Introduction
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- Figure 1: Photo of the LP GEM module (click to see larger version)
In the framework of the LCTPC collaboration our group built the field cage of a TPC, prototype of a portion of the final ILD TPC, called the Large Prototype TPC, or LP TPC. This prototype was developed to allow the groups forming the collaboration to test the new technologies and techniques that will be necessary to validate to build a big TPC with the performances required by the ILD project. To this goal the anode endplate of the TPC can be equipped with up to seven active and exchangeable readout modules, developed by the members of the collaboration and tested on the DESY Test Beam line T24.
Our group had also previous, extensive expertise in the development of TPC detectors based on Micro Pattern Gas Detectors, in particular the Gas Electron Multiplier or GEM, thus we decided to exploit this expertise design and building a readout module for the LP to validate GEMs with pad readout and a ceramic integrated support system as a gas amplification device for the ILD TPC (see Figure 1).
Module Requirements
The requirements that drove the design of the DESY Module for the Large Prototype are:
- It must mechanically fit the LP.
- Its presence should produce negligible distortions of the electric field in the sensitive volume of the LP.
- It must use GEMs as gas amplification device.
- It should be able to fit an optional gating device.
- It must use the ALTRO based DAQ system developed for the LCTPC collaboration.
- The GEMs must be mounted using a support system built out of ceramics, developing on the previous research done on small TPC prototypes.
First Prototype: Korund
The first iteration of the DESY Module for the LP features:
- A readout board with almost 1000 pads as small as 1,2 x 5,7 mm2 laid out in a central column to be able to reconstruct the ionization trail of particles crossing the module perpendicular to the pad rows. The rest of the board is filled with 8 times bigger pads, to reduce the routing complexity of this first prototype and to increase the energy resolution when the module is commissioned with radioactive sources. On the top side of the board 20 slots provide the HV power supply for the GEM stack.
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- Figure 2: Detail of the Korund Module Pad Board showing the small pads on the central area on the right, the bigger one at their left and the HV power supply slots on top (click on an image to see larger version).
- Each GEM of the stack mounted in a sandwich of ceramic pieces which provide for the support, the stretching, the spacing and the aligning of the stack.
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- Figure 3: GEM mounted in a ceramic support piece (click on an image to see larger version).
- The possibility to stack up to 4 GEMs and power them, independently from one another, from the readout board. Each GEM can be easily installed, removed or replaced.
- A sensitive area of more than 95% of the available surface for a module of the large prototype.
