A Time Projection Chamber with Gas Electron Multipliers
For the future Linear Collider project TESLA the Technical Design Report (TDR) foresees a Time Projection Chamber (TPC) as the central tracking device. Due to the large variety of physics expected at TESLA the detector has to fulfill stringent requirements. The TPC has been chosen due to its high number of space points, the high granularity and the low material thickness. The aims for resolution etc. can be found in the for TESLA.
To come up with these challenging aims it is proposed to use the so called Gas Electron Multipliers (GEMs) instead of wires for the gas amplification. Besides the better spatial resolution GEMs provide an intrinsically suppressed ion feedback. Here in Karlsruhe a prototype for such a GEM-TPC was built to do some R&D work for the future detector. In the following sections we will give further information on GEMs, on our TPC prototype and on results from different measurements with the prototype.
The GEM foils were developed in 1996 by Fabio Sauli. Originally planned as preamplifiers for Micro Strip Gas Counters (MSGCs) it worked out, that their gas amplification could be strong enough to use them as the unique gas amplification device in gas detectors. By using two or three GEMs in series the needed gas amplifiations of A=10^3 to 10^4 can be achieved.
GEMs consist of a very thin, two-side metal-clad polymer foil (56mm total thickness), which is perforated with a high density of photolithographically etched holes. On application of a potential difference (usually 300-500 Volts) between upper and lower metal layer, a strong electric field is built up inside the holes. Inserted in the drift gap of a gas detector, drifting electrons from ionizing particles are guided into these holes, where they undergo proportional gas amplification. A major fraction of all electrons is then released into the volume below the GEM foil, where they can be collected by readout electronics or, in order to achieve higher total gains, transferred to another GEM.
GEMs are classified by the type and the distances of their holes. We distinguish between different
arrangements of the holes (quadratic or hexagonal), between different sectional views and between the following GEM parameters:
The so called Standard GEM is of the following type:
The electrical transparency tau of GEMS is very important for their use in TPCs. It depends on the above geometrical quantities but also on the electrical fields under and above the foil. A simulated (MagBoltz) example for this statement is shown in the following picture:
In these simulations the GEM voltage was chosen to 500V, the field under the GEM to 5000 V/cm and the field above the GEM as written in the examples. We easily see that the electrical transparency for electrons is bigger for lower fields above the GEM.
The above text shows clearly why it is considered to use GEMs as the gas amplification method in TPCs, but there are other advantages of the usage of GEMs in TPCs in comparison to wires, e.g. the intrinsically suppressed ion feedback. For a better understanding of this point we use the following picture:
The approaching electrons are amplified in the holes and are able to continue their way to the endplate. In contrast to this are the ions that are built during the amplification process mostly absorbed on the upper side of the GEM foil. This process has to be examined by further measurement. If the intinsic suppression of the ion feedback is not enough, there are other possibilties to suppress the ion feedback through e.g. a third GEM above the other GEMs with a configuration that is ideal for an ion absorption or through the well known gating wires.
For a GEM-TPC the well-known pad structure of the endplate should be maintained. The principal difference between the formed signal in a conventional wire TPC and a GEM-TPC is shown in the following picture:
While a wire TPC induces a wide signal on the underlying pads, the signal width of a GEM-TPC is only given by the transverse diffusion coefficient. On application of a magnet field (4T as it will be in TESLA) this coefficient can be rather small and it is possible that only one pad is hit by an event, what is not wanted, because the spatial resolution can be improved by calculating the hit by the COG method (center of gravity). By changing the form of the pads you can achieve artificially that more than one pad is hit, e.g. through so called chevrons (middle of the following picture) or different formed pads.
In the year 2002 a new GEM-TPC prototype was built in Karlsruhe. A description of its construction can be found in IEEE 2003 (Kappler). For this reason here will only be presented some pictures and short additional explanations.
In order to study different questions on GEM-TPCs there were built a short (12.5cm) and a long (25cm) drift cylinder. All parts of the detector prototype are easily replaceable, so that the TPC can be equipped with different types of gas detectors and pad geometries.
In this picture you can see a schematic image of one of our possible readout endcaps for the TPC. It is equipped with a double-GEM structure and a readout system for measuring drift velocities, diffusion coefficients and temperature dependencies of the gain etc.
Here you see the profile of our drift cylinder. It consists of two resistor chains that are positioned outside of the drift volume so that possible heating of this volume is avoided. There are left some windows in most of the layers, so that it is possible to radiate the sensible volume with conventional radioactive sources.
Soon you will find here a short summary of possible methods and tools for our measurements
Look at our publication part to see the results now ... or wait till this will be finished
Thanks for your interest, TESLA-TPC group Karlsruhe