Gas-filled detectors are simple, portable, and rugged. They operate by utilizing the ionization produced by radiation as it passes through a gas. They generally consist of two electrodes to which a certain electrical potential is applied. The space between the electrodes is then filled with a gas. Ionizing radiation loses energy to the gas by creating excited molecules and generating electron-ion pairs of positive ions and electrons. On average, about 30-35 eV of energy is lost per electron-ion pair created. If they existed in an environment without an external electrical field, these charges would simply recombine.
With an external electrical field (EV) applied, the positive and negative charges drift in opposite directions, this motion results in an electric current on the electrodes that can be measured externally. The detector signal is generated by the motion of charge in the electric field, and not by the arrival of charges at the electrodes
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