Cernoch, C. (2003). Performance of ATLAS muon precision drift chambers in a high radiation environment [Dissertation, Technische Universität Wien]. reposiTUm. https://resolver.obvsg.at/urn:nbn:at:at-ubtuw:1-11882
E141 - Atominstitut der Österreichischen Universitäten
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Date (published):
2003
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Number of Pages:
129
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Abstract:
ATLAS (A Large Toroidal LHC ApparatuS) is a general-purpose experiment for the future Large Hadron Collider LHC) at CERN which will offer the opportunity to explore new aspects of fundamental physics by advancing to an energy scale well beyond that achieved by current experiments. The muon spectrometer of the ATLAS detector requires an area of around 5500 m?2 of precision tracking chambers, which will reconstruct the muon tracks in a magnetic field of ~ 0.5 T. Approximately 370000 high pressure drift tubes (MDTs), filled with 800 m?3 drift gas (Ar-CO_2 gas mixture), form about 1200 of these tracking chambers, aiming to reach a momentum resolution of 10 % for muons with a transverse momentum of p_T = 1 TeV, to exploit the LHC physics discovery range. This demanding performance requirement has to be fulfilled in the presence of a very high particle background of photons and neutrons leading to background counting rates of up to 300 kHz per drift tube. This background irradiation deteriorates the detector performance and may induce ageing processes on the electrodes. In the presented work for the first time a long-term study with a MDT chamber in a high-background environment, similar to the ATLAS operational environment, has been performed. The high purity gas prototype and its control system was designed, realised and tested for stability and system behaviour in permanent operation. The gained experiences certified the feasibility of the final MDT gas system. Moreover the impact of gas recirculation on the ageing behaviour of the drift chamber was studied in detail, by operating half of the drift chamber in recirculated and the other half in flushed gas mode. In the course of the ageing test a charge corresponding to 4 years of ATLAS operation, assuming a conservative worst-case irradiation, were accumulated by the drift tubes. The signal pulseheight spectra of the MDTs were followed up with reference measurements of cosmic muons during this long-term test. The test proved that no ageing occurred in the drift tubes of the open gas system within the accumulated irradiation dose. The drift tubes connected to the circulated gas system showed a local pulseheight drop on the gas inlet side. This effect was studied intensively and found to be caused by silicone polymerisation on the anode wire - an evidence that the ageing was not caused by the drift gas and gas circulation itself, but by accumulating gas impurities. The source of the contamination was found eventually. The results of this test has a significant influence on the construction of the MDT gas system.