2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/15474A synthesis model for the cosmic X-ray Background (XRB) is presented, which includes a proper treatment of Compton scattering in the absorbing matter for type 2 AGN. Evidence for a decrease of the relative importance of type 2 AGN at high redshift is found, which may be due either to a decrease of the relative number of obscured sources, or (more plausibly) to an increase of the fraction of Compton-thick absorbed sources. The XRB spectrum, soft X-rays and hard X-rays source counts can be simultaneously fitted only if the XRB normalization as derived from BeppoSAX/MECS measurements is adopted.4 pages. To be published in the proc. of the workshop "Large Scale Structure in the X-ray Universe", Santorini, September 1999Astrophysicstext