First results with the new PTB cryogenic radiometer for the vacuum-ultraviolet spectral range

In the radiometry laboratory of the Physikalisch-Technische Bundesanstalt (PTB) at the Berlin electron storage ring BESSY, a new cryogenic electrical-substitution radiometer with a cavity absorber is now in operation as the primary detector standard for the vacuum-ultraviolet spectral range. The instrument is optimized for use with narrowband synchrotron radiation, as dispersed by a monochromator. It is capable of measuring spectral radiant powers of a few microwatts with a resolution of 1,5 nW and a relative uncertainty of 0,2%. The cryogenic radiometer has been employed at a new detector-calibration facility for the normal-incidence spectral range. Calibrations of Hamamatsu S1337 and G1127 photodiodes have been performed in the 170 nm to 500 nm spectral range with a relative uncertainty of the spectral responsivity less than 1%. At four laser wavelengths, agreement was found with a calibration against the laser-operated cryogenic radiometer of the PTB at Berlin.