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16 Radiation for Radionuclide Users C. Measurement of Ionizing Radiation Ionizing radiation cannot be detected by the human senses. Its detection requires the identification of physical or chemical changes in a medium (gas, liquid, or solid) through which the radiation passes. Measurement of ionizing radiation requires the quantification of these physical or chemical changes. In general, detection and measurement methods can be categorized as being either active or passive. Active Detection Methods Active radiation detection systems can be loosely defined as those that require an electrical power source for operation. Such detectors are generally used to characterize dose rates, count rates, etc. though some also have the capability of measuring cumulative dose. There are two principal types of active detectors; gas ionization and scintillation. Gas Ionization Detectors Most gas ionization detectors consist of a gas-filled chamber with a voltage applied such that a central wire becomes the anode and the chamber wall the cathode (See Figure 3). Any ion pairs produced by radiation interacting with the chamber move to the electrodes where they are collected to form an electronic pulse which can be measured and quantified. Depending upon the voltage applied to the chamber, the detector may be considered an ion chamber , a proportional counter , or a Geiger-Mueller (GM) detector . Because of its versatility and dependability, the Geiger-Mueller detector is the most widely used portable survey instrument. It Diagram of a Gas Ionization Detector γ , x rays cathode (-) anode (+) α, β + - - - + +