  Aims. The statistical properties of radio halos can be used to discriminate
among the possible models for their origin. Therefore an unbiased and
exhaustive investigation in this direction is crucial.
  Methods. With this goal in mind in this paper we revise the occurrence of
radio halos in the redshift range 0-0.4, combining the low redshift (z<0.2)
statistical study of XBACs clusters with the NVSS (by Giovannini et al. 1999)
with our recent results from the radio follow up of REFLEX and eBCS clusters,
the GMRT radio halo survey, at higher redshift (0.2<z<0.4).
  Results. We find a significant statistical evidence (at 3.7 sigma) of an
increase of the fraction of clusters with Radio Halos with the X-ray luminosity
(mass) of the parent clusters and show that this increase is in line with
statistical calculations based on the re-acceleration scenario. We argue that a
fundamental expectation of this scenario is that the probability to have radio
halos emitting at hundred MHz is larger than that at GHz frequencies and thus
that future radio interferometers operating at low frequencies, such as LOFAR
and LWA, should detect a larger number of radio halos with respect to that
caught by present GHz observations. We also show that the expected increase of
the fraction of clusters with radio halos with the cluster mass as measured
with future LOFAR and LWA surveys should be less strong than that in present
surveys.
