High-intensity X-ray Research Articles

Determination of multiwavelength anomalous diffraction coefficients at high x-ray intensity

The high-intensity version of multiwavelength anomalous diffraction (MAD) has a potential for solving the phase problem in femtosecond crystallography with x-ray free-electron lasers (XFELs). For MAD phasing, it is required to calculate or measure the MAD coefficients involved in the key equation, which depend on XFEL pulse parameters. In this work, we revisit the generalized Karle–Hendrickson equation to clarify the importance of configurational fluctuations of heavy atoms induced by intense x-ray pulses, and investigate the high-intensity cases of transmission and fluorescence measurements of samples containing heavy atoms. Based on transmission/fluorescence and diffraction experiments with crystalline samples of known structures, we propose an experimental procedure to determine all MAD coefficients at high x-ray intensity, which can be used in ab initio phasing for unknown structures.

Photoelectron spectroscopy method to reveal ionization potential lowering in nanoplasmas

Here we propose a scheme for probing the outer-shell atomic energy levels within a laser-created nanoplasma, using photoelectron spectroscopy data obtained from the irradiation of the nanoplasma with an ultraintense ‘probing’ pulse from a soft x-ray free-electron laser. The proposed method can then detect shifts of outer-shell energy levels of an atom or an ion within a plasma, due to the effect of charged plasma environment on atomic potentials, known as the ‘plasma screening effect’. Various theoretical models exist that estimate the magnitude of the screening effect. However, the first experimental data that can verify theoretical models have become available only recently (Vinko et al 2012 Nature 482 59). They were obtained with a hard x-ray-based experimental method which uses the information encoded in fluorescence spectra and is, therefore, restricted to deep atomic shells. Below, we show that our photoelectron spectroscopy method of probing a nanoplasma with a destructive, high-intensity soft x-ray pulse that brings the irradiated system to the regime of ‘massively parallel’ ionization (Gnodtke et al 2012 Phys. Rev. Lett. 108 175003) enables access to the information on the energy level (and ultimately energy level shift) of the valence orbital. In particular, the result of such a photoelectron spectroscopy experiment could help to clarify the discrepancy between the ion abundances in nanoplasmas observed during the recent high-harmonic-generation and free-electron-laser experiments with intense soft x-ray pulses.

Optically-dressed resonant Auger processes induced by high-intensity x-rays

We have unveiled coherent multiphoton interferences originating from different quantum paths taken by the Auger electron induced by a high-intensity x-ray/extreme ultraviolet pulse under the presence of a strong optical field. These interferences give rise to a clear signature in the angle-resolved Auger electron spectrum: an asymmetry with respect to the energy of the Auger decay channel. In order to illustrate this effect we have considered the resonant Auger decay of the transition in Ne+. The simulations show that these interferences are very sensitive to the parameters of the x-ray and optical fields.

Current saturation in free-air ionization chambers with chopped synchrotron radiation

Saturation curves for free-air ionization chambers with electrode gap widths of 4.2, 8.4 and 18 mm were obtained for 10 and 15 keV undulator synchrotron radiation thinned with a 230 Hz rotating-disk chopper. Ion recombination in free-air ionization chambers was found to be inversely proportional to the applied electric field, and an expression that satisfactorily reproduced the ion-recombination rate is determined. A comparison of the expressions for continuous and pulsed X-rays revealed that chopped high-intensity X-rays require a higher voltage to attain saturation when the product of the pulse width and electric field exceeds a value that depends on the X-ray energy. This behaviour was observed explicitly for 10 keV X-rays in measurements with the ionization chamber placed before and after the chopper.

Mesoscopic hydro-thermodynamics of phonons

A generalized Hydrodynamics, referred to as Mesoscopic Hydro-Thermodynamics, of phonons in semiconductors is presented. It involves the descriptions of the motion of the quasi-particle density and of the energy density. The hydrodynamic equations, which couple both types of movement via thermo-elastic processes, are derived starting with a generalized Peierls-Boltzmann kinetic equation obtained in the framework of a Non-Equilibrium Statistical Ensemble Formalism, providing such Mesoscopic Hydro-Thermodynamics. The case of a contraction in first order is worked out in detail. The associated Maxwell times are derived and discussed. The densities of quasi-particles and of energy are found to satisfy coupled Maxwell-Cattaneo-like (hyperbolic) equations. The analysis of thermo-elastic effects is done and applied to investigate thermal distortion in silicon mirrors under incidence of high intensity X-ray pulses in FEL facilities. The derivation of a generalized Guyer-Krumhansl equation governing the flux of heat and the associated thermal conductivity coefficient is also presented.

