  IXPE (Imaging X-ray Polarimetry Explorer) is a NASA Small Explorer mission --
in partnership with the Italian Space Agency (ASI) -- dedicated to X-ray
polarimetry in the 2--8 keV energy band. The IXPE telescope comprises three
grazing incidence mirror modules coupled to three detector units hosting each
one a Gas Pixel Detector (GPD), a gas detector that allows measuring the
polarization degree by using the photoelectric effect. A wide and accurate
ground calibration was carried out on the IXPE Detector Units (DUs) at
INAF-IAPS, in Italy, where a dedicated facility was set-up at this aim. In this
paper, we present the results obtained from this calibration campaign to study
the IXPE focal plane detector response to polarized radiation. In particular,
we report on the modulation factor, which is the main parameter to estimate the
sensitivity of a polarimeter.
