The PERCIVAL detector: first user experiments

The PERCIVAL detector is a CMOS imager designed for the soft X-ray regime at photon sources. Although still in its final development phase, it has recently seen its first user experiments: ptychography at a free-electron laser, holographic imaging at a storage ring and preliminary tests on X-ray photon correlation spectroscopy. The detector performed remarkably well in terms of spatial resolution achievable in the sample plane, owing to its small pixel size, large active area and very large dynamic range; but also in terms of its frame rate, which is significantly faster than traditional CCDs. In particular, it is the combination of these features which makes PERCIVAL an attractive option for soft X-ray science.

AXSIS: Exploring the frontiers in attosecond X-ray science, imaging and spectroscopy

X-ray crystallography is one of the main methods to determine atomic-resolution 3D images of the whole spectrum of molecules ranging from small inorganic clusters to large protein complexes consisting of hundred-thousands of atoms that constitute the macromolecular machinery of life. Life is not static, and unravelling the structure and dynamics of the most important reactions in chemistry and biology is essential to uncover their mechanism. Many of these reactions, including photosynthesis which drives our biosphere, are light induced and occur on ultrafast timescales. These have been studied with high time resolution primarily by optical spectroscopy, enabled by ultrafast laser technology, but they reduce the vast complexity of the process to a few reaction coordinates. In the AXSIS project at CFEL in Hamburg, funded by the European Research Council, we develop the new method of attosecond serial X-ray crystallography and spectroscopy, to give a full description of ultrafast processes atomically resolved in real space and on the electronic energy landscape, from co-measurement of X-ray and optical spectra, and X-ray diffraction. This technique will revolutionize our understanding of structure and function at the atomic and molecular level and thereby unravel fundamental processes in chemistry and biology like energy conversion processes. For that purpose, we develop a compact, fully coherent, THz-driven atto-second X-ray source based on coherent inverse Compton scattering off a free-electron crystal, to outrun radiation damage effects due to the necessary high X-ray irradiance required to acquire diffraction signals. This highly synergistic project starts from a completely clean slate rather than conforming to the specifications of a large free-electron laser (FEL) user facility, to optimize the entire instrumentation towards fundamental measurements of the mechanism of light absorption and excitation energy transfer. A multidisciplinary team formed by laser-, accelerator,- X-ray scientists as well as spectroscopists and biochemists optimizes X-ray pulse parameters, in tandem with sample delivery, crystal size, and advanced X-ray detectors. Ultimately, the new capability, attosecond serial X-ray crystallography and spectroscopy, will be applied to one of the most important problems in structural biology, which is to elucidate the dynamics of light reactions, electron transfer and protein structure in photosynthesis.

Spectrometer for shot-to-shot photon energy characterization in the multi-bunch mode of the free electron laser at Hamburg

The setup and first results from commissioning of a fast online photon energy spectrometer for the vacuum ultraviolet free electron laser at Hamburg (FLASH) at DESY are presented. With the use of the latest advances in detector development, the presented spectrometer reaches readout frequencies up to 1 MHz. In this paper, we demonstrate the ability to record online photon energy spectra on a shot-to-shot base in the multi-bunch mode of FLASH. Clearly resolved shifts in the mean wavelength over the pulse train as well as shot-to-shot wavelength fluctuations arising from the statistical nature of the photon generating self-amplified spontaneous emission process have been observed. In addition to an online tool for beam calibration and photon diagnostics, the spectrometer enables the determination and selection of spectral data taken with a transparent experiment up front over the photon energy of every shot. This leads to higher spectral resolutions without the loss of efficiency or photon flux by using single-bunch mode or monochromators.

Intensity interferometry of single x-ray pulses from a synchrotron storage ring

We report on measurements of second-order intensity correlations at the high-brilliance storage ring PETRA III using a prototype of the newly developed adaptive gain integrating pixel detector. The detector records individual synchrotron radiation pulses with an x-ray photon energy of 14.4 keV and repetition rate of about 5 MHz. The second-order intensity correlation function is measured simultaneously at different spatial separations, which allows us to determine the transverse coherence length at these x-ray energies. The measured values are in a good agreement with theoretical simulations based on the Gaussian Schell model.

