Formation of carbon deposit on a scintillator screen exposed to x-ray FEL radiation

A surface darkening of optical and imaging elements exposed to intense short-wavelength radiation for a long time represents a serious problem not only at large scale facilities (e.g., synchrotron radiation sources and free-electron lasers) but also for a sustainable operation of EUV lithography (EUVL) equipment and other lab-scale devices (capillary discharge lasers; sources of high-order harmonics). This kind of damage is usually explained by the radiolytic reduction of carbon-containing residual gases leading to the deposition of carbonaceous material on the irradiated surface. At the European XFEL facility, such a darkened damage pattern appeared on the surface of a hexagonal boron nitride (h-BN) phosphor screen. The area covered by a thicker layer of the deposit is sharply demarcated by a projection of the aperture through which the x-ray laser beam passes. In addition to these circular patterns, a halo without any sharp boundary can be seen by naked eye. Both photoelectron (XPS) and Raman spectroscopy revealed all-carbon character of the deposit in both areas. However, it has also been observed that scintillator screens made of different materials, but irradiated under the same conditions, do not seem to develop the same sort of damage. Laboratory simulations of the deposit formation on the h-BN surface have been performed with an XUV capillary-discharge laser, i.e., with significantly softer radiation delivered in pulses much longer than those generated at the European XFEL facility. Possible formation mechanisms of the deposit are discussed in this contribution.