Design and construction of a GEM-TPC prototype for research and development purposes

For a number of running and future particle-physics experiments, studies on the readout of time projection chambers (TPCs) based on the gas electron multiplier (GEM) are ongoing. This technology offers higher granularity and intrinsically suppressed ion feedback as the two major advantages compared to a conventional readout with multiwires. We describe the design and construction of a small and flexible TPC prototype, whose cylindrical drift volume can be equipped with endcaps of different gas detector types. Its field cage is designed in a double-ring layout to guarantee high homogeneity of the electric field. All materials facing the gas volume have been selected with special respect to detector aging properties. An endcap with multi-GEM readout is currently set up and successfully operated with a modified version of the STAR-TPC electronics. The functionality of the detector has been verified in system checks, and its performance could recently be tested in high-intensity particle beams.

I. Introduction

Time projection chambers (TPCs) are attractive central tracking devices because they provide a large number of spatial coordinates and energy loss measurements along the tracks of charged particles [1]. In order to further improve point resolution and double-hit separation, micropattern gas detectors (MPGDs) of high granularity and with intrinsic ion feedback suppression are being considered to replace the conventional TPC readout based on multiwires. Fig. 1. Electron microscope photograph of a GEM in standard geometry ( m pitch, m outer diameter, and m inner hole diameter, compare the schematic cut through the foil). The plot of electric field lines on the right illustrates that electrons are guided into the holes and undergo proportional gas amplification in the locally strong electric field.