The aging investigation results of CMS EMU prototype chambers are presented. Aging tests under sustained irradiation by a 90 Sr (2 Ci) were performed with three gas gain reduction was not observed up to the accumulated charge Q = 13.56 C/cm. However, the dark current in the chamber was noticeable starting already from Q = 2 C/cm. The absence of the CF 4 (in the Ar=CO 2 gas mixture) caused fast aging deterioration, and Q = 0.2 C/cm was enough to obtain 50% of the gas gain reduction. In the test with Ar(40%)+CO 2 (50%)+CF 4 (10%) mixture the gas gain reduction and the dark current increase were not observed up to Q = 13.35 C/cm. Reduction of the gas ow rate to 0.1 volume per day caused 50% of the gain decrease due to the gas pollution by fragments of dissociated gas molecules. The gain restored after switching back to the 1 volume per day ow rate. Studies with electronic microscope suggest that in all the tests a silicon contained in RTV sealing and FR4 is the major source of the chamber contamination.
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