M. Richwin

Imaging by parabolic refractive lenses in the hard X-ray range

The manufacture and properties of compound refractive lenses (CRLs) for hard X-rays with parabolic profile are described. These novel lenses can be used up to ∼60 keV. A typical focal length is 1 m. They have a geometrical aperture of 1 mm and are best adapted to undulator beams at synchrotron radiation sources. The transmission ranges from a few % in aluminium CRLs up to about 30% expected in beryllium CRLs. The gain (ratio of the intensity in the focal spot relative to the intensity behind a pinhole of equal size) is larger than 100 for aluminium and larger than 1000 for beryllium CRLs. Due to their parabolic profile they are free of spherical aberration and are genuine imaging devices. The theory for imaging an X-ray source and an object illuminated by it has been developed, including the effects of attenuation (photoabsorption and Compton scattering) and of the roughness at the lens surface. Excellent agreement between theory and experiment has been found. With aluminium CRLs a lateral resolution in imaging of 0.3 µm has been achieved and a resolution below 0.1 µm can be expected for beryllium CRLs. The main fields of application of the refractive X-ray lenses are (i) microanalysis with a beam in the micrometre range for diffraction, fluorescence, absorption, scattering; (ii) imaging in absorption and phase contrast of opaque objects which cannot tolerate sample preparation; (iii) coherent X-ray scattering.

A MICROSCOPE FOR HARD X RAYS BASED ON PARABOLIC COMPOUND REFRACTIVE LENSES

We describe refractive x-ray lenses with a parabolic profile that are genuine imaging devices, similar to glass lenses for visible light. They open considerable possibilities in x-ray microscopy, tomography, microanalysis, and coherent scattering. Based on these lenses a microscope for hard x rays is described, that can operate in the range from 2 to 50 keV, allowing for magnifications up to 50. At present, it is possible to image an area of about 300 μm in diameter with a resolving power of 0.3 μm that can be increased to 0.1 μm. This microscope is especially suited for opaque samples, up to 1 cm in thickness, which do not tolerate sample preparation, like many biological and soil specimens.

Mapping the chemical states of an element inside a sample using tomographic x-ray absorption spectroscopy

Hard x-ray absorption spectroscopy is combined with scanning microtomography to reconstruct full near-edge spectra of an elemental species at each location on an arbitrary virtual section through a sample. These spectra reveal the local concentrations of different chemical compounds of the absorbing element inside the sample and give insight into the oxidation state, the local atomic structure, and the local projected free density of states. The method is implemented by combining a quick scanning monochromator and data acquisition system with a scanning microprobe setup based on refractive x-ray lenses.

Piezo-QEXAFS: advances in time-resolved X-ray absorption spectroscopy.

The Piezo-QEXAFS technique is a novel tool for time-resolved X-ray absorption spectroscopy in the hard X-ray range. Monochromator components consisting of specialized tilt stages to perform fast energy scans, lightweight crystal holders, bending mechanics, and control electronics are being installed and commissioned. It is planned to perform fast EXAFS scans with time resolution in the millisecond range. With Piezo-QEXAFS all typical X-ray absorption experiments will be possible as it retains the standard linear geometry. The achieved time resolution opens interesting insights into the dynamics of phase transitions and chemical reactions.

Piezo-XAFS–time-resolved x-ray absorption spectroscopy

The piezo-x-ray absorption spectroscopy technique is a novel tool for time-resolved x-ray absorption spectroscopy in the hard x-ray range. It makes use of piezo tilt tables mounted below the crystals in a double crystal or channel cut crystal monochromator. Repetitive energy scans are performed by applying an oscillatory high voltage to the piezo translators of the tilt tables. Currently, this allows one to scan an energy range of several hundred eV in the hard x-ray range with repetition frequencies of typically 10 Hz. The capability to record full extended x-ray absorption fine structure spectra on a subsecond time scale is demonstrated.

Status And New Applications Of Time-Resolved X-Ray Absorption Spectroscopy

Time‐resolved X‐ray absorption spectroscopy is a valuable tool for detailed investigations of fast chemical reactions, thin film deposition phenomena, solid state reactions, phase transformations etc. We will report on latest experimental developments which employ a new monochromator design consisting of a cam driven tilt table for rapid angular oscillations and a fixed‐exit channel cut crystal. Using cryogenic cooling, the silicon crystal can cope with the full heat load from third generation undulator sources such as the APS, SPring‐8 or the ESRF. Depending on photon flux and sample quality, repetition rates of about 100 Hz can be realized for the XANES range, while the acquisition of full EXAFS spectra with a scan range of typically 1 keV are feasible at a repetition rate of 10 Hz. An excellent data quality can be obtained. Since a fast sequential energy scanning technique is used, the detection of fluorescence radiation or surface sensitive techniques can be applied, and a reference sample can be monitored simultaneously with each measurement to detect even minor edge shifts reliably. Furthermore, XANES microtomography in transmisson and fluorescence becomes feasible using the fast scanning monochromator in combination with refractive X‐ray lenses for beam focusing.

A novel crystal bender for x-ray synchrotron radiation monochromators

A new bending mechanism for an indirectly water cooled monochromator crystal has been developed. The main design goals were a lightweight construction, ease of manufacture, and control. The construction consists of a U-shaped first Si(111) crystal, which can be bent by compressed air to compensate the bowing induced by the heat load from the impinging white synchrotron radiation. The performance of the system was tested at the x-ray undulator beamline BW1 at HASYLAB (Hamburg, Germany). For heat loads between ∼100 and 400 W, rebending of the crystal significantly increases the intensity of the monochromatic beam while the rocking curve is narrowed accordingly, typical widths obtained at 8.9 keV photon energy amount to about 10 arc sec.