  Large area Gas Electron Multiplier (GEM) detectors have been the preferred
choice for tracking devices in major nuclear and particle physics experiments.
Uniformity over surface of the detector in terms of gain, energy resolution and
efficiency is crucial for the optimum performance of these detectors. In the
present work, detailed performance study of a 10x10 cm^2 triple GEM detector
operated using Ar and CO_2 gas mixtures in proportions of 70:30 and 90:10, has
been made by making a voltage scan of the efficiency with 106^Ru-Rh beta-source
and cosmic rays. The gain and energy resolution of the detector were studied
using the X-ray spectrum of 55^Fe source. The uniformity of the detector has
been investigated by dividing the detector in 7x7 zones and measuring the gain
and energy resolution at the center of each zone. The variations of the gain
and energy resolution have been found to be 8.8% and 6.7%, respectively. These
studies are essential to characterise GEM detectors before their final use in
the experiments.
