The CAST time projection chamber

One of the three x-ray detectorsof the CERN Axion Solar Telescope(CAST) experiment searching for solar axionsis a time projection chamber(TPC) with a multi-wire proportional counter(MWPC) as a readout structure. Its design has been optimized to provide high sensitivity to the detection of the low intensity x-ray signal expected in the CAST experiment. A low hardware threshold of 0.8?keV is set to a safe level during normal data taking periods, and the overall efficiency for the detection of photons coming from conversion of solar axionsis 62%. Shielding has been installed around the detector, lowering the background level to 4.10 ? 10?5?counts?cm?2?s?1?keV?1 between 1 and 10?keV. During phase I of the CAST experiment the TPC has provided robust and stable operation, thus contributing with a competitive result to the overall CAST limit on axion?photon coupling and mass.