  Neutron oscillation into mirror neutron, a sterile state exactly degenerate
in mass with the neutron, could be a very rapid process, even faster than the
neutron decay itself. It can be observed by comparing the neutron lose rates in
an ultra-cold neutron trapping experiment for different experimental magnetic
fields. We developed a Monte Carlo code that simulates many of the features of
this kind of experiment with non-uniform magnetic fields. The aim of the
simulation is to provide all necessary tools, needed for analyzing experimental
results for neutron traps with different geometry and different configurations
of magnetic field. This work contain technical details on the Monte Carlo
simulation used for the analysis in arXiv:1712.05761 not presented in it.
