BOLUX: A cryogenic electrical-substitution radiometer as high accuracy primary detector in the 150-11,000Â eV range

We have measured synchrotron radiant power at the laboratory of the Physikalisch-Technische Bundesanstalt (PTB) at the Berlin electron storage ring BESSY II. We reported experiments in DC mode and showed the possibility to study the drift and the temporal stability at the exit of a synchrotron beamline. For a helium bath temperature of 1.33 K and in the spectral range of 150 eV-11 keV, we have measured a high limit of detection (in 1 s integrated) to be 10−10 W, with a DC responsivity of 1.3×105 V/W, spatial nonuniformity over 4 mm diameter below 1% and possibility to use over five decades between a few nW and 100 μW with a good linearity (<1%). Typically available radiant power of 50 nW can be measured with a standard uncertainty as low as 1%. BOLUX response has been directly compared with qualified photodiodes [1]; we have observed a standard deviation between the two detectors lower than the total uncertainty of the detectors.