Non-Maxwellian electron distributions in time-dependent simulations of low-Z materials illuminated by a high-intensity X-ray laser

The interaction of high intensity X-ray lasers with matter is modeled. A collisional-radiative time-dependent module is implemented to study radiation transport in matter from ultrashort and ultraintense X-ray bursts. Inverse bremsstrahlung absorption by free electrons, electron conduction or hydrodynamic effects are not considered. The collisional-radiative system is coupled with the electron distribution evolution treated with a Fokker-Planck approach with additional inelastic terms. The model includes spontaneous emission, resonant photoabsorption, collisional excitation and de-excitation, radiative recombination, photoionization, collisional ionization, three-body recombination, autoionization and dielectronic capture. It is found that for high densities, but still below solid, collisions play an important role and thermalization times are not short enough to ensure a thermal electron distribution. At these densities Maxwellian and non-Maxwellian electron distribution models yield substantial differences in collisional rates, modifying the atomic population dynamics.

Ceria Nanocubes: Dependence of the Electronic Structure on Synthetic and Experimental Conditions

The properties of nanostructured ceria are both size- and shape-dependent. Here, ∼3–10 nm ceria nanocubes having exposed (002) surfaces were hydrothermally synthesized. For the first time, the electronic properties of such nanocubes were systematically studied using X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. Comparisons of the collected Ce 3d XPS spectra after a single sweep revealed that the concentration of Ce3+ present within the nanocubes is independent of the particle size and the Ce precursor used. Ce 3d spectra collected after averaging 20 sweeps demonstrated the reduction of Ce4+, and that the degree of reduction was also independent of the synthetic conditions. This observed surface reduction/damage is mainly the result of exposure to high intensity X-ray radiation. These conclusions were further confirmed through examination of Ce M4,5- and N4,5-edge XANES spectra. On the basis of these results, it is clear that the concentration of Ce3+ on the surface of ceria nanocubes...

Nonequilibrium electron dynamics in materials driven by high-intensity x-ray pulses

We calculated the evolution of the electron system in solid-density matter irradiated by high-intensity x-ray pulses between 2 and 8 keV using molecular dynamics. For pulses shorter than 40 fs, the kinetic energy distribution of the electrons is highly nonthermal during and right after the pulse, and a large fraction of the absorbed x-ray energy resides with the fast photoelectrons which equilibrate on the timescale of the pulse length. The average ionization and electron temperature of the bulk of the electrons are significantly lower than their equilibrium values.

Using in Situ X-ray Reflectivity to Study Protein Adsorption on Hydrophilic and Hydrophobic Surfaces: Benefits and Limitations

We have employed in situ X-ray reflectivity (IXRR) to study the adsorption of a variety of proteins (lysozyme, cytochrome c, myoglobin, hemoglobin, serum albumin, and immunoglobulin G) on model hydrophilic (silicon oxide) and hydrophobic surfaces (octadecyltrichlorosilane self-assembled monolayers), evaluating this recently developed technique for its applicability in the area of biomolecular studies. We report herein the highest resolution depiction of adsorbed protein films, greatly improving on the precision of previous neutron reflectivity (NR) results and previous IXRR studies. We were able to perform complete scans in 5 min or less with the maximum momentum transfer of at least 0.52 Å(-1), allowing for some time-resolved information about the evolution of the protein film structure. The three smallest proteins (lysozyme, cytochrome c, and myoglobin) were seen to deposit as fully hydrated, nondenatured molecules onto hydrophilic surfaces, with indications of particular preferential orientations. Time evolution was observed for both lysozyme and myoglobin films. The larger proteins were not observed to deposit on the hydrophilic substrates, perhaps because of contrast limitations. On hydrophobic surfaces, all proteins were seen to denature extensively in a qualitatively similar way but with a rough trend that the larger proteins resulted in lower coverage. We have generated high-resolution electron density profiles of these denatured films, including capturing the growth of a lysozyme film. Because the solution interface of these denatured films is diffuse, IXRR cannot unambiguously determine the film extent and coverage, a drawback compared to NR. X-ray radiation damage was systematically evaluated, including the controlled exposure of protein films to high-intensity X-rays and exposure of the hydrophobic surface to X-rays before adsorption. Our analysis showed that standard measuring procedures used for XRR studies may lead to altered protein films; therefore, we used modified procedures to limit the influence of X-ray damage.

