Th. Tschentscher

Evaluation of Residual Stresses in the Bulk of Materials by High Energy Synchrotron Diffraction

High energy synchrotron diffraction is introduced as a new method for residual stress analysis in the bulk of materials. It is shown that energy dispersive measurements are sufficiently precise so that strains as small 10−4 can be determined reliably. Due to the high intensity and the high parallelism of the high energy synchrotron radiation the sample gauge volume can be reduced to approximately 50 μm×1 mm×1 mm compared to gauge volume of one mm3 up to several mm3 achievable by neutron diffraction. The benefits of the high penetration depth and the small gauge volume are demonstrated by the results of stress studies performed on a fiber reinforced ceramic, a functional gradient material and a metal-ceramic compound. Furthermore, it is shown that in case of a cold extruded metal specimen the energy dispersive measurement technique yields simultaneous information about texture and residual stresses and thus allows a detailed investigation of elastic and plastic deformation gradients.

Focusing Optics for High-Energy X-ray Diffraction

Novel focusing optical devices have been developed for synchrotron radiation in the energy range 40-100 keV. Firstly, a narrow-band-pass focusing energy-tuneable fixed-exit monochromator was constructed by combining meridionally bent Laue and Bragg crystals. Dispersion compensation was applied to retain the high momentum resolution despite the beam divergence caused by the focusing. Next, microfocusing was achieved by a bent multilayer arranged behind the crystal monochromator and alternatively by a bent Laue crystal. A 1.2 micro m-high line focus was obtained at 90 keV. The properties of the different set-ups are described and potential applications are discussed. First experiments were performed, investigating with high spatial resolution the residual strain gradients in layered polycrystalline materials. The results underline that focused high-energy synchrotron radiation can provide unique information on the mesoscopic scale to the materials scientist, complementary to existing techniques based on conventional X-ray sources, neutron scattering or electron microscopy.

Experiments with Very High Energy Synchrotron Radiation

The use of synchrotron radiation with very high photon energies has become possible only with the latest generation of storage rings. All high-electron-energy synchrotron sources will have a dedicated program for the use of very high photon energies. The high-energy beamline ID15 at the ESRF was the first beamline built and dedicated to this purpose, and it has now been in user operation for more than three years. The useful energy range of this beamline is 30-1000 keV and the superconducting insertion device for producing the highest attainable photon energies is described in detail. The techniques most often used today are diffraction and Compton scattering; an overview of the most important experiments is given. Both techniques have been used in the investigation of magnetic systems, and, additionally, the high resolution in reciprocal space, which can be achieved in diffraction, has led to a series of applications. Other fields of research are addressed, and attempts to indicate possible future research areas of high-energy synchrotron radiation are made.

On High-Resolution Reciprocal-Space Mapping with a Triple- Crystal Diffractometer for High-Energy X-rays

High-energy X-rav diffraction by means of triple-crystal techniques is a powerful tool for investigating dislocations and strain in bulk materials. Radiation with an energy typically higher than 80 keV combines the advantage of low attenuation with high resolution at large momentum transfers. The triple-crystal diffractometer at the High Energy Beamline of the European Synchrotron Radiation Facility is described. It is shown how the transverse and longitudinal resolution depend on the choice of the crystal reflection, and how the orientation of a reciprocal-lattice distortion in an investigated sample towards the resolution element of the instrument can play an important role. This effect is demonstrated on a single crystal of silicon where a layer of macro pores reveals satellites around the Bragg reflection. The resulting longitudinal distortion can be investigated using the high transverse resolution of the instrument when choosing an appropriate reflection.

Coherent X-ray scattering and lensless imaging at the European XFEL Facility.

Coherent X-ray diffraction imaging is a rapidly advancing form of lensless microscopy. The phase information of the diffraction pattern is embedded in a sufficiently sampled coherent diffraction pattern. Using advanced computational methods, this diffraction pattern can be inverted to produce an image of a sample with diffraction-limited resolution. It is attractive to use high-power coherent X-ray beams produced by future X-ray free-electron lasers for imaging nanoscale condensed matter, materials and biological samples. Here, the scientific case, requirements and the possible realisation of the coherent X-ray diffraction imaging beamlines at the European XFEL Facility are presented.

