Monte Carlo simulations of microgap gas-filled proportional counters

Monte Carlo calculations have been widely employed to model the interactions of electrons and photons as they travel through and collide with matter. This approach has been applied with some success to the problem of simulating the response of gas-filled proportional counters, mapping out electron transport through the electric field on an interaction-by-interaction basis. These studies focus on the multiplication of electrons as they drift into the high electric field region of the detector and subsequently avalanche. We are using this technique in our new simulation code to depict avalanching in microgap gas-filled proportional counters, in order to investigate the variation elf two principle detector properties with the anode pitch used in the detector. Spatial resolution information can be obtained by measuring the lateral diffusion distance of an electron from the point where it is liberated to the point in the detector where it initiates an avalanche. By also modeling the motion of the positive ions that are left behind fi om the initial avalanche, we are able to gauge the effect of space charge distortion on subsequent avalanches. This effect is particularly important at the high X-ray count rates that we are interested in for our ultimate aim, which is to use the detectors as part of a high-speed tomography system for imaging multiphase oil/water/gas flows.