The next NASA Small Explorer (SMEX) mission, IXPE ( Imaging X-ray Polarimetry Explorer) , will be launched in 2021, with three identical X-ray mirror module assemblies and Gas Pixel Detectors (GPD). IXPE introduces the capability for X-ray polarimetric imaging, uniquely enabling the measurement of X-ray polarisation with scientifically meaningful spatial, spectral, and temporal resolution, helping answer the fundamental questions.
A series of simulation activities together with the calibration for flight units are in progress. As an important tool to understand the background of the space instruments, Geant4 background simulation will be reported. This work foresee the study of background rejection to be implemented in on-ground analysis software, which is especially importance for faint extended sources, like molecular clouds and supernova remnants.