Gerd Wellenreuther

Avian Magnetoreception: Elaborate Iron Mineral Containing Dendrites in the Upper Beak Seem to Be a Common Feature of Birds

The magnetic field sensors enabling birds to extract orientational information from the Earths magnetic field have remained enigmatic. Our previously published results from homing pigeons have made us suggest that the iron containing sensory dendrites in the inner dermal lining of the upper beak are a candidate structure for such an avian magnetometer system. Here we show that similar structures occur in two species of migratory birds (garden warbler, Sylvia borin and European robin, Erithacus rubecula) and a non-migratory bird, the domestic chicken (Gallus gallus). In all these bird species, histological data have revealed dendrites of similar shape and size, all containing iron minerals within distinct subcellular compartments of nervous terminals of the median branch of the Nervus ophthalmicus. We also used microscopic X-ray absorption spectroscopy analyses to identify the involved iron minerals to be almost completely Fe III-oxides. Magnetite (Fe II/III) may also occur in these structures, but not as a major Fe constituent. Our data suggest that this complex dendritic system in the beak is a common feature of birds, and that it may form an essential sensory basis for the evolution of at least certain types of magnetic field guided behavior.

Hard X-ray nanoprobe at beamline P06 at PETRA III

We describe the hard X-ray scanning microscope planned for the new synchrotron radiation source PETRA III at DESY in Hamburg, Germany. It is based on nanofocusing refractive X-ray lenses and is designed for two-dimensional mapping and scanning tomography. It supports X-ray fluorescence and (coherent) diffraction contrast, yielding elemental and structural information from inside the sample. Spatial resolutions down to well below 50 nm are aimed for in direct space. A further increase in spatial resolution is expected by applying ptychographic scanning schemes. The optical scheme with a two-stage focusing optic is described.

Kinoform diffractive lenses for efficient nano-focusing of hard X-rays

A nano-focusing module based on two linear Fresnel zone plates is presented. The zone plates are designed to generate a kinoform phase profile in tilted geometry, thus overcoming the efficiency limitations of binary diffractive structures. Adjustment of the tilt angle enables tuning of the setup for optimal efficiency over a wide range of photon energies, ranging from 5 to 20 keV. Diffraction efficiency of more than 50% was measured for the full module at 8 keV photon energy. A diffraction limited spot size of 100 nm was verified by ptychographic reconstruction for a lens module with a large entrance aperture of 440 μm × 400 μm.

Non-negative factor analysis supporting the interpretation of elemental distribution images acquired by XRF

Stacks of elemental distribution images acquired by XRF can be difficult to interpret, if they contain high degrees of redundancy and components differing in their quantitative but not qualitative elemental composition. Factor analysis, mainly in the form of Principal Component Analysis (PCA), has been used to reduce the level of redundancy and highlight correlations. PCA, however, does not yield physically meaningful representations as they often contain negative values. This limitation can be overcome, by employing factor analysis that is restricted to non-negativity. In this paper we present the first application of the Python Matrix Factorization Module (pymf) on XRF data. This is done in a case study on the painting Saul and David from the studio of Rembrandt van Rijn. We show how the discrimination between two different Co containing compounds with minimum user intervention and a priori knowledge is supported by Non-Negative Matrix Factorization (NMF).

Dual detection X-ray fluorescence cryotomography and mapping on the model organism Daphnia magna

