Abstract
An irreversible response of inorganic scintillators to intense soft x-ray laser radiation was investigated at the FLASH (Free-electron LASer in Hamburg) facility. Three ionic crystals, namely, Ce:YAG (cerium-doped yttrium aluminum garnet), PbWO4 (lead tungstate), and ZnO (zinc oxide), were exposed to single 4.6 nm ultra-short laser pulses of variable pulse energy (up to 12 μJ) under normal incidence conditions with tight focus. Damaged areas produced with various levels of pulse fluences, were analyzed on the surface of irradiated samples using differential interference contrast (DIC) and atomic force microscopy (AFM). The effective beam area of 22.2 ± 2.2 μm2 was determined by means of the ablation imprints method with the use of poly(methyl methacrylate) - PMMA. Applied to the three inorganic materials, this procedure gave almost the same values of an effective area. The single-shot damage threshold fluence was determined for each of these inorganic materials. The Ce:YAG sample seems to be the most radiation resistant under the given irradiation conditions, its damage threshold was determined to be as high as 660.8 ± 71.2 mJ/cm2. Contrary to that, the PbWO4 sample exhibited the lowest radiation resistance with a threshold fluence of 62.6 ± 11.9 mJ/cm2. The threshold for ZnO was found to be 167.8 ± 30.8 mJ/cm2. Both interaction and material characteristics responsible for the damage threshold difference are discussed in the article.
Highlights
Scintillators play an important role in short-wavelength radiation research with respect to development, characterization, and utilization of sources generating intense radiation in the extreme ultraviolet and soft x-ray spectral regions
Since the damage threshold fluence is defined as: Fth where Fth [J/cm2], Eth [J], and Aeff [cm2] represent the threshold fluence, threshold pulse energy, and effective area of the beam in the plane of the sample surface, respectively, the knowledge of the beam size is indispensable for the process of threshold fluence determination
Determined critical doses (Table 2) are in all three cases higher than energy densities needed for thermal melting of the material, calculated from values of specific heat capacities, melting points and latent heats of melting found in the literature for a particular material investigated here. This indicates that material ablation is the key process responsible for the damage in all three investigated inorganic materials irradiated by 4.6 nm Free-electron lasers (FELs) radiation
Summary
Scintillators play an important role in short-wavelength radiation research with respect to development, characterization, and utilization of sources generating intense radiation in the extreme ultraviolet and soft x-ray spectral regions. The rapid development of new extreme ultraviolet and soft x-ray lasers during the last two decades [1,2,3,4] has opened new possibilities in research of the interaction of short-wavelength radiation with matter This raises new challenges for the applicability of scintillator materials. Radiation resistance of scintillating crystals has to be considered when being used as short-wavelength radiation monitors, in order to prevent irreversible changes of their crystal structure or even damage This follows from the fact that currently available free-electron lasers (FELs) generate 10-fs – 100-fs pulses, each carrying an energy equal to hundreds of microjoules [11,12,13,14]. It is understood this picture represents only a first approximation
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