  Liquid-metal jet X-ray sources promise to deliver high photon fluxes, which
are unprecedented for laboratory based X-ray sources, because the regenerating
liquid-metal anode is less sensitive to damage caused by an increased electron
beam power density. For some quantitative X-ray analysis techniques, knowledge
of the absolute photon flux is needed. However, a precise experimental
determination of the photon flux of high-performance X-ray sources is
challenging, because a direct measurement results in significant dead time
losses in the detector or could even harm the detector. Indirect determinations
rely on data base values of attenuation or scattering cross sections leading to
large uncertainties. In this study we present an experimental determination of
the photon flux of a liquid-metal jet X-ray source by means of elastic and
inelastic photon scattering. Our approach allows for referencing the
unpolarized output radiation of the liquid-metal jet X-ray source to polarized
synchrotron radiation in a simple setup. Absolute photon fluxes per solid angle
are presented with a detailed uncertainty budget for the characteristic
emission lines of Ga K$\alpha$ and In K$\alpha$ for two different acceleration
voltages of the X-ray source. For a nominal setting of 200 W (70 kV, 2.857 mA)
the determined values for Ga K$\alpha$ and In K$\alpha$ are 6.0(5)$\times
10^{12}$ s$^{-1}$sr$^{-1}$ and 3.8(4)$\times 10^{11}$ s$^{-1}$sr$^{-1}$,
respectively.
