Aging Studies of Large Area Proportional Chambers under High-Rate Irradiation with $CF_4$-based Mixtures (Part 2)

Experimental conditions at the HERA-B experiment impose very strong requirements for gaseous detectors. The charged particle fluxes through the HERA-B tracking system, varying with the radial distance R from the beam line, are about $2 \times 10^{7}/R^{2}$ particles per second, and comparable to those that will be encountered by LHC experiments. The severe radiation environment of the HERA-B experiment leads to a maximum charge deposit on a wire, within the muon detector, of 200 mC/cm per year. We report recent results of aging studies performed by irradiating proportional wire chambers filled with $Ar/CF_4/CH_4$ (74:20:6), $Ar/CF_4/CH_4$ (67:30:3), $Ar/CF_4/CO_2$ (65:30:5), $Ar/CF_4$ (70:30), $CF_4/CH_4$ (90:10), $CF_4/CH_4$ (80:20) mixtures in a three different experimental setups. The size of the irradiation zone varied in the tests from 1 cm up to 500 cm. Our experience shows that the aging rate depends not only on the total collected charge, but, in addition, on the mode of operation and area of irradiation. The possible application of these results to the construction of a large area gaseous detectors for operation in high rate environments is presented.