ABSTRACT
The physical principle behind a gas-filled detector is quite simple. Radiation interacts in either the chamber gas or the chamber walls, thereby liberating electrons from parent nuclei. Gas detectors can be operated in either a pulse mode or current mode. In this case, a single radiation particle, such as an alpha particle, beta particle, or gamma ray, interacts in the chamber volume, producing a charge cloud around the interaction site. Radiation interactions in gas can cause excitation and ionization. Excitation is a process in which the energy transferred to a gas atom or molecule is less than the ionization energy. Columnar recombination is of most concern for gas-filled detectors operated in pulse mode, because the background or interaction rate under most situations is usually high enough to cause appreciable volumetric recombination. Volumetric recombination occurs upon irradiation of a gas medium in high radiation fields, such that negative and positive ions encounter oppositely polarized charge carriers generated from multiple random events.
