Improvement of a cryogenic radiometer for XFEL absolute intensity measurement

A cryogenic radiometer was improved for measurements of the absolute radiant power of x-ray Free Electron Laser, which provides intense radiation with an ultra-short pulse duration. Based on simulation results obtained by the Monte Carlo program EGS 5 code, a new cavity absorber of the cryogenic radiometer was developed. The simulation results show that the new cavity absorber achieves absorptance close to unity for hard x-rays up to photon energies of 40 keV. The excellent performance of the new cavity absorber, as well as the consistency between the new and the former cavity, was confirmed by calibrating two different types of silicon photodiodes. The calibration results agreed well within their relative expanded uncertainties. To confirm the performance of the new cavity absorber in the high radiant power region, the radiant powers obtained with the cryogenic radiometer and an x-ray beam monitor were also compared. A strong correlation between the two detectors was obtained. With the new cavity absorber, the absolute radiant power of XFEL for photon energies of up to 40 keV with low uncertainties is expected to be measured.