Radiation hardness of luminescent screens under FEL radiation

The European X-Ray Free-Electron Laser (EuXFEL) generates highly brilliant x-ray radiation with photon energies of typically 0.3 to 20 keV, with pulse energies of several mJ, and a beam diameter of approximately 0.5 mm at 9 keV at a distance of 250 m from the source. The beam is pulsed with pulse durations smaller than 100 fs. Luminescent screens along the beam transport path are used for beam diagnostics, to visualise the beam position and transverse shape. The EuXFEL beam can reach the damage threshold of the different screen materials already within a few pulses. Care has to be taken about permissible beam conditions, and the most suitable screens must be selected for a desired application. This contribution reports on the experiences with radiation damage of luminescent screens from different materials (YAG:Ce, diamond:B, BN) during the recently accomplished 5 years of user operation of EuXFEL and gives examples of different types of degradation: from slight degradations by structural changes of screens and surface damage up to screen fracture. Under the particular pulse structure of the EuXFEL radiation at 9 keV photon energy, YAG screens withstand up to 0.5 J/mm2 of incident pulse energy accumulated over successive radiation pulses, respectively 40 J/mm2 for diamond.