Characterization studies of a GEM detector prototype for the ALICE TPC upgrade for LHC Run 3 and beyond

Abstract

The multiwire proportional chamber signal readout of the ALICE Time Projection Chamber (TPC) will be replaced by Gas Electron Multiplier (GEM) detectors to meet the new requirements due to higher collision rate and energies during LHC Run 3 in 2018 and beyond. The GEM detector used in the ALICE TPC will vary from standard GEM detectors because it is designed to have a low ion back flow in order to have minimal distortion of the electric field in the TPC. The focus of this thesis has been to characterize a GEM detector prototype using different types of radioactive sources. A goal has also been to make at setup for discharge studies with the GEM detector using both an external alpha source and an internal gaseous source. The gain of the GEM detector prototype was calibrated at different operating voltages, detector gases and gas flow rates using a 5.9 keV photon source. The same source was used to measure the relative resolution. It was measured to be around 12 % at nominal gain of 2000. This is low compared to standard GEM detectors which might have resolution around 8 % at a 5.9 keV photon peak. This is due to the special specifications of the GEM detector prototype which aims to reduce the ion back flow. The detector was also tested with minimum ionizing particles (MIPs) using a beta source. A setup with a plastic scintillator as an external trigger was used to discriminate the low energy electrons emitted from the beta source. The system had high noise when reading out the largest readout pad due to high input capacitance of the large readout area. It did not perform well and the gain had to be above the nominal gain of 2000 to get a clear Landau distribution from the MIPs. An external alpha source was used to induce discharges in the detector. No discharge was observed at nominal gain but increasing the gain by a factor of about five made discharges happen. This setup only gives discharges in a small area where the source is pointing. A setup for discharge studies with gaseous alpha sources was made. Radioactive radon gas from a rock and thorium enriched mantles was added to the detector gas flow and used as sources. Using radioactive gas makes particles decay in the whole gaseous volume of the detector. The rate of alpha decays from these source was measured up to about 8 Hz with a 27 × 27 mm2 readout pad. After a lot of testing with different radioactive sources the relative energy resolution of 5.9 keV photons was measured again. The resolution for a detector setting was 15 % before all the testing but had decreased to 20 % afterwards. The detector may have been damaged by the alpha particles from the radon sources.