Probability of forming radio halos with ν_s > 140 and ν_s > 600 MHz (red and yellow lines, respectively) as a function of the cluster virial mass in the redshift range 0.6–0.7. Magnetic fields of B = 5 μGauss and B = 2 μGauss are assumed (solid and dashed lines, respectively). The shadowed regions represent the 1σ uncertainty derived through Monte Carlo calculations. Top panel: expected fraction of USSRH visible at 144 MHz with steepening frequency ν_s < 600 MHz as a function of the cluster virial mass, assuming B = 5 μGauss and B = 2 μGauss (solid and dashed lines, respectively). The shadowed area indicates the observed fraction of USSRH in our sample.