  We investigate the role of the radiative effects for the temporal evolution
of the reionization fraction by using cosmological Smooth Particle
Hydrodynamics (SPH) simulations. We find that the increase of photo-ionization
and photo-heating rates due to optical depth effects results in a significantly
contribute to the heating of the IGM before and during the reionization. The
main effect of the UV radiation spectrum on the temporal evolution of the
ionization fraction is given by the value of the reionization redshift and the
redshift interval, in which the reionization is completed. We evaluate the
effects of the UV radiation background on the CMB angular power spectrum taking
into account different temporal evolutions of the ionization fraction. We show
that through E-mode CMB polarization power spectrum measurements, the Planck
experiment will have the sensitivity to distinguish between different
reionization histories even when they imply the same optical depth to electron
scattering and degenerated temperature anisotropy power spectra.