Fractal morphology, imaging and mass spectrometry of single aerosol particles in flight

The morphology of micrometre-size particulate matter is of critical importance in fields ranging from toxicology to climate science, yet these properties are surprisingly difficult to measure in the particles' native environment. Electron microscopy requires collection of particles on a substrate; visible light scattering provides insufficient resolution; and X-ray synchrotron studies have been limited to ensembles of particles. Here we demonstrate an in situ method for imaging individual sub-micrometre particles to nanometre resolution in their native environment, using intense, coherent X-ray pulses from the Linac Coherent Light Source free-electron laser. We introduced individual aerosol particles into the pulsed X-ray beam, which is sufficiently intense that diffraction from individual particles can be measured for morphological analysis. At the same time, ion fragments ejected from the beam were analysed using mass spectrometry, to determine the composition of single aerosol particles. Our results show the extent of internal dilation symmetry of individual soot particles subject to non-equilibrium aggregation, and the surprisingly large variability in their fractal dimensions. More broadly, our methods can be extended to resolve both static and dynamic morphology of general ensembles of disordered particles. Such general morphology has implications in topics such as solvent accessibilities in proteins, vibrational energy transfer by the hydrodynamic interaction of amino acids, and large-scale production of nanoscale structures by flame synthesis.

Nanoplasma dynamics of single large xenon clusters irradiated with superintense x-ray pulses from the linac coherent light source free-electron laser

The plasma dynamics of single mesoscopic Xe particles irradiated with intense femtosecond x-ray pulses exceeding 10(16)  W/cm2 from the Linac Coherent Light Source free-electron laser are investigated. Simultaneous recording of diffraction patterns and ion spectra allows eliminating the influence of the laser focal volume intensity and particle size distribution. The data show that for clusters illuminated with intense x-ray pulses, highly charged ionization fragments in a narrow distribution are created and that the nanoplasma recombination is efficiently suppressed.

Femtosecond dark-field imaging with an X-ray free electron laser

The emergence of femtosecond diffractive imaging with X-ray lasers has enabled pioneering structural studies of isolated particles, such as viruses, at nanometer length scales. However, the issue of missing low frequency data significantly limits the potential of X-ray lasers to reveal sub-nanometer details of micrometer-sized samples. We have developed a new technique of dark-field coherent diffractive imaging to simultaneously overcome the missing data issue and enable us to harness the unique contrast mechanisms available in dark-field microscopy. Images of airborne particulate matter (soot) up to two microns in length were obtained using single-shot diffraction patterns obtained at the Linac Coherent Light Source, four times the size of objects previously imaged in similar experiments. This technique opens the door to femtosecond diffractive imaging of a wide range of micrometer-sized materials that exhibit irreproducible complexity down to the nanoscale, including airborne particulate matter, small cells, bacteria and gold-labeled biological samples.

Micrometre resolution of a charge integrating microstrip detector with single photon sensitivity

A synchrotron beam has been used to test the spatial resolution of a single-photon-resolving integrating readout-chip coupled to a 320 µm-thick silicon strip sensor with a dedicated readout system. Charge interpolation methods have yielded a spatial resolution of σ(x) ≃ 1.8 µm for a 20 µm-pitch strip.

Diamond X-ray beam-position monitoring using signal readout at the synchrotron radiofrequency

Single-crystal diamond is a material with great potential for the fabrication of X-ray photon beam-position monitors with submicrometre spatial resolution. Low X-ray absorption combined with radiation hardness and excellent thermal-mechanical properties make possible beam-transmissive diamond devices for monitoring synchrotron and free-electron laser X-ray beams. Tests were made using a white bending-magnet synchrotron X-ray beam at DESY to investigate the performance of a position-sensitive diamond device using radiofrequency readout electronics. The device uniformity and position response were measured in a 25 microm collimated X-ray beam with an I-Tech Libera ;Brilliance' system. This readout system was designed for position measurement and feedback control of the electron beam in the synchrotron storage ring, but, as shown here, it can also be used for accurate position readout of a quadrant-electrode single-crystal diamond sensor. The centre-of-gravity position of the F4 X-ray beam at the DORIS III synchrotron was measured with the diamond signal output digitally sampled at a rate of 130 Msample s(-1) by the Brilliance system. Narrow-band filtering and digital averaging of the position signals resulted in a measured position noise below 50 nm (r.m.s.) for a 10 Hz bandwidth.

