Claudio Ferrero

McXtrace: a Monte Carlo software package for simulating X-ray optics, beamlines and experiments

This article presents the Monte Carlo simulation package McXtrace, intended for optimizing X-ray beam instrumentation and performing virtual X-ray experiments for data analysis. The system shares a structure and code base with the popular neutron simulation code McStas and is a good complement to the standard X-ray simulation software SHADOW. McXtrace is open source, licensed under the General Public License, and does not require the user to have access to any proprietary software for its operation. The structure of the software is described in detail, and various examples are given to showcase the versatility of the McXtrace procedure and outline a possible route to using Monte Carlo simulations in data analysis to gain new scientific insights. The studies performed span a range of X-ray experimental techniques: absorption tomography, powder diffraction, single-crystal diffraction and pump-and-probe experiments. Simulation studies are compared with experimental data and theoretical calculations. Furthermore, the simulation capabilities for computing coherent X-ray beam properties and a comparison with basic diffraction theory are presented.

Revealing letters in rolled Herculaneum papyri by X-ray phase-contrast imaging

Hundreds of papyrus rolls, buried by the eruption of Mount Vesuvius in 79 AD and belonging to the only library passed on from Antiquity, were discovered 260 years ago at Herculaneum. These carbonized papyri are extremely fragile and are inevitably damaged or destroyed in the process of trying to open them to read their contents. In recent years, new imaging techniques have been developed to read the texts without unwrapping the rolls. Until now, specialists have been unable to view the carbon-based ink of these papyri, even when they could penetrate the different layers of their spiral structure. Here for the first time, we show that X-ray phase-contrast tomography can reveal various letters hidden inside the precious papyri without unrolling them. This attempt opens up new opportunities to read many Herculaneum papyri, which are still rolled up, thus enhancing our knowledge of ancient Greek literature and philosophy.

Computer simulation of bent perfect crystal diffraction profiles

Various theoretical methods for calculating diffraction profiles of perfect crystals are available in literature. Although these methods hold within certain validity ranges due to their inherent approximations, they constitute the current state-of-the-art of numerical computation of diffraction profiles. In this paper we summarize the theory of Zachariasen for flat crystals, the multi-lamellar approximation for bent crystals and the Penning-Polder approximation for bent Laue crystals. Some examples of their results are presented. Another method to calculate the diffraction profile consists in solving the Takagi-Taupin equations. The finite difference method, that provides a numerical solution of these equations, is briefly discussed. A new method for solving numerically these equations using the finite element method is proposed. This method is very flexible, because it can consider a crystal with an arbitrary shape and cover the case of critical regime (i.e., inhomogeneities and deformations) with fine elements. In addition, it can couple naturally the diffraction calculation with thermal or mechanical crystal deformations. These deformations are generally induced by the x-ray beam (heat load), the crystal bender (mechanical stress) or are intrinsic to the crystal (inhomogeneities, impurities, dislocations, etc.). An example of the feasibility of this method is shown.

GENFIT: software for the analysis of small-angle X-ray and neutron scattering data of macro­molecules in solution

GENFIT is a new computer code featuring an advanced model-fitting capability to analyse small-angle X-ray and neutron scattering data of macromolecular systems. Batches of experimental curves can be simultaneously best fitted using common parameters issued from combinations of models and, if applicable, constrained by physical and/or phenomenological relations.

Thermally induced self-assembly of cylindrical nanodomains in low molecular weight PS-b-PMMA thin films.

The phase behaviour in thin films of an asymmetric polystyrene-b-polymethylmethacrylate (PS-b-PMMA) block copolymer with a molecular weight of 39 kg mol(-1) was assessed at a wide range of temperatures and times. Cylindrical PMMA structures featuring a diameter close to 10 nm and perpendicularly oriented with respect to the substrate were obtained at 180 °C in relatively short annealing times (t ≤ 30 min) by means of a simple thermal treatment performed in a standard rapid thermal processing machine.

