Exposures are recorded and the records are retained for each person. Instruments for measuring ionized radiation typically include a sensing device and a readout device. Sensors are the most critical, because different types of sensors are appropriate for different types of radiation. Sensors include Geiger-Mueller tubes which are used in Geiger Counters, ionization chambers, luminescent detectors, scintillation detectors and photographic emulsions. Radiation entering the tube ionizes the gas and creates small currents that the instrument measures.
Ionization chambers measure beta, gamma and x-rays using a charge placed on an electrode in a tube.
Radiation ionizes the air surrounding the electrode and allows charge to leak away. The amount of charge lost is related to the amount of radiation arriving at the chamber. Pocket ionization detectors are common dosimetry devices. Radiation sensitive photographic film detects gamma, and x-rays. The radiation affects the emulsion similar to light.