  We report results from Monte Carlo simulations of systems of magnetic dipoles
that relax through quantum tunneling, much as Fe_8 crystals at very low
temperature.For short times, a hole develops in suitably defined magnetic field
pseudo--distributions, which matches the shape of the tunnel window (TW). Much
later, ordinary field distributions P(h) develop similar holes if thermal
energies are not much larger than the TW's energy. Still later, below the
long--range ordering (LRO) temperature, P(h) exhibits the signature of LRO.
