Numerical studies of transient gain reduction process in a multi-wire proportional chamber

A gain reduction process caused by successive beam irradiation in a multi-wire proportional chamber was numerically investigated to clarify the relations between the gas gain variation and the ion density distribution. A numerical code was developed based on a two-dimensional drift-diffusion model in order to evaluate the ion and electron density distributions and the electric field variation caused by the space charge effect. In order to consider the gain reduction process which occurs under the high rate and successive irradiation, the simulations were performed for the time period of ∼10-100 μs, which is much longer than the time required for ions to travel from an anode to a cathode. The numerical simulation results showed that for the low gas gain regime of ∼10, quasi-stationary density distribution of the ions was formed by the high-rate beams of ∼10(8)-10(10) particles per second, and that the transient variation of the gas gain became constant after establishment of the quasi-stationary ion density distributions.