Electron storage ring BESSY as a source of calculable spectral photon flux in the x-ray region

The electron storage ring BESSY is shown to be a primary standard of spectral photon flux ΦE from the infrared to the x-ray region. During the last years we have improved the measurements of the important storage ring parameters involved in the determination of ΦE and extended the usable spectral range to the x-ray region. We have achieved a relative uncertainty for ΦE of 0.18% (√3σ level) at a photon energy E=1 eV, 0.35% at E=5 keV and 0.57% at E=15 keV. In order to investigate and prove possible deviations between the calculated photon flux ΦE of BESSY and the flux of well established calibration sources in the x-ray region, the radionuclide x-ray standards, an intercomparison was made using a Si(Li) detector. At two different photon energies, 6.4 and 8.0 keV, agreement within the combined uncertainties was found.