Spatially‐Resolved In‐Situ Structural Study of Organic Electronic Devices with Nanoscale Resolution: The Plasmonic Photovoltaic Case Study

A novel high spatial resolution synchrotron X-ray diffraction stratigraphy technique has been applied in-situ to an integrated plasmonic nanoparticle-based organic photovoltaic device. This original approach allows for the disclosure of structure-property relations linking large scale organic devices to length scales of local nano/hetero structures and interfaces between the different components.

A GPU-based architecture for real-time data assessment at synchrotron experiments

Current imaging experiments at synchrotron beam lines often lack a real-time data assessment. X-ray imaging cameras installed at synchrotron facilities like ANKA provide millions of pixels, each with a resolution of 12 bits or more, and take up to several thousand frames per second. A given experiment can produce data sets of multiple gigabytes in a few seconds. Up to now the data is stored in local memory, transferred to mass storage, and then processed and analyzed off-line. The data quality and thus the success of the experiment, can, therefore, only be judged with a substantial delay, which makes an immediate monitoring of the results impossible. To optimize the usage of the micro-tomography beam-line at ANKA we have ported the reconstruction software to modern graphic adapters which offer an enormous amount of calculation power. We were able to reduce the reconstruction time from multiple hours to just a few minutes with a sample dataset of 20 GB. Using the new reconstruction software it is possible to provide a near real-time visualization and significantly reduce the time needed for the first evaluation of the reconstructed sample. The main paradigm of our approach is 100% utilization of all system resources. The compute intensive parts are offloaded to the GPU. While the GPU is reconstructing one slice, the CPUs are used to prepare the next one. A special attention is devoted to minimize data transfers between the host and GPU memory and to execute I/O operations in parallel with the computations. It could be shown that for our application not the computational part but the data transfers are now limiting the speed of the reconstruction. Several changes in the architecture of the DAQ system are proposed to overcome this second bottleneck. The article will introduce the system architecture, describe the hardware platform in details, and analyze performance gains during the first half year of operation.

Curved Crystal Transmission Optics for Energy-Dispersive X-ray Absorption Spectroscopy.

The development of a curved crystal monochromator of the Laue type for energy-dispersive X-ray absorption spectroscopy is presented. The quality of the X-ray absorption spectra at high photon energies is compared with spectra measured with silicon crystals in the more frequently used Bragg geometry. In the Bragg case, an asymmetric broadening of the reflectivity profile leads to strong distortions of the near-edge fine structure and to a reduction in spectral resolution. The reflectivity profiles of fiat and curved crystals for Laue and Bragg geometry have been calculated using dynamical theory and are compared with experimental data. The new optics have been used for in situ time-resolved X-ray absorption spectroscopy. An example of the application of the technique for the characterization of a Pd catalyst is given. The X-ray absorption fine structure at the Pd K-edge has been measured during the activation and during the heterogeneous catalytic oxidation of carbon monoxide.

Rare Earth doped ceria: a combined X-ray and neutron pair distribution function study

Abstract Rare Earth doped ceria materials (Ce1–xRExO2–x/2) are widely studied for their application in solid oxide fuel cell devices. In this work, RE(Yb, Y, Nd, La)-doped ceria samples at constant (x = 0.25) doping rate were subjected to a combined synchrotron radiation and neutron powder diffraction study. The dopants were chosen in order to cover a wide range of dopant-ionic radii. The effect of doping on the average structure is investigated using conventional Rietveld analysis, while the Pair Distribution Function technique is used to explore the spatial extent of disorder as well as the local structure. Two models for mapping the local structure, in terms of oxygen relaxation and nano-phase separation, are presented.

Bent crystals in Laue geometry: dynamical focusing of a polychromatic incident beam

The main limitation of Laue geometry for the achievement of a small focus size is the focus broadening caused by the intrinsic Darwin width and the spread of the beam in the Borrmann triangle, resulting from propagation inside the crystal. A method, based on dynamical focusing, is suggested that allows improvement of the quality of high-energy polychromatic focusing by bent crystals in Laue geometry.