HARPO: a TPC as a gamma-ray telescope and polarimeter
  A gas Time Projection Chamber can be used for gamma-ray astronomy with excellent angular-precision and sensitivity to faint sources, and for polarimetry, through the measurement of photon conversion to $e^+e^-$ pairs. We present the expected performance in simulations and the recent development of a demonstrator for tests in a polarized photon beam. 
Denis Bernard, Philippe Bruel, Mickael Frotin, Yannick Geerebaert,   Berrie Giebels, Philippe Gros, Deirdre Horan, Marc Louz...
astro-ph.IM physics.ins-det