Initial structure and growth dynamics of YBa(2)Cu(3)O(7-delta) during pulsed laser deposition

The initial heteroepitaxial growth of YBa{2}Cu{3}O{7-delta} films on SrTiO3(001) substrates during pulsed laser deposition shows a growth-mode transition and a change of growth unit. The growth starts with two blocks, each two-thirds the size of the complete unit cell. The first of these blocks grows in a step-flow fashion, whereas the second grows in the layer-by-layer mode. Subsequent deposition occurs layer-by-layer of complete unit cells. These results suggest that the surface diffusion in the heteroepitaxial case is strongly influenced by the competition with formation energies, which is important for the fabrication of heteroepitaxial devices on the unit cell scale.

Improving the performance of high-resolution X-ray spectrometers with position-sensitive pixel detectors

A dispersion-compensation method to remove the cube-size effect from the resolution function of diced analyzer crystals using a position-sensitive two-dimensional pixel detector is presented. For demonstration, a resolution of 23 meV was achieved with a spectrometer based on a 1 m Rowland circle and a diced Si(555) analyzer crystal in a near-backscattering geometry, with a Bragg angle of 88.5 degrees . In this geometry the spectrometer equipped with a traditional position-insensitive detector provides a resolution of 190 meV. The dispersion-compensation method thus allows a substantial increase in the resolving power without any loss of signal intensity.

Polarization switching in BaTiO3thin films measured by X-ray diffraction exploiting anomalous dispersion

Films of BaTiO3ranging from 20 nm to 300 nm in thickness were grown with pulsed laser deposition on Nb:SrTiO3. The quality of the layers was investigated using atomic force microscopy, X-ray reflectivity and X-ray diffraction. Both the micrographs and the X-ray reflectivity spectra indicate a smooth surface of the layers. The X-ray diffraction profiles measured using synchrotron radiation show features characteristic for highly crystalline thin films. The application of an external electric field parallel to thecaxis changes anhklreflection of BaTiO3to anhk\bar{l} reflection. Due to the anomalous dispersion, the intensities of these two reflections are not equal and the atomic displacements can be determined from the intensity differences. The electric field-induced intensity changes can be as large as a few percent, which makes data collection from a 100 nm film using CuKα radiation from an X-ray tube feasible. The ΔI/Ivalues of a number of reflections from the 20 and 50 nm films were measured using synchrotron radiation. The observed ΔI/Ivalues were in good agreement with the intensity changes expected for polarization switching in the bulk.

Electric-field-induced structural changes measured with a CCD-coupled X-ray image intensifier

The conventional method to measure small induced changes in integrated intensity utilizes a zero-dimensional detector, which greatly limits the data collection speed. This paper shows that the use of an area detector in combination with an X-ray chopper decreases the data collection time significantly. A monochromatic diffraction setup using a charge-coupled device (CCD) detector coupled to an X-ray image intensifier was constructed and tested. The setup proved to be sufficiently stable to measure changes in integrated intensity well below 0.1%. Subsequently, a data set of a piezoelectric KD2PO4crystal in an external electric field was collected. The data were merged to yield 77 unique reflections. The induced structural changes were determined by a least-squares refinement. The results agree very well with experiments in which a zero-dimensional detector was used. The major improvement is the decrease in data collection time by one order of magnitude, without any degradation of the data quality, offering new possibilities for this type of perturbation study.

