The Physikalisch-Technische Bundesanstalt (PTB) operates a radiometry laboratory at the synchrotron radiation facility BESSY. A beamline for detector calibration provides monochromatic radiation of tunable photon energy, high spectral purity (less than 1% false light contribution) and high radiant power in the photon energy range from 50 eV to 1500 eV. With that source of monochromatic radiation a cryogenic electrical substitution radiometry (ESR) is operated as a primary detector standard, which is capable of measuring radiant power in the order of some (mu) W with a relative uncertainty below 0.2%. The spectral responsivity of silicon photodiodes can be measured with a relative uncertainty of 0.3% by comparison to the ESR. On the other hand, silicon photodiodes have been introduced as easy-to- operate detector standards in the soft x-ray region in the PTB laboratory some years ago. The spectral responsivity of these photodiodes can be determined using a self-calibration technique, which is based on the knowledge of mean electron- hole pair creation energy, w, and the measurement of the angle of incidence dependence of the spectral responsivity. We deduced a constant value of w equals (3.64 plus or minus 0.03) eV for silicon in the soft x-ray region and improved the method to determine the charge collection efficiency of the photodiode. That allows us now to use silicon detectors as detectors of calculation spectral responsivity with a relative uncertainty of about 2%.
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