  PolarLight is a gas pixel X-ray polarimeter mounted on a CubeSat, which was
launched into a Sun-synchronous orbit in October 2018. We build a mass model of
the whole CubeSat with the Geant4 toolkit to simulate the background induced by
the cosmic X-ray background (CXB) and high energy charged particles in the
orbit. The simulated energy spectra and morphologies of event images both
suggest that the measured background with PolarLight is dominated by high
energy electrons, with a minor contribution from protons and the CXB. The
simulation reveals that, in the energy range of 2-8 keV, there are roughly 28%
of the background events are caused by energy deposit from a secondary electron
with an energy of a few keV, in a physical process identical to the detection
of X-rays. Thus, this fraction of background cannot be discriminated from X-ray
events. The background distribution is uneven on the detector plane, with an
enhancement near the edges. The edge effect is because high energy electrons
tend to produce long tracks, which are discarded by the readout electronics
unless they have partial energy deposits near the edges. The internal
background rate is expected to be around 6 x 10^-3 counts/s/cm2 in 2-8 keV if
an effective particle discrimination algorithm can be applied. This indicates
that the internal background should be negligible for future focusing X-ray
polarimeters with a focal size in the order of mm.
