  We study the effect of a magnetic field on the thermal relaxation of
non-interacting small monodomain particle systems particles with a distribution
of anisotropy constants and random easy-axes directions. Numerical calculations
of the relaxation curves for different distribution widths, and under different
magnetic fields $H$ and temperatures $T$, have been performed in the framework
of a two-state approximation. We show how the obtained data can be analyzed in
terms of an modified $Tln(t/\tau_0)$ scaling from which the field dependence of
the mean relaxing energy barriers can be extracted, a microscopic information
which is not easily obtainable by other methods.
