The Simulation And Experiment Research Of Micromegas

Micromegas detector (MICRO MEsh GAseous Structure) is a gaseous detector developed since 1990's. Micromegas is a kind of plane-structure gas detector. It consists of drift gap, amplification gap, and a thin mesh which separates the two regions. The drift gap is 3mm～10mm, and the amplification gap is near 100μm. It shows high rate capability up to 108mm-2.s-1, excellent spatial resolution and good performance of radiation hardness, which make it a suitable candidate in high-intensity X-ray detection and tomography. Its simple structure and low cost have been adopted by many experiments in particle physics experiments, such as charged particle localization, TPC, neutron or X-ray imaging.The performance of Micromegas is studied by the 3D simulation and the experiments carefully and particularly.We combine the Maxwell field calculation software and Garfield to simulate the performance of Micromegas in detail. The column model as the line of mesh is used in the 3D model building for the first time, and the column model is more similar to the mesh in the experiment than the cubic model. We calculate the electron drift velocity, the diffusion coefficients and the Townsend coefficient change with the electric field in different gas mixtures. In Argon90% + Isobutane10% gas mixture the applied voltage of drift electrode mesh and amplification electrode mesh are -510V, -490V, respectively, and near 180μm of the minimum of space resolution is got when the diffusion effect is considered only. The electric field lines end on the mesh of amplification electrode, and most ions produced in the avalanche process drift to the mesh, and are absorbed by the mesh. So Micromegas shows high rate capability. The electron collection efficiency changes with the electric field ratio of amplification gap and drift gap. For the 20~30μm diameter mesh, the simulation result shows that the electron collection efficiency rises up to the plateau when the ratio of amplification gap and drift gap more than 200. And the electron collection efficiency can be larger with smaller diameter mesh. The gain can get the maximum when the amplification gap is 60~80μm. So the amplification gap is chosen in the range of 50~100μm usually. The fluctuation of amplification gap would change the gain relatively little. The amplitude of the signal is due to the electron movement. The signal after the pre-amplifier Pspice simulation is consistent with the signal observed by the oscillograph. We can understand the property of Micromegas by the simulation very well. The results of simulations provide important information on the choice of the gas, mesh and the structure.Based on the previous experiments, we summarize the facture technique of Micromegas, whose amplification gap is separated by fish lines, and used the 500LPI bias form mesh as amplification electrode, and 120μm fish lines as the amplification gap. The 350LPI tabby form mesh is chosen as the drift electrode. The performance of Micromegas, such as the uniformity amplification gap, the electron collection efficiency, the gain, energy resolution in different gas mixtures are measured and studied. In the experiment we measure the electron collection efficiency with changing the applied voltage of drift electrode and the constant voltage of amplification electrode. By the result comparison of two groups with different mesh voltage, we conclude that the change of the electron collection efficiency is independent with the voltage of amplification electrode and is related to the electric field ratio of amplification gap and drift gap, and the result is consistent with the simulation. The change of the electron collection efficiency in different gas mixtures shows the electron compound effect reduces with the decrease of Isobutane. We calculate the percents of Argon atoms which participate in the Penning effect in Argon and Isobutane gas mixtures. We find that the energy resolution of the maximum gain is not the best resolution value, for the electron compound effect on low electric field. When the Isobutane percent is decreased, the fluctuation of electron number is weakened by the increasing of the electric field ratio of amplification gap and drift gap, and the ratio plateau is prolonged and the energy resolution of the maximum gain is improved. We measured the Micromegas using the hot stamping foils as the amplification gap. The energy resolution of 5.9keV X-ray is better than 22%, and it is a valuable new technique.There is an important problem on the technique of improving the uniformity of the amplification gap. The research of bulk technique should be studied to prepare for the X-ray imaging experiment. It is expected to reach the international standards and bring forth some new ideas for more application in the future experiments including the third-generation synchronization radiation light source and the spallation neutron source. With good time and space resolution, Micromegas can be widely applied in medical engineering and industrial X-ray tomography.