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FP5

FAMTO Sintesi della relazione

Project ID: HPRI-CT-1999-50004
Finanziato nell'ambito di: FP5-HUMAN POTENTIAL
Paese: Germany

Ultrafast x-ray optics

New x-ray monochromators (crystals) have been designed, constructed and tested to provide the highest flux of photons that can be collected and focused onto a sample using femtosecond laser-produced plasma x-ray sources. These developments are crucial for the success of time-resolved x-ray diffraction experiments with a laser-plasma x-ray source as the number of useable x-ray photons is weak at this present stage of the source development. The main challenge was to keep the ultrafast nature of the radiation unperturbed. Extended simulations of potential new bent crystals were done. They have addressed new materials, thin crystals, and new bending shapes. It resulted in the production and the tests of selected crystals like the toroïdal Germanium 100, the toroïdal thin CdTe and the ellipsoidal HOPG as examples.

These crystals have been used within the FAMTO project by different partners to achieve their successful experiments. Some of them are now part of the new ultrafast x-ray workstation developed at LOA. It is now possible for teams outside of the consortium to benefit from the expertise gained in the development of such x-ray crystals by having the possibility to use them in close collaboration with the University of Jena.

Upgraded toroïdal crystals: The 4th order of (100) reflection of germanium crystals and the 400 reflection in Gallium arsenide crystals were selected following extensive simulation of x-ray properties in different materials. Compared to the crystals used so far, a higher reflectivity and a not too wide reflection curve for a good angular resolution was expected. Furthermore, no additional temporal smearing of the x-ray pulse by the absorption depth inside the bent crystal is produced. As x-ray line for pump-probe experiments, Ti Kalpha was chosen.

Two Ge 400 crystals with different bending radii have been produced (300mm and 500mm) and are now in use. Gallium arsenide has almost the same x-ray diffraction properties as well as the same reflection angle (Bragg angle) than Germanium. Two tests of crystal bending of GaAs standard wafer were very successful with respect to the bending of the crystal surface. The surface bending of these crystals is much better than for germanium. One crystal which was bent with a bending radius of 500mm has been produced. This crystal showed reasonable x-ray focusing and is now in use.

Ellipsoïdal crystals: A new generation of x-ray optics has been developed for time-resolved x-ray diffraction experiments with laser-produced plasma x-ray sources, with the goal to increase by orders of magnitude the x-ray flux that can be focused onto a sample compared to the existing technologies. Bent crystal of a Highly Oriented Pyrolytic Graphite (HOPG) crystal foil have been produced with an ellipsoidal shape. The crystal was designed for Ti K radiation at 4.5keV. The former has appeared to be a key point for that purpose and has been produced using a high precision turning machine.

The new crystal has the following properties. The efficiency of the optic, determined by the ratio of photons focused by the crystal to the photons emitted by an x-ray source, is 3x10-3. This is more than 30 times of the efficiency of crystals used so far in ultrafast experiments using laser-produced plasma sources. This is really a new quality with respect to the brightness. A solid angle of 0.43 steradians can be used to collect the x-ray radiation. The smallest focus of a 120µm fluorescence source was measured to be 480 µm. This is not as good as for the perfect crystals like GaAs or silicon, but is still sufficient for a number of experiments. The rocking curve of the graphite foil was measured by using Ti Kalpha. The integrated reflectivity was 5 times higher than for the strongest reflection of quartz at this photon energy. The width of the rocking curve was determined to be 0.1°. This is, to our knowledge, the best value for such a mosaic crystals and explains the lower limit of 480µm spot size. Due to the crystal thickness, a time smearing of the x-ray pulse of not more than 200fs can be achieved. This is very reasonable for many time resolved experiments in the subpicosecond regime.

A test experiments has shown that 10+7 photons can be detected. This result represents an outstanding increase of the x-ray flux by a factor 100 compared to the previous generation of x-ray optics (toroïdal crystals). This will significantly improve the running time-resolved x-ray diffraction experiments.

Informazioni correlate

Reported by

Friedrich-Schiller University Jena
Max-Wien-Platz 1
07743 Jena
Germany
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