Absolute photon power measurements at the European XFEL instruments

The average photon flux is one of the main parameters of any photon source. At the European XFEL (EuXFEL), this is continuously monitored in the tunnels by X-ray Gas Monitors (XGMs) [1-3]. However, to measure the absolute value of the X-ray flux at the sample location in the scientific end-stations, in general a smaller device is required since the XGMs and their associated differential pumping stages are very space demanding. Therefore, a miniature solid-state room-temperature calorimeter based on a design by AIST, Japan [4-8] was commissioned with beam at the Spectroscopy and Coherent Scattering (SCS) instrument where it could be located directly downstream of an XGM. This allowed for validation of the gas-based measurements with a solid-state physics method with an independent absolute calibration. The calorimeter was then applied at the High Energy Density (HED) instrument to characterize the beamline transmission and to calibrate a commercial laser power meter (LPM) [9] as a secondary relative monitor for X-ray measurements. This contribution describes the technical parameters and capabilities of the calorimeter and LPM and presents the measurements taken at SCS and HED.