| The gas proportional counter technology has seen vast changes in the last decade and has lead to the introduction of micro-pattern gas detectors that offer much more improved x-ray counting rate capability and better position resolution than traditional wire chambers. In this study, we investigated the Gas Electron Multiplier, a member of micro-pattern gas detector generation, as an energy sensitive, single photon-counting x-ray detector for digital radiography. We built a high pressure detector with two cascaded Gas Electron Multipliers to evaluate its performance characteristics in Neon, Argon, Krypton and Xenon mixtures. The determination of detector gain as a function of pressure and the filling gas formed the major part of this evaluation. In the second part of the study, we employed position sensitive electronics to demonstrate the imaging capability of the detector. For the first time, we acquired 2D images and examined the spatial resolution of the detector for Krypton and Xenon mixtures as a function of gas pressure using double and quadruple GEM configurations up to 7 atmospheres gas pressure. Besides the experimental studies, we constructed a theoretical model based on the Cascaded Linear Systems approach to study the imaging performance of single photon counting gas detectors. In the model, we gave the emphasis to the intrinsic properties of the gas absorber; therefore we considered the contribution of quantum efficiency, primary electron range and reabsorption of K fluorescence photons on detector MTF and DQE. |
