Development of large-aperture GEMs as a gating device of the ILC-TPC for blocking positive ion feedback

The feedback of positive ions in gas amplification detectors of a time projection chamber (TPC) causes adverse effects on the electric field in the drift region, thereby degrading the spatial resolution of the TPC. Instead of employing a conventional wire-grid as a gating device, as often done to alleviate this problem, there is a proposed solution to use a gas electron multiplier (GEM) operated in low voltage mode as gate. The gate is required to block the positive ions when it is closed and to have high transparency to drift electrons when it is open. A TPC for the international linear collider (ILC) will be operated in a 3.5 tesla axial magnetic field, and in a gas with a long mean-free-time of drift electrons between collisions with gas molecules. Consequently, the motion of electrons is strongly restricted to the direction of the magnetic field. Therefore high optical transparency of the gate is required to ensure its high transmission rate of the electrons in the open state. A gate having a GEM-like structure would be most suitable for a readout module structure of the ILC-TPC since it is easier to implement and allows a low switching voltage of a few tens of volts. Ordinary amplifying GEMs are, however, not suitable because of their poor optical transparency. To achieve high electron transmission, several different types of large-aperture GEMs which have 75% to 85% optical transparency were produced using various processing methods (e.g. UV-laser ablation technology) by a Japanese company. These samples were tested with a test chamber installed in a 1-tesla solenoid magnet, and the high electron transmission was confirmed. Also presented is the evaluation of their performance by simulations using an ANSYS-Garfield framework.