Piezo-modulated active grating for selecting X-ray pulses separated by one nanosecond.

We present a novel method of temporal modulation of X-ray radiation for time resolved experiments. To control the intensity of the X-ray beam, the Bragg reflection of a piezoelectric crystal is modified using comb-shaped electrodes deposited on the crystal surface. Voltage applied to the electrodes induces a periodic deformation of the crystal that acts as a diffraction grating, splitting the original Bragg reflection into several satellites. A pulse of X-rays can be created by rapidly switching the voltage on and off. In our prototype device the duty cycle was limited to ∼1 ns by the driving electronics. The prototype can be used to generate X-ray pulses from a continuous source. It can also be electrically correlated to a synchrotron light source and be activated to transmit only selected synchrotron pulses. Since the device operates in a non-resonant mode, different activation patterns and pulse durations can be achieved.

Gratings for synchrotron and FEL beamlines: a project for the manufacture of ultra-precise gratings at Helmholtz Zentrum Berlin.

Blazed gratings are of dedicated interest for the monochromatization of synchrotron radiation when a high photon flux is required, such as, for example, in resonant inelastic X-ray scattering experiments or when the use of laminar gratings is excluded due to too high flux densities and expected damage, for example at free-electron laser beamlines. Their availability became a bottleneck since the decommissioning of the grating manufacture facility at Carl Zeiss in Oberkochen. To resolve this situation a new technological laboratory was established at the Helmholtz Zentrum Berlin, including instrumentation from Carl Zeiss. Besides the upgraded ZEISS equipment, an advanced grating production line has been developed, including a new ultra-precise ruling machine, ion etching technology as well as laser interference lithography. While the old ZEISS ruling machine GTM-6 allows ruling for a grating length up to 170 mm, the new GTM-24 will have the capacity for 600 mm (24 inch) gratings with groove densities between 50 lines mm-1 and 1200 lines mm-1. A new ion etching machine with a scanning radiofrequency excited ion beam (HF) source allows gratings to be etched into substrates of up to 500 mm length. For a final at-wavelength characterization, a new reflectometer at a new Optics beamline at the BESSY-II storage ring is under operation. This paper reports on the status of the grating fabrication, the measured quality of fabricated items by ex situ and in situ metrology, and future development goals.

Density minimum of confined water at low temperatures: a combined study by small-angle scattering of X-rays and neutrons.

A simple explanation is given for the low-temperature density minimum of water confined within cylindrical pores of ordered nanoporous materials of different pore size. The experimental evidence is based on combined data from in-situ small-angle scattering of X-rays (SAXS) and neutrons (SANS), corroborated by additional wide-angle X-ray scattering (WAXS). The combined scattering data cannot be described by a homogeneous density distribution of water within the pores, as was originally suggested from SANS data alone. A two-step density model reveals a wall layer covering approximately two layers of water molecules with higher density than the residual core water in the central part of the pores. The temperature-induced changes of the scattering signal from both X-rays and neutrons are consistent with a minimum of the average water density. We show that the temperature at which this minimum occurs depends monotonically on the pore size. Therefore we attribute this minimum to a liquid-solid transition of water influenced by confinement. For water confined in the smallest pores of only 2 nm in diameter, the density minimum is explained in terms of a structural transition of the surface water layer closest to the hydrophilic pore walls.

Application of conventional and microbeam synchrotron radiation x-ray fluorescence and absorption for the characterization of human nails.

The synchrotron radiation based spectroscopies X-ray fluorescence and X-ray absorption fine structure are used to detect illness-related changes in the elemental distribution and bonding environment of metals in human nails. The effective atomic number of a collection of nails is determined using two methods, the X-ray transmittance and the scattering method, and is found equal to 7.5 +/- 0.5. X-ray fluorescence maps of the elemental distributions, recorded with a lateral resolution of 5 microm, reveal that S, Ca and Zn are distributed homogeneously while Fe tends to cluster. In the Fe-rich clusters, which have a diameter in the range 15-30 microm, the Fe concentration is 10 times higher than in the matrix. The Zn K edge X-ray Absorption Fine Structure spectra reveal that Zn, in the nails from healthy donors and patients suffering from lung diseases, is four-fold coordinated with N and S and the Zn-N and Zn-S distances are equal to 2.03 A and 2.25 A, respectively. Finally the signature of various bonds in the C-, O- and N- K near edge X-ray absorption fine structure spectra is discussed.

Mineralized microstructure of calcified avian tendons: a scanning small angle X-ray scattering study.

The micrometer level spatial distribution of the size, shape, and orientation of mineral crystallites in the calcifying matrix of tendons near the edge of the mineralizing front was investigated by scanning small angle X-ray scattering using synchrotron X-ray radiation. Using a special microbeam arrangement enabling 20 microm beam resolution and short measurement times, linear diffraction scans were made on sections from the normally calcifying tendons (tibialis cranialis) from the domestic turkey, which calcify in the distal to proximal direction. A change in shape and arrangement of mineral crystals was observed within the first 200 microm of the mineralization front, and the mineral crystal distribution was highly anisotropic with crystals aligned parallel to the fiber axis. In a cross-section of the tendon cut at right angles to the fiber axis, the orientation distribution of crystals was not azimuthally symmetric, and showed a small but nonzero anisotropy and a continuous change in mean orientation angle across the width of the tendon cross-section.

