The BeppoSAX High Energy Large Area Survey (HELLAS)
  We have surveyed about 50 deg-2 of sky in the largely unexplored 5-10 keV band using the BeppoSAX MECS instrument, finding 180 sources. After correction for the non uniform sky coverage we find about 18 sources deg-2 with F(5-10keV) >= 5E-14 cgs, and resolve 30-40 % of the hard 5-10 keV Cosmic X-ray Background (XRB). Optical identification of a first small sample of sources show that most (11 out of 14) are AGN. Six of these show evidence of absorption/extinction in X-ray/optical, thus providing support to the scenario in which the hard XRB is largely made by obscured AGN (Setti & Woltjer 1989, Comastri et al. 1995). 
F. Fiore, P. Giommi, F. La Franca, G. Matt, G.C. Perola, A. Comastri,   S. Molendi, M. Elvis, I Gioia, F. Tamburelli, D. Ric...