Radio Halos and Relics in Clusters of Galaxies and Detection Statistics
  New sensitive VLA observations confirm the existence of halo and relic sources in 6 Abell clusters where a diffuse emission was found in the NVSS. We find evidence that the frequency of clusters with halos and relics is larger in clusters with high X-ray luminosity. The evidence that the occurrence of a halo source is larger at high redshifts is marginal. 
Gabriele Giovannini, Luigina Feretti and Federica Govoni