  Using the Geant4 toolkit, a Monte-Carlo code to simulate the detector
background of the Simbol-X focal plane instrument has been developed with the
aim to optimize the design of the instrument. Structural design models of the
mirror and detector satellites have been built and used as baseline for our
simulations, to evaluate the different background contributions that must be
taken into account to determine the sensitivity of the Simbol-X detectors. We
work towards a simulation based background and mass model which can be used
before and during the mission.
  For different material compositions, material thicknesses, locations etc. the
response of the instrument to the diffuse cosmic hard X-ray background and to
the cosmic proton induced background have been calculated. As a result we
present estimates of the background count rate expected in the low and high
energy detector, and anti-coincidence rates. The effect of induced
radioactivity in the detector and shielding materials and soft proton
scattering in the mirror shells are also under study.
