  A systematic study is performed to measure the ion backflow fraction of the
GEM detectors. The effects of different voltage configurations and Ar/CO_2 gas
mixtures, in ratios of 70:30, 80:20 and 90:10, on positive ion fraction are
investigated in detail. Moreover, a comparative study is performed between
single and quadruple GEM detectors.The ion current with detector effective gain
is measured with various field configurations and with three proportions of gas
mixtures. The ion backflow fraction for the GEM is substantially reduced with
the lower drift field. A minimum ion backflow fraction of 18 % is achieved in
the single GEM detector with Ar/CO_2 80:20 gas mixture, however, a minimum ion
backflow fraction of 3.5 %, 3.0%, and 3.8 % are obtained for a drift field of
0.1kV/cm with Ar/CO_2 70:30, 80:20 and 90:10 gas mixtures, respectively for
quadrupole GEM detector. Similar values of effective gain and ion backflow
fraction have been found by calculating the current from pulse height spectrum
method, obtained in the Multi Channel Analyser.
