Behaviour of scintillators under XUV free electron laser radiation

Abstract

Free electron lasers (FEL) are new generation accelerator-based short wavelength light sources providing high pulse intensity and femtosecond pulse duration, which enable investigation of interaction of elementary excitations in solids under extreme conditions. Using the FLASH facility of HASYLAB at DESY (Hamburg, Germany), we investigated the response of different materials with scintillating properties based on intrinsic emissions to the 25.6 and 13.8 nm FEL radiation by means of time-resolved luminescence spectroscopy. FLASH delivered single pulses of 25 fs duration having energy per pulse up to 30 mJ resulting in power densities of 10 12 W/cm 2 on crystals. As a function of excitation density we observed the shortening of lifetime and non-exponential behaviour of emission decays in CaWO 4 , while the emission spectra recorded are comparable to those obtained at conventional excitation sources. r 2007 Elsevier B.V. All rights reserved.Keywords: Free electron laser; XUV radiation; Scintillators; Luminescence decay; Excitation density effects; CaWO 4Free electron lasers (FEL) are new generation accelerator-based short wavelength light sources with advanced properties (high pulse intensity, femtosecond pulse duration, monochromatic radiation, coherence, etc.), enabling new kinds of investigations in different scientific fields under extreme conditions. A pioneering test-experiment at TTF-1 facility in Hamburg had shown that interaction of 89 nm FEL radiation with scintillators leads to a remarkable shortening of the decay time and a drop in quantum yield of luminescence (e.g., in BaF 2 and YAG:Ce) [1]. Analogous observation was reported for a number of inorganic scintillators in Ref. [2], where polychromatic hard X-ray synchrotron radiation was applied in experiments. It is obvious that, due to increasing peak power of new light sources, scintillator screens leave the region of linear response, in which they have been used so far to visualise intense radiation and particle beams. In the nonlinear regime effects arising from mutual interaction of elementary excitations can be investigated in more detail in comparison with conventional radiation sources as discussed in Ref. [3].Using the BL1 of the FLASH facility of HASYLAB at DESY (Hamburg, Germany) [4], we investigated the response of different scintillation materials based on intrinsic or activator emission to 25.6 and 13.8 nm FEL radiation in single bunch mode with a pulse duration of 25 fs and maximum energy of 30 mJ at 5 Hz repetition rate. By applying a N 2 gas absorber, the incident beam could be attenuated up to two orders of magnitude from the estimated maximum value of 10 12 W/cm 2 . Luminescence spectra, typically averaged over 2000 FEL pulses, were recorded with a CCD camera mounted on the exit arm of the ARC SpectraPro308i imaging spectrograph. Emission spectra are presented as recorded without any correction to the sensitivity of detector and transmission of spectrograph. A XP2020Q PMT connected to a digital scope was used to record luminesc...