Cryogenic Radiometers

To absolutely determine radiant power, cryogenic radiometers are in common use in the optical regime for more than 20 years now. Basically, it is an electrical substitution radiometer (ESR), i. e. a thermal detector based on the equivalence of electrical and radiant heating.

In the substitution mode, first the absorber is electrically heated to the working temperature (bias heating). In a second step, the radiation shutter is opened and monochromatized synchrotron radiation from one of the beamlines falls onto the absorber. The working temperature is kept constant, so that the electrical power is reduced by exactly the value of the incident radiant power. The absorber is operated near liquid helium temperature (4 K) to improve both the sensitivity and the accuracy of the radiometer. To minimize reflection losses, in the vacuum-ultraviolet to x-ray range cavity absorbers are in use. For the operation with monochromatized synchrotron radiation, dedicated adaptations with regard to conventional systems operated with lasers have been made, e. g. to enhance sensitivity and adapt the response time.

Currently, PTB operates two different cryogenic radiometers as primary detector standards with synchrotron radiation (“SYRES”: Synchrotron Radiation Cryogenic Electrical Substitution Radiometer). SYRES I is used for the EUV- and X-ray regime, and has a cavity absorber with an X-ray absorbing backplate which allows to measure radiant powers at photon energies up to 60 keV. SYRES II is adapted to the conditions in the VUV spectral range. These instruments are in operation to maintain and disseminate a scale of spectral responsivity of semiconductor photo detectors between 3 eV and 60 keV.