Micro X-ray fluorescence (-XRF) is a rapidly evolving analytical technique which allows visualising the trace level metal distributions within a specimen in an essentially non-destructive manner. At second generation synchrotron radiation sources, detection limits at the sub-ppm level can be obtained with micrometer resolution, while at third generation sources the spatial resolution can be better than 100 nm. Consequently, the analysis of metals within biological systems using micro and nano X-ray fluorescence imaging is a quickly developing field of research. Since X-ray fluorescence is a scanning technique, the elemental distribution within the sample should not change during analysis. Biological samples pose challenges in this context due to their high water content. A dehydration procedure is commonly used for sample preparation, enabling an analysis of the sample under ambient temperature conditions. Unfortunately, a potential change of elemental redistribution during the sample preparation is difficult to verify experimentally and therefore can not be excluded completely. Creating a cryogenic sample environment allowing an analysis of the sample under cryogenic condition is an attractive alternative, but not available on a routine basis. In this article, we make a comparison between the elemental distributions obtained by micro SR-XRF within a chemically fixed and a cryogenically frozen Daphnia magna, a model organism to study the environmental impact of metals. In what follows, we explore the potential of a dual detector set-up for investigating a full ecotoxicological experiment. Next to conventional 2D analysis, dual detector X-ray fluorescence cryotomography is illustrated and the potential of its coupling with laboratory absorption micro-CT for investigating the tissue specific elemental distributions within this model organism is highlighted.

Fast Strain Mapping of Nanowire Light-Emitting Diodes Using Nanofocused X‑ray Beams

X-ray nanobeams are unique nondestructive probes that allow direct measurements of the nanoscale strain distribution and composition inside the micrometer thick layered structures that are found in most electronic device architectures. However, the method is usually extremely time-consuming, and as a result, data sets are often constrained to a few or even single objects. Here we demonstrate that by special design of a nanofocused X-ray beam diffraction experiment we can (in a single 2D scan with no sample rotation) measure the individual strain and composition profiles of many structures in an array of upright standing nanowires. We make use of the observation that in the generic nanowire device configuration, which is found in high-speed transistors, solar cells, and light-emitting diodes, each wire exhibits very small degrees of random tilts and twists toward the substrate. Although the tilt and twist are very small, they give a new contrast mechanism between different wires. In the present case, we image complex nanowires for nanoLED fabrication and compare to theoretical simulations, demonstrating that this fast method is suitable for real nanostructured devices.

Non-negative matrix factorization for the near real-time interpretation of absorption effects in elemental distribution images acquired by X-ray fluorescence imaging.

Elemental distribution images acquired by imaging X-ray fluorescence analysis can contain high degrees of redundancy and weakly discernible correlations. In this article near real-time non-negative matrix factorization (NMF) is described for the analysis of a number of data sets acquired from samples of a bi-modal α+β Ti-6Al-6V-2Sn alloy. NMF was used for the first time to reveal absorption artefacts in the elemental distribution images of the samples, where two phases of the alloy, namely α and β, were in superposition. The findings and interpretation of the NMF results were confirmed by Monte Carlo simulation of the layered alloy system. Furthermore, it is shown how the simultaneous factorization of several stacks of elemental distribution images provides uniform basis vectors and consequently simplifies the interpretation of the representation.

Hard x-ray nano-beam characterization by ptychographic imaging

Modern hard x-ray scanning microscopes generate x-ray beams with lateral sizes well below 100 nm. Characterizing these beams in terms of shape and size by conventional techniques is tedious, requires highly accurate test objects and stages, and yields only incomplete information. Since recently, we use a ptychographic scanning coherent diffraction imaging technique in order to characterize hard x-ray nano beams in x-ray scanning microscopes, obtaining a detailed quantitative picture of the complex wave field in the nano focus and allowing one to reconstruct the exit wave field behind the nano-focusing optic, giving detailed insight into its aberrations.

Fast EXAFS in synchronous scanning mode at PETRA P06

First fast EXAFS spectra have been successfully measured at the Hard X-ray Micro-probe Beamline P06 (PETRA III) synchronously scanning the U32 undulator gap and the monochromator Bragg axis. Stability and spatial homogeneity of the beam are proved to be the limiting factors for the quality of the EXAFS spectra, whereas the performance of the data acquisition electronics limits the energy resolution when total scan time is reduced to 30 seconds. Results of the test measurements give us the estimate of the utmost performance of 510 seconds per full EXAFS scan considering certain hardware modifications.