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How do ion chambers measure gamma-ray exposure rates?

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detector output connected to a measuring circuit — radiation measurement Figure 1: (A) A simple equivalent circuit for the development of a voltage pulse at the output of a detector. R represents the resistance and C the capacitance of the circuit; V(t) is the time (t)-dependent voltage produced. (B) A representative current pulse due to the interaction of a single quantum in the detector. The total charge Q is obtained by integrating the area of the current, i(t), over the collection time, t_c. (C) The resulting voltage pulse that is developed across the circuit of (A) for the case of a long circuit time constant. The amplitude (V_max) of the pulse is equal to the charge Q divided by the capacitance C.

Ion chambers measure gamma-ray exposure rates by utilizing a gas-filled volume between two electrodes maintained at a specific voltage. When gamma rays enter the chamber, they ionize the gas, creating ion pairs. The applied electric field causes these charges to drift toward the electrodes, generating an electric current in an external circuit.1

These devices are typically operated in current mode within the ion-saturation region, where the measured current is directly proportional to the rate at which radiation forms charges in the gas. Because the historical unit of gamma-ray exposure, the roentgen, is defined by the amount of ionization charge produced per unit mass of air, air-filled ion chambers provide an accurate measurement of exposure rates. Due to the very low magnitude of these currents, sensitive electrometers are required to record the data. Alternatively, some chambers operate in integration mode, where the total ionization charge created over a period is determined by measuring the voltage drop across the chamber's capacitance.1