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

Abstract

We have measured the electrical and radiometric properties of a cryogenic absolute radiometer BOLUX (Bolomètre pour l’Utilisation dans le domaine de rayons X). BOLUX is intended for use as a primary detector standard for radiant power measurement of a synchrotron beam from EUV to X-ray spectral ranges. The absolute radiometer uses a composite bolometer with a built-in electrical heating element. The bolometer's absorber, optimized for beam diameter and for X-ray absorption, is coupled both to a Ge doped thermometer and to the electrical resistance calibrator with similar thermal paths so that electrical and radiant heating are equivalent. This device operates between 4 K the liquid helium temperature, and 1.25 K obtained by pumping helium. The main advantage of BOLUX over other radiometers is its portability (weight 12 kg) which means it can be used with any synchrotron beamline. The second advantage is its very small time constant (20 ms) compared with other apparatus (typically a few min). We can use it in DC mode or in AC mode and Alpha sources calibration experiments have already been realized. 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 1.3×10 5 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.