We describe the science motivation and development of a pair production
telescope for medium-energy gamma-ray polarimetry. Our instrument concept, the
Advanced Energetic Pair Telescope (AdEPT), takes advantage of the
Three-Dimensional Track Imager, a low-density gaseous time projection chamber,
to achieve angular resolution within a factor of two of the pair production
kinematics limit (~0.6 deg at 70 MeV), continuum sensitivity comparable with
the Fermi-LAT front detector (<3x10-6 MeV cm-2 s-1 at 70 MeV), and minimum
detectable polarization less than 10% for a 10 millicrab source in 106 seconds.
