  In a dense gas plasma a short laser pulse propagates in relativistic
self-trapping mode, which enables effective conversion of laser energy to the
accelerated electrons. This regime sustains effective loading which maximizes
the total charge of the accelerating electrons, that provides a large amount of
betatron radiation. The 3D particle-in-cell simulations demonstrate how such
regime triggers X-ray generation with 0.1-1 MeV photon energies, low
divergence, and high brightness. It is shown that a 135 TW laser can be used to
produce $3\times 10^{10}$ photons of $>10$ keV energy and a 1.2 PW laser makes
it possible generating about $10^{12}$ photons in the same energy range. The
laser-to-gammas energy conversion efficiency is up to $10^{-4}$ for the
high-energy photons, $\sim 100$ keV, while the conversion efficiency to the
entire keV-range x-rays is estimated to be a few tenths of a percent.
