  The performance of the Time Projection Chamber (TPC) polarimeter for the
Polarimeter for Relativistic Astrophysical X-ray Sources (PRAXyS) Small
Explorer was evaluated using polarized and unpolarized X-ray sources. The
PRAXyS mission will enable exploration of the universe through X-ray
polarimetry in the 2-10 keV energy band. We carried out performance tests of
the polarimeter at the Brookhaven National Laboratory, National Synchrotron
Light Source (BNL-NSLS) and at NASA's Goddard Space Flight Center. The
polarimeter was tested with linearly polarized, monochromatic X-rays at 11
different energies between 2.5 and 8.0 keV. At maximum sensitivity, the
measured modulation factors at 2.7, 4.5 and 8.0 keV are 27%, 43% and 59%,
respectively and the measured angle of polarization is consistent with the
expected value at all energies. Measurements with a broadband, unpolarized
X-ray source placed a limit of less than 1% on false polarization in the PRAXyS
polarimeter.
