  The Space Variable Object Monitor (SVOM) is a forthcoming Chinese - French
astrophysics space mission dedicated to the study of Gamma-ray bursts and
high-energy transients. ECLAIRs, a wide-field hard X-ray coded mask imager, is
the leading instrument for the transient detection and their first
localisation. The sensitivity of such instruments is usually limited by the
background, either of instrumental or astrophysical origin. Detailed
estimations of the background are obtained by simulating the interaction of
particles with the matter using, in the present case, the GEANT4 Monte-Carlo
toolkit. However, this is a time consuming process, especially when it is
needed to carry out all possible geometrical and orbital configurations.
Instead, we present a much faster method that allows computing the background
in either a static or dynamic (time dependent) way. The method is based on the
preliminary calculation of a large particle database using the GEANT4 toolkit
followed by a selection process based on the incoming direction and energy of
the particles. This approach is as accurate as direct Monte-Carlo methods,
while it reduces the computation time by a factor of $10^3 - 10^4$ for our
application. We apply this method to compute the SVOM/ECLAIRs dynamic
background.
