A laboratory study of radiation damage to drift chambers

Abstract

A laboratory study of aging effects under intense radiation in Ar/C 2H 6 and Ar/CO 2/CH 4 filled drift chambers was carried out in order to establish detector lifetimes. In both gas mixtures stable operation in proportional mode and for Ar/C 2H 6 also in the limited streamer regime was possible up to total collected charges of several times 10 17 e − per mm of anode wire. Gain reductions were observed at rates between 0 and 1% per 10 16 e −/mm for Ar/C 2H 6 depending on the test conditions. Under conditions, where gas amplification was stable in Ar/C 2H 6, gain losses in Ar/CO 2/CH 4 ranged between 1.3 and 3.4% per 10 16 e −/mm. Gain nonuniformities were inferred from broadenings and distortions of the 55Fe 5.9 keV γ line. These gain variations are caused by typically less than 100 nm thick deposits on the aged anode wires of poorly conductive material containing O, Si and C. Admixtures of H 2O did not halt the aging processes, but smoothed local gain variations. Contaminations with organic vapor from soft PVC surfaces initiated and accelerated radiation damage. Spark and glow discharges in Ar/C 2H 6 led to growth of grains and flakes of material containing C as the only detectable element on anode and cathode wires within minutes. In contrast discharges in Ar/CO 2CH 4 did not result in any detectable deposits.