A single CERN GEM was prepared by Purdue and installed in the TPC. The GEM is operated at 400V, with the top electrode biased at -400V and the bottom electrode biased at 0. The drift field is 300V/cm with the termination biased at -900V. The gap between the field termination and the GEM is 0.5cm with a field of 1000V/cm. Pads are spaced 0.3cm from the GEM and biased at +1500V to provide an extraction field of 5000V/cm. Electronic amplifiers are the same units used with the wire gas-amplification. The observed pulse shape should be due to the amplifiers, which leads me to believe that this is 100MHz data, not 25MHz. Spectra have been processed to extract the signals. First, within each spectra, the pulse height of each time bin is replaced with the average of 5 bins. Then, the common noise is computed as the average of all 32 time spectra. The common noise, displayed in the bottom red trace, was subtracted from each time spectra.
