Carbon coated gas electron multipliers for time projection chamber prototype

A Time Projection Chamber (TPC) is one of the two main proposed tracking detector concepts for the planned International Linear Collider (ILC). At university of Siegen Gas Electron Multipliers (GEMs) are used for gas amplification inside a TPC. To avoiding charging up of the holes of GEM and to maintain a constant gain, GEMs have been coated with a high resistivity 1012-1013 Ω/□ carbon layer. Previous measurements showed, that a thin layer of carbon on both sides of a GEM have a positive influence on some of its parameters. An overcoating with carbon should increase the time stability of the gain and resistance. Other advantages are the reduction of electrostatical charge of the kapton layer and the possibility to increase the gain of each GEM by operating the GEM on higher voltages since the coating with a thin carbon layers leads to a spark reduction at high electrical fields between the upper and lower side of the GEM. At University of Siegen we are able to measure in our lab inside a small test chamber by using a pad readout to measure amplification. Fe55 source is used to ionize the gas. The GEMs, which we tested, were coated at the ''Fraunhofer Gesellschaft fuer Schicht und Oberflaechentechnik'' with two different type of carbon layers. We operated these GEMs up to 500 V and achieved a gain of 755 for a single GEM.