The electron storage ring BESSY II as a primary source standard from the visible to the the X-ray range

The Physikalisch-Technische Bundesanstalt (PTB) has used the BESSY I 800 MeV electron storage ring as a primary source standard since 1984. The calculable spectral photon flux ΦE of a bending magnet with relative uncertainties of 4 × 10−4 to 4 × 10−3 for the photon energy 1 eV to 15 keV was used in a broad spectral range from the visible to the soft X-ray range for the calibration of radiation sources and energy-dispersive X-ray detectors. In 1998, BESSY II was brought into operation. Owing to its higher electron energy, of 1.7 GeV, the useful spectral range could be extended significantly into the X-ray region up to a photon energy of about 50 keV, while special storage-ring operation at 900 MeV results in a spectrum up to about 7 keV. The PTB has established BESSY II as a European primary source standard from the visible to the X-ray range by setting up equipment for the measurement of all storage-ring parameters and geometrical quantities involved in the calculation of ΦE according to Schwinger. At BESSY II, ΦE from a bending magnet can be calculated with a relative standard uncertainty of 3 × 10−4 for photon energies below 3 keV, rising to 2 × 10−3 at 50 keV.