  We report the results of detailed Monte Carlo simulations of the performance
expected both at balloon altitudes and at the probable satellite orbit of a
hard X-ray coded-aperture camera being developed for the MIRAX mission. Based
on a thorough mass model of the instrument and detailed specifications of the
spectra and angular dependence of the various relevant radiation fields at both
the stratospheric and orbital environments, we have used the well-known package
GEANT4 to simulate the instrumental background of the camera. We also show
simulated images of source fields to be observed and calculated the detailed
sensitivity of the instrument in both situations. The results reported here are
especially important to researchers in this field considering that we provide
important information, not easily found in the literature, on how to prepare
input files and calculate crucial instrumental parameters to perform GEANT4
simulations for high-energy astrophysics space experiments.
