition per mass.