  The High Energy Physics group of the University of Texas at Arlington Physics
Department has been developing Gas Electron Multiplier (GEM) detectors for use
as the sensitive gap detector in digital hadron calorimeters (DHCAL) for the
future International Linear Collider. In this study, two kinds of prototype GEM
detectors have been tested. One has 30x30 cm2 active area double GEM structure
with a 3 mm drift gap, a 1 mm transfer gap and a 1 mm induction gap. The other
one has two 2x2 cm2 GEM foils in the amplifier stage with a 5 mm drift gap, a 2
mm transfer gap and a 1 mm induction gap. We present characteristics of these
detectors obtained using high-energy charged particles, cosmic ray muons and
106Ru and 55Fe radioactive sources. From the 55Fe tests, we observed two
well-separated X-ray emission peaks and measured the chamber gain to be over
6500 with a high voltage of 395 V across each GEM electrode. Both the spectra
from cosmic rays and the 106Ru fit well to Landau distributions as expected
from minimum ionizing particles. We also present the chamber performance after
high dosage exposure to radiation as well as the pressure dependence of the
gain and correction factors. Finally, we discuss the quality test results of
the first set of large scale GEM foils and discuss progress and future plans
for constructing large scale (100cmx100cm) GEM detectors.
