Design and construction of a TPC using GEM foils for gas amplification

We report on the development of a large TPC for the e⁺e^- linear collider. Our studies concentrate on the use of gas electron multiplier (GEM) foils for the gas amplification in the TPC. Charge transfer measurements are performed using a triple-GEM structure installed into a small test chamber which is irradiated with an ⁵⁵Fe source. This device is operated in high magnetic fields up to 5 T. No severe drop in primary ionisation statistics is observed, but un improved suppression of ion backdrift is achieved. For optimum settings the relative ion backdrift drops to only 2.5 per mill. Additionally, the width of the charge cloud created by individual ⁵⁵Fe photons is measured using a finely segmented strip readout behind the triple-GEM structure. Charge widths between 0.2 and 0.5 mm RMS are observed. This charge broadening is reduced in high magnetic fields. Based on these experiments a prototype GEM-TPC has been constructed for further tests in the 5 T magnet and in testbeams. This chamber will help to answer the question, if the design goals of the linear collider TPC can be reached using the GEM technology.