  The Muon Detector of the HERA-B experiment at DESY is a gaseous detector that
provides muon identification in a high-rate hadronic environment. We present
our studies on the properties of several fast gases, Ar/CF4/CH4 (74:20:6),
Ar/CF4/CH4 (67:30:3) and Ar/CF4/CO2 (65:30:5), which have been found to fulfill
muon detection requirements. The severe radiation environment of the HERA-B
experiment leads to the maximum charge deposit on a wire, within the muon
detector, of 200 mC/cm per year. For operation in such an environment, the main
criteria for the choice of gas turned out to be stability against aging. An
overview of aging results from laboratory setups and experimental detectors for
binary and ternary mixtures of Ar, CH4, CF4 and CO2 is presented and the
relevance of the various aging results is discussed. Since it is not clear how
to extrapolate aging results from small to large areas of irradiation, the
lifetime of aluminum proportional chambers was studied under various
conditions. In this paper we provide evidence that aging results depend not
only upon the total collected charge. It was found that the aging rate for
irradiation with $Fe^{55}$ X-rays and 100 MeV $\alpha$-particles may differ by
more than two orders of magnitude.
