  We report on an accurate measurement of the CXB in the 15-50 keV range
performed with the Phoswich Detection System (PDS) instrument aboard the
BeppoSAX satellite. We establish that the most likely CXB intensity level at
its emission peak (26-28 keV) is ~40 keV/cm2/s/sr, a value consistent with that
derived from the best available CXB measurement obtained over 25 years ago with
the first High Energy Astronomical Observatory satellite mission (HEAO-1;
Gruber et al. 1999), whose intensity, lying well below the extrapolation of
some lower energy measurements performed with focusing telescopes, was
questioned in the recent years. We find that 90% of the acceptable solutions of
our best fit model to the PDS data give a 20-50 keV CXB flux lower than 6.5E-08
erg/cm2/s/sr, which is 12% higher than that quoted by Gruber et al. (1999) when
we use our best calibration scale. This scale gives a 20-50 keV flux of the
Crab Nebula of 9.22E-09 erg/cm2/s, which is in excellent agreement with the
most recent Crab Nebula measurements and 6% smaller than that assumed by Gruber
et al. (1999). In combination with the CXB synthesis models we infer that about
25% of the intensity at ~30 keV arises from extremely obscured, Compton thick
AGNs (absorbing column density N_H > 1.0E+24 H/cm2), while a much larger
population would be implied by the highest intensity estimates. We also infer a
mass density of supermassive BHs of ~3.0E+05 Msol/Mpc3. The summed contribution
of resolved sources (Moretti et al. 2003) in the 2-10 keV band exceeds our best
fit CXB intensity extrapolated to lower energies, but it is within our upper
limit, so that any significant contribution to the CXB from sources other than
AGNs, such as star forming galaxies and diffuse Warm-Hot Intergalactic Medium
(WHIM), is expected to be mainly confined below a few keV.
