A self-consistent model of the discharge in halogen-quenched Geiger-Mueller radiation detectors

The Geiger-Mueller detector is one of the oldest gas-filled radiation detector and its production process is well established. The classical model of a Townsend avalanche can be used to explain the discharge mechanism of first generation detectors (long cylinder with a very thin anode, rare gas -- organic filling, working at about 1000 V). However modern detectors using rare gas -- halogen mixtures have considerably lower working voltages ({le} 500 V), and sometimes use a rather large anode or different shapes (i.e., {open_quotes}umbrella{close_quotes}) Clearly in these detectors other mechanisms are responsible for the discharge The present work reports the results of a 2D self-consistent model of the discharge in halogen-quenched GM detectors. The model is built on the local field approximation in cylindrical geometry, and solves the continuity equations for charged and excited species, the Poisson equation, and chemical kinetics equations. The response of the external electric circuit is taken into account. The mixtures studied are Ne-Cl{sub 2}-Cl (Cl being formed from the dissociation of Cl{sub 2}) with neon concentrations between 80% and 99% and Cl{sub 2} dissociation fractions between 0 and 30%. Transport parameters and collision frequencies were computed using a Boltzmann code based on the first moments 3more » of the Boltzmann equation in the hydrodynamic regime. A power-law scheme for the charged particles and an upwind scheme for the excited species, are used with a variable grid. In the description of the chemical kinetics, the species: Ne{sup +}, Cl{sup -}, Cl{sup +}, Cl{sub 2}{sup +}, were taken account of as well as radiative and metastable states, these ones related to Penning ionization. Photoionization in the gas and the photoelectric effect at the cathode were included but the imprisonment of the resonance radiation was only dealt with through the classical {open_quote}escape factor{close_quote}.« less