A prototype GEM-TPC for PANDA

Abstract

The PANDA fixed-target experiment planned at the future international Facility for Antiproton and Ion Research (FAIR) in Darmstadt, Germany, will investigate fundamental questions in non-perturbative QCD. It features a cooled antiproton beam with momenta from 1.5 to 15 GeV/c and will reach luminosities of up to 2⋅1032 cm−2 s−1 corresponding to a p̄p-annihilation rate of 2⋅107 s−1. One option for the central tracker of the target spectrometer was a cylindrical, ungated, continuously running TPC with GEM-based amplification stage surrounding the interaction point.To show the feasibility of such a detector, a prototype with a drift length of 727.8 mm and an inner (outer) radius of 52 mm (154 mm) was built. The pad plane of the detector has 10254 hexagonal pickup electrodes which are read out using 42 front-end cards, each equipped with four AFTER chips, originally developed for the T2K experiment.The prototype was installed and tested in the FOPI spectrometer at GSI (Darmstadt, Germany). The central drift chamber and the RPC-TOF system of FOPI give excellent reference for momentum and energy loss measurements with the TPC. A superconducting solenoid magnet creates a magnetic field of 0.6 T. Gas mixtures of Ar/CO2 (90/10) and Ne/CO2 (90/10) were used with different drift fields ranging from 150 to 350 V/cm. A 83mKr source was used for gain calibration.The GEM-TPC will significantly improve the vertex resolution and secondary-vertex detection of the FOPI spectrometer, where it was used in a three-week physics campaign with a π− beam in June 2011. A detailed overview of the detector hardware and electronics will be presented together with first results from the commissioning of the detector.