Refinement of framework disorder in dehydrated CaA zeolite from single-crystal synchrotron data

An accurate knowledge of zeolite structure is required for understanding their selective sorption capacities and their catalytic properties. In particular, the positions of the exchangeable cations and their interactions with the framework are essential. The present study deals with the accurate crystal structure determination of a fully exchanged and fully dehydrated CaA zeolite (Ca(48)Al(96)Si(96)O(384), Fm3c, a = 24.47 A) using single-crystal high-resolution synchrotron X-ray diffraction [(sin straight theta/lambda)(max) = 1.4 A(-1)]. It is shown that cation exchange severely distorts the skeleton, especially around the O2 atom. The high-resolution synchrotron data reveal that this latter O atom is disordered and lies out of the mirror plane it occupies in other A-type zeolites. This feature is related to that observed for Ca(2+) cations.

Deconvolution of the two-dimensional point-spread function of area detectors using the maximum-entropy algorithm

The maximum-entropy method (MEM) has been applied for the deconvolution of the point-spread function (PSF) of two-dimensional X-ray detectors. The method is robust, model and image independent, and only depends on the correct description of the two-dimensional point-spread function and gain factor of the detector. A significant enhancement of both the spatial resolution and the contrast ratio has been obtained for two phase-contrast images recorded with an ultra-high-resolution X-ray imaging detector. The method has also been applied to a Laue diffraction image of a protein crystal, showing an important improvement in both the peak separation of closely spaced diffraction peaks and the signal-to-noise ratio of medium and weak peaks. The principle of the method is explained and examples of its application are presented.

Towards extinction-free experimental diffraction data on Al2O3

Using 58 keV (0.214 Å) synchrotron radiation we have obtained virtually extinction-free data on Al2O3. Refinement of a multipole model against these data produced deformation densities of high quality, which compare well with theoretical maps.

The piezoelectric tensor element d 33 of KTiOPO4 determined by single crystal X-ray diffraction

The d 33 piezoelectric constant of KTiOPO4 (potassium titanium orthophosphate, KTP) has been determined for two different samples and at temperatures between 100 and 220 K, using high-resolution X-ray diffraction of a single-crystal in an electric field. The observed value of 15 (2) \times 10−12 m V−1 is between the two values of 10.4 and 25.8 \times 10−12 m V−1 found in the literature. The value of d 33 is shown to be constant over the temperature range 100–220 K and no anomaly was observed at the conductor–insulator transition at 150 K. The results obtained are believed to be sample-independent, since the same value was obtained for two different crystals, measured at different sources.

Applications of high-energy synchrotron radiation for structural studies of polycrystalline materials

The large penetration power of high-energy X-rays (>60 keV) raises interesting prospects for new types of structural characterizations of polycrystalline materials. It becomes possible in a non-destructive manner to perform local studies, within the bulk of the material, of the fundamental materials physics properties: grain orientations, strain, dislocation densities etc. In favourable cases these properties may be mapped in three dimensions with a spatial resolution that matches the dimensions of the individual grains. Imbedded volumes and interfaces become accessible. Moreover, the high energies allow better in-situ studies of samples in complicated environments (industrial process optimization). General techniques for research in this energy range have been developed using broad-band angle-dispersive methods, on-line two-dimensional detectors and conical slits. Characterizations have been made at the level of the individual grains and grain boundaries as well as on ensembles of grains. The spatial resolution is presently of the order of 10-100 micom. Four examples of applications are presented along with an outlook.

Wide dynamic range germanium detector for perturbation crystallography

An IR detector based on a cooled germanium photodetector has been tested for applications in X-ray diffraction. The detector can be used simultaneously in photon-counting mode and current mode giving a dynamic range from < 1 to 10(9) photons s(-1). Since germanium is used as the photodetector, its efficiency at energies above 25 keV is much better than the silicon equivalents. The detector proved to be highly linear both in the low-flux region (< 10(5) photons s(-1)) where photon counting is used and in the high-flux region (> 10(5) photons s(-1)) where the detector is run in current mode. The response time of the detector is of the order of 1 mus, making it suitable for studies in perturbation crystallography, especially when coupled to a lock-in amplifier. As an example, the shift of a reflection of LiNbO(3) induced by an external electric field was determined with the germanium detector and lock-in amplifier.