  In this work we present an active Compton scattering polarimeter as a focal
plane instrument able to extend the X-ray polarimetry towards hard X-rays.
Other authors have already studied various instrument design by means of Monte
Carlo simulations, in this work we will show for the first time the
experimental measurements of "tagging efficiency" aimed to evaluate the
polarimeter sensitivity as a function of energy. We performed a
characterization of different scattering materials by measuring the tagging
efficiency that was used as an input to the Monte Carlo simulation. Then we
calculated the sensitivity to polarization of a design based on the laboratory
set-up. Despite the geometry tested is not optimized for a realistic focal
plane instrument, we demonstrated the feasibility of polarimetry with a low
energy threshold of 20 keV. Moreover we evaluated a Minimum Detectable
Polarization of 10% for a 10 mCrab source in 100 ks between 20 and 80 keV in
the focal plane of one multilayer optics module of NuSTAR. The configuration
used consisted of a doped p-terphenyl scatterer 3 cm long and 0.7 cm of
diameter coupled with a 0.2 cm thick LaBr3 absorber.
