  Gas-based time projection chambers (TPCs) have been shown to be highly
sensitive X-ray polarimeters having excellent quantum efficiency while at the
same time achieving large modulation factors. To observe polarization of the
prompt X-ray emission of a Gamma-ray burst (GRB), a large area detector is
needed. Diffusion of the electron cloud in a standard TPC could be prohibitive
to measuring good modulation when the drift distance is large. Therefore, we
propose using a negative ion TPC (NITPC) with Nitromethane (CH3NO2) as the
electron capture agent. The diffusion of negative ions is reduced over that of
electrons due to the thermal coupling of the negative ions to the surrounding
gas. This allows for larger area detectors as the drift distance can be
increased without degrading polarimeter modulation. Negative ions also travel
~200 times slower than electrons, allowing the readout electronics to operate
slower, resulting in a reduction of instrument power. To optimize the NITPC
design, we have measured gas gain with SciEnergy gas electron multipliers
(GEMs) in single and double GEM configurations. Each setup was tested with
different gas combinations, concentrations and pressures: P10 700 Torr, Ne+CO2
700 Torr at varying concentrations of CO2 and Ne+CO2+CH3NO2 700 Torr. We report
gain as a function of total voltage, measured from top to bottom of the GEM
stack, and as a function of drift field strength for the gas concentrations
listed above. Examples of photoelectron tracks at 5.9 keV are also presented.