High energy scattering beamlines at European Synchrotron Radiation Facility

The scientific background and scope of research at the High Energy Scattering Beamlines of the European Synchrotron Radiation Facility are reviewed briefly. The beamline has two different insertion devices (IDs), a permanent magnet multipole wiggler and a superconducting wavelength shifter. The IDs can be used alternatingly, and both provide circularly polarized radiation off the orbit plane up to energies of several hundreds of keV. Measurements of the spectral brightness and the polarization components by energy dispersive powder diffraction and Compton scattering from iron in an alternating magnetic field are presented and compared with calculations based on source parameters. The beamline optics are based on use of horizontally reflecting bent Si crystals. These provide various focusing geometries, and the calculated flux of monochromatic radiation is given at energies between 30 and 175 keV. The maximum of 2×1012 photons/s is reached at 80 keV. The main experimental apparatus in the initial operation...

Orbital and spin magnetism in US : comparison with USe and UTe

We report the results of two experiments performed on uranium monosulphide: measuring the magnetic Compton scattering (MCS) and x-ray magnetic circular dichroism (XMCD) at the M4,5 uranium absorption edges. From the MCS experiment we get the spin moment of both the localized (5f electrons) and the diffuse (mainly 6d electrons) contributions. Combining these results with bulk magnetization and published neutron diffraction data, we can separate the orbital and spin contributions to the localized and diffuse moments. Using the XMCD measurements, we deduce the expectation value of the magnetic dipole operator. We finally compare our results on US with published measurements made on USe and UTe.

Uranium-sensitive tomography with synchrotron radiation

Element-sensitive tomography produces quality information in the field of medical imaging. This method, also known as dichromatic tomography, can be useful to visualize the distribution of heavy elements, such as actinides, without destroying the sample. One of the problems is to obtain a monochromatic photon beam of sufficiently high energy; the other is to have a way of recording these high-energy photons with a good spatial resolution. Here, the results of a first experiment on uranium mapping with synchrotron radiation are reported. Various natural and artificial samples of a few centimetres in size with uranium concentration between 0.008 g cm−3 and 2 g cm−3 were scanned using photon beams around 115 keV and a specially designed camera. The data were then analysed using a conventional fast reconstruction technique. This yielded excellent results with spatial resolutions down to 50 µm. For the first time it was shown that element-sensitive tomography using synchrotron radiation could be extended to the heaviest natural element. Therefore, in principle, the spatial distribution of any element can now be reconstructed using synchrotron radiation. Extension of this technique to very heavy elements can be important for geology, health physics and nuclear waste storage.

Femtosecond time-resolved powder diffraction experiments using hard X-ray free-electron lasers

In the next decade the scientific community expects a strong impact in physics, chemistry, biology, material research and life sciences by the availability of high-brilliance X-ray radiation from free-electron laser (FEL) sources. In particular, in the field of ultrafast science these new sources will allow new types of experiments, enabling new phenomena to be discovered. Whereas today ultrafast X-ray diffraction experiments are strongly restricted by the limited X-ray flux of current sources of sub-picosecond X-ray pulses, FELs will provide short pulses of typically 10(12) photons with a duration of the order of 100 fs and monochromaticity of 10(-3). Here, the feasibility of time-resolved single-shot powder diffraction experiments using these intense pulses, and the requirements of these experiments, are discussed. The detector count rates are estimated for diffraction from a model compound in a wide q-regime under the special consideration of high resolving power. In the case of LCLS radiation parameters, single-shot experiments will be feasible although high-resolution powder diffraction will require a reduction of the intrinsic FEL radiation bandwidth.

(γ,eγ) spectroscopy: A new technique to determine electron momentum densities of solids (invited)

In the past γ‐ray Compton scattering experiments proved their value for the investigation of many‐body effects in the electronic structure of solids by measuring the projection of the electron momentum density onto the scattering vector, the so‐called Compton profile. Due to the availability of modern synchrotron radiation facilities the momentum resolution of the technique was improved substantially and, by using circular polarized photons, ‘‘magnetic’’ Compton profiles could be determined in ferromagnetic materials. A new approach, where the Compton scattered photon is measured in coincidence with the recoiling electron, allows for a direct determination of electron momentum densities in solids. This (γ,eγ) scattering technique will reach its full potential once synchrotron radiation from undulators in electron storage rings operating at energies above 10 GeV will be available.