Characteristics of mineral particles in the human bone/cartilage interface.

Bone and cartilage consist of different organic matrices, which can both be mineralized by the deposition of nano-sized calcium phosphate particles. We have studied these mineral particles in the mineralized cartilage layer between bone and different types of cartilage (bone/articular cartilage, bone/intervertebral disk, and bone/growth cartilage) of individuals aged 54 years, 12 years, and 6 months. Quantitative backscattered electron imaging and scanning small-angle X-ray scattering at a synchrotron radiation source were combined with light microscopy to determine calcium content, mineral particle size and alignment, and collagen orientation, respectively. Mineralized cartilage revealed a higher calcium content than the adjacent bone (p<0.05 for all samples), whereas the highest values were found in growth cartilage. Surprisingly, we found the mineral platelet width similar for bone and mineralized cartilage, with the exception of the growth cartilage sample. The most striking result, however, was the abrupt change of mineral particle orientation at the interface between the two tissues. While the particles were aligned perpendicular to the interface in cartilage, they were oriented parallel to it in bone, reflecting the morphology of the underlying organic matrices. The tight bonding of mineralized cartilage to bone suggests a mechanical role for the interface of the two elastically different tissues, bone and cartilage.

Viscoelastic properties of collagen: synchrotron radiation investigations and structural model.

Collagen type I is the most abundant structural protein in tendon, skin and bone, and largely determines the mechanical behaviour of these connective tissues. To obtain a better understanding of the relationship between structure and mechanical properties, tensile tests and synchrotron X-ray scattering have been carried out simultaneously, correlating the mechanical behaviour with changes in the microstructure. Because intermolecular cross-links are thought to have a great influence on the mechanical behaviour of collagen, we also carried out experiments using cross-link-deficient tail-tendon collagen from rats fed with beta-APN, in addition to normal controls. The load-elongation curve of tendon collagen has a characteristic shape with, initially, an increasing slope, corresponding to an increasing stiffness, followed by yielding and then fracture. Cross-link-deficient collagen produces a quite different curve with a marked plateau appearing in some cases, where the length of the tendon increases at constant stress. With the use of in situ X-ray diffraction, it was possible to measure simultaneously the elongation of the collagen fibrils inside the tendon and of the tendon as a whole. The overall strain of the tendon was always larger than the strain in the individual fibrils, which demonstrates that some deformation is taking place in the matrix between fibrils. Moreover, the ratio of fibril strain to tendon strain was dependent on the applied strain rate. When the speed of deformation was increased, this ratio increased in normal collagen but generally decreased in cross-link-deficient collagen, correlating to the appearance of a plateau in the force-elongation curve indicating creep. We proposed a simple structural model, which describes the tendon at a hierarchical level, where fibrils and interfibrillar matrix act as coupled viscoelastic systems. All qualitative features of the strain-rate dependence of both normal and cross-link-deficient collagen can be reproduced within this model. This complements earlier models that considered the next smallest level of hierarchy, describing the deformation of collagen fibrils in terms of changes in their molecular packing.

Analysis of the hierarchical structure of biological tissues by scanning X-ray scattering using a micro-beam.

The outstanding mechanical properties of biological tissues such as wood or bone are mainly due to their hierarchical structure and to their optimization at all levels of hierarchy. It is therefore essential to characterize the structure at all levels to understand the complex behavior of such tissues. Structures down to the micrometer level are accessible to light or scanning electron microscopic observation. In the case of bone this includes, for example, morphometry of the trabecular architecture or the bone mineral density distribution in cortical and trabecular bone. To characterize the sub-micrometer structure of, e.g., the collagen-mineral composite in the case of bone or the cellulose microfibrils in the case of wood, other methods, such as transmission electron microscopy or X-ray scattering are necessary. The recent availability of extremely brilliant synchrotron X-ray sources has led to the development of the new techniques of scanning small-angle X-ray scattering and scanning X-ray microdiffraction, which are capable of providing structural information on the micrometer and the nanometer level, simultaneously. As a basic principle of the method the specimen is scanned across an X-ray beam which has a diameter of few micrometers. Measuring the X-ray absorption at each position provides an image of the specimen (microradiography) with resolution similar to light microscopy, in the micrometer range. Moreover, the X-ray scattering pattern is analyzed at each specimen position to provide parameters characterizing the structure in the nanometer range. The present paper reviews the principles of the techniques and demonstrates their application to biological materials, such as wood or bone.

Fibrillar structure and mechanical properties of collagen.

Collagen type I is among the most important stress-carrying protein structures in mammals. Despite their importance for the outstanding mechanical properties of this tissue, there is still a lack of understanding of the processes that lead to the specific shape of the stress-strain curve of collagen. Recent in situ synchrotron X-ray scattering experiments suggest that several different processes could dominate depending on the amount of strain. While at small strains there is a straightening of kinks in the collagen structure, first at the fibrillar then at the molecular level, higher strains lead to molecular gliding within the fibrils and ultimately to a disruption of the fibril structure. Moreover, it was observed that the strain within collagen fibrils is always considerably smaller than in the whole tendon. This phenomenon is still very poorly understood but points toward the existence of additional gliding processes occurring at the interfibrillar level.