2001-11-15Dissertation DOI: 10.18452/14703
Aging studies of drift chambers of the HERA-B outer tracker using CF 4 -based gases
Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät I
ÿþDThe intense radiation environment in the HERA-B experiment, being comparable to that of LHC experiments, requires that the detector be very resistant to high radiation loads. The Outer Tracker of HERA-B consists of drift tubes folded from polycarbonate foils (honeycomb) and is operated with a CF4-containing gas mixture. Aging tests made in HERA-B, under hadronic irradiation, using prototype drift chambers showed a rapid rise of self-sustaining dark currents (Malter effect). Extensive aging studies were carried out with the objective to find an appropriate set of construction materials as well as optimal operational parameters. It was shown that the gain loss (anode aging) and the Malter effect could be avoided after replacing the methane in the counting gas, Ar/CF4/CH4, by CO2 and applying cathode coating. However, other, relatively less known aging effects appeared: rising conductivity of the wire-supporting strips and wire corrosion. These two aging effects could also be prevented keeping water at a proper concentration. The aging tests showed that a too high water content (above about 500 ppm) gives rise to strip conductivity, conversely, a too low water content (below about 100 ppm) enhances anode wire corrosion. In an attempt to interpret the test results, plasma phenomena like quenching by different admixtures, production of molecular species, concentration of long-lived HF produced via CF4 dissociation in the avalanche were semi-quantitatively estimated. This work contains also a comprehensive summary of all aging studies for the Outer Tracker and an overview of the literature on wire chamber aging, in particular with CF4-based counting gas mixtures.
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