  While X-ray Spectroscopy, Timing and Imaging have improved verymuch since
1962, when the first astronomical non-solar source was discovered, especially
with the launch of Newton/X-ray Multi-Mirror Mission, Rossi/X-ray Timing
Explorer and Chandra/Advanced X-ray Astrophysics Facility, the progress of
X-ray polarimetry has been meager. This is in part due to the lack of sensitive
polarization detectors, in part due to the fate of approved missions and in
part because the celestial X-ray sources appeared less polarized than expected.
Only one positive measurement has been available until now. Indeed the eight
Orbiting Solar Observatory measured the polarization of the Crab nebula in the
70s.
  The advent of techniques of microelectronics allowed for designing a detector
based on the photoelectric effect in gas in an energy range where the optics
are efficient in focusing X-rays. Herewe describe the Instrument, which is the
major contribution of the Italian collaboration to the SmallExplorer mission
called IXPE, the Imaging X-ray Polarimetry Explorer, which will be flown in
late 2021. The instrument, is composed of three Detector Units, based on this
technique, and a Detector Service Unit. Three Mirror Modules provided by
Marshall Space Flight Center focus X-rays onto the detectors. In the following
we will show the technological choices, their scientific motivation and the
results from the calibration of the Instrument.
  IXPE will perform imaging, timing and energy resolved polarimetry in the 2-8
keV energy band opening this window of X-ray astronomy to tens of celestial
sources of almost all classes.
