Synchrotron-radiation-based cryogenic radiometry in the X-ray range

The Physikalisch-Technische Bundesanstalt (PTB) has installed an X-ray radi ometry beamline at the new electron storage ring BESSY II. The synchrotron radiation is monochromatized by a four-crystal monochromator in the photon- energy range from 1.75 keV to 10 keV. The known radiant power, determined w ith a cryogenic electrical-substitution radiometer (ESR) as primary detecto r standard, is used for the calibration of detectors such as semiconductor photodiodes. Relative uncertainties of spectral responsivity well below 1% have been achieved.