CALCULATION OF FIELD DISTORTION BY A LARGE-GAINED AVALANCHE IN A GAS COUNTER

Simulation performed by using the continuity equation of electrons and positive ions is applied to examine the three-dimensional formation a nd the time evolution of an electron avalanche in a cylindrical gas co unter. The field made by space charge is calculated exactly and concur rently by the relaxation method in the evolution. The gas multiplicati on for photon detection is consistently estimated for an Ar-based mixt ure by not using any adjustable parameter for geometrical dependency. Field distortion at the time of the finish of the evolution is studied systematically. The distortion is concentrated in a small region of a ngle in case of a large diameter of the anode. The shape of the distor tion is calculated for several distributions of the initial charge. Th e distortion is enhanced by the distribution that occupies small area in the transverse plain in the avalanche. Change of the field distorti on depending on the anode diameter and the initial charge distribution is shown quantitatively by these calculations. The transition to a st reamer by the continuous supply of electrons is suggested to depend st rongly on the charge distribution in an avalanche.