  The long-term stability in terms of gain and energy resolution of a prototype
triple Gas Electron Multiplier (GEM) detector has been investigated with high
rate X-ray irradiation. Premixed Ar/CO2 (80:20) and (90:10) gases have been
used for this stability study. A strong Fe55 X-ray source is used to irradiate
the chamber. The uniqueness of this work is that the same source has been used
to irradiate the GEM prototype and also to monitor the spectra. This
arrangement is important since it reduces the mechanical complexity of using an
X-ray generator as well as the cost of the setup. A small area of the chamber
is exposed continuously to the X-ray for the entire duration of the operation.
The effect of temperature and pressure on the gain and energy resolution is
monitored. The result of the long-term stability test for a triple GEM detector
using Ar/CO2 (70:30) gas mixture has been reported earlier [1]. The results
with Ar/CO2 (80:20) and (90:10) gas mixtures for the same chamber are presented
in this article.
