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"TPC Resolution studies using the concept of charge dispersion in MPGDs with a resistive anode"

A TPC read out with a system of Micro Pattern Gas Detectors (MPGD) such as the Micromegas or the Gas Electron Multiplier (GEM) will not have the systematic problems of existing wire/pad TPCs. The MPGD-TPC could, in principle, reach the diffusion limit. However, charge position determination methods that work for the wire/pad TPC are less effective and make the MPGD TPC resolution significantly worse than the diffusion limit. Narrower pads indeed lead to a better spatial resolution, but also lead to an increase in the number of readout channels and in complexity, which ultimately affect the overall cost of the detector.

Recent R&D at Carleton has focused on developing new techniques to improve the MPGD-TPC resolution over that achievable with normal techniques. A new concept based on the phenomenon of charge dispersion in MPGDs with a resistive anode has been developed which could enable one to approach the statistical limit of resolution from transverse diffusion. The resistive anode allow a controlled dispersal of track avalanche charge over a larger area to improve the determination of position centroids with wide pads. Our recent studies with the GEM and the Micromegas detectors instrumented with a resistive anode are quite promising as a possible readout option for the LC TPC. Using 2 mm wide pads, we have demonstrated better GEM/Micromegas-TPC resolution with a resistive anode readout than has been achieved with conventional MPGD TPC readout systems. The resolution is near the diffusion limit of resolution for a gaseous TPC. In cosmic tests with no magnetic field, the measured resolution follows the expectations from transverse diffusion and electron statistics. Beam tests in a magnet is next to demonstrate good resolution for a MPGD instrumented TPC in a magnetic field. A resolution of ~100 microns for all tracks (2.5 m drift) using ~2 mm wide pads appears feasible with a resistive anode for the ILC TPC readout. The next step is an active participation in the construction of a large TPC prototype to investigate many readout schemes, including the readout concept of charge dispersion in MPGDs with a resistive anode.

Continuing studies with small prototypes [demo-consolidation]

Our measurements to date have demonstrated the feasibility of achieving the diffusion limit of resolution for a TPC with charge dispersion readout in the absence of a magnetic field. We are presently in the process of demonstrating the same in the presence of a 1 T magnetic in a beam test at KEK. The beam tests with hadrons are not ideal for TPC 2-track resolution studies. Two track resolution studies can be carried out more effectively in an electron beam. We plan and request funds to pursue these initiatives at DESY (Germany) in an electron beam next year (FY2006). At the end of the beam tests, we expect to have demonstrated the feasibility of achieving ~100 m resolution with ~ 2 mm wide pads in a 4 T magnetic field.

Large prototype effort [design-construction]

The large prototype is needed to demonstrate that performance of relatively small prototypes can be achieved in a large-scale device. The large prototype will also be essential to provide input to designing the ILC TPC. Carleton plans to work on developing MPGD segmentation schemes. There are also R&D activities related to process development and quality control for resistive anode readout with charge dispersion. The Carleton group has built up valuable experience in producing the first generation prototypes is well positioned to effectively contribute to the design and construction of endplate of the large ILC prototype TPC. A particularly challenging aspect in the endplate design is the scheme to tile many MPGD readout systems while minimising the dead regions.

As part of the LDC detector concept effort, the Carleton group will also participate in the design and simulation of a full size TPC. This design will evolve over the coming years, and will take into account the lessons learned from small scale prototypes. The Carleton group has developed a complete stand-alone simulation of MPGD-TPC signals with charge dispersion. Next, this full response simulation of the MPGD-TPC should be interfaced to the GEANT4 detector geometry and then to the tracking code within the Linear Collider Condition Data (LCCD) toolkit. This framework will serve to optimize the sector design of the endplates for acceptance, energy loss, tracking efficiency, and pattern recognition.

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