High-Gain Thompson-Scattering X-Ray Free-Electron Laser by Time-Synchronic Laterally Tilted Optical Wave

A novel approach to generating coherent x rays with 10(9)-10(10) photons and femtoseconds duration per laser pulse is proposed. This high intensity x-ray source is realized first by the pulse front tilt of a lateral fed laser to extend the electron-laser synchronic interaction time by several orders, which accomplishes the high-gain free-electron-laser-type exponential growth process and coherent emission with highly microbunched electron beam. Second, two methods are presented to enhance the effective optical undulator strength parameter. One is to invoke lenses to focus two counterpropagating lasers that are at normal incidence to the electron beam as a transverse standing wave; the other is to invent a periodic microstructure that can significantly enhance the center electromagnetic field realized by a resonant standing wave and the quadrupole waveguides. The energy coupling efficiency between the electron beam and laser is therefore greatly improved to generate the high brightness x rays, which is demonstrated by analytical and simulation results.

Blowhole Generation Phenomenon and Quality Improvement in GMAW of Galvanized Steel Sheet

In the automotive industry, galvanized steel sheets have been applied for quality improvement. However, it has a problem of the weldability debasement by porosity defect (blowhole, pit) and spatter increase. In this study, the blowhole generating behavior which was not clarified until now has been observed by the high-intensity X-ray imaging apparatus. And the method to make reduce both of the porosity defects and the spatter has been examined. In the result, the measure was effective that the porosity defect is released as zinc vapor from just under the arc to outside of the molten pool. Based on this knowledge, a solution to problem was accomplished by optimizing (a) composition and size of welding wire, (b) shielding gas composition and (c) current pulse wave pattern.

コヒーレントX線回折イメージング

Coherent X-ray diffractive imaging (CXDI) allows us to observe thick objects with a high spatial resolution, also providing us with unique structural information, i.e., electron density distribution and/or strain distribution. We have developed high-resolution diffractive imaging apparatus using the high-intensity X-ray beam focused by total reflection mirrors at SPring-8 and have demonstrated high-resolution plane-wave CXDI and scanning CXDI (i.e. X-ray ptychography). In addition, we have demonstrated element-specific X-ray ptychography using anomalous scattering around a specific element. In the near future, CXDI will become a promising tool for structure investigation in various fields including high-pressure science.

高分解能・高感度X線タイコグラフィー

X-ray ptychography, which is one of the coherent X-ray diffractive imaging techniques, allows us to observe extended objects with a high-spatial resolution and a wide field of view. We have developed high-resolution and high-sensitivity X-ray ptychography using the high-intensity X-ray beam focused by total reflection mirrors at SPring-8. In addition, we have applied it to the elemental mapping of metallic nanoparticles and the imaging of the dislocation strain fields of a silicon film. This method will be available to observe various materials and biological specimen.

Effect of high-intensity x-ray radiation on Bragg diffraction in silicon and diamond

We simulated the Bragg reflection of high-intensity short x-ray pulses from single-crystal silicon and diamond by coupling tight-binding-molecular dynamics with a simple atomic kinetics model. We found that even when the pulse-averaged Bragg intensity degrades significantly, the reflectivity drops only slightly at the beginning of the pulse until the lattice is disordered by non-thermal melting. These results suggest that Bragg reflectors could produce shortened x-ray pulses through temporal slicing.

MultiphotonL-shell ionization of H2S using intense x-ray pulses from a free-electron laser

Sequential multiphoton $L$-shell ionization of hydrogen sulfide exposed to intense femtosecond pulses of 1.25-keV x rays has been observed via photoelectron, Auger electron, and ion time-of-flight spectroscopies. Monte Carlo simulations based on relativistic Dirac-Hartree-Slater calculations of Auger decay rates in sulfur with single and double $L$-shell vacancies accurately model the observed spectra. While single-vacancy-only calculations are surprisingly accurate even at the high x-ray intensity used in the experiment, calculations including double-vacancy states improve on yield estimates of highly charged sulfur ions. In the most intense part of the x-ray focal volume, an average molecule absorbs more than five photons, producing multiple $L$-shell vacancies in 17$%$ of photoionization events according to simulation. For 280-fs pulse duration and \ensuremath{\sim}10${}^{17}$ W cm${}^{\ensuremath{-}2}$ focal intensity, the yield of S${}^{13+}$ is \ensuremath{\sim}1$%$ of the S${}^{3+}$ yield, in good agreement with simulations. An overabundance of S${}^{12+}$, and S${}^{14+}$ observed in the experimental ion spectra is not predicted by either single-vacancy or double-vacancy calculations.

Strongly driven resonant Auger effect treated by an open-quantum-system approach

We present theoretical studies of a two-step resonant Auger process at high x-ray intensity. Tuning a short x-ray pulse to the initially closed resonant channel of the 1s-2p transition in singly ionized neon, the initially neutral neon target is valence ionized. Subsequently, the strong resonant x-ray field transfers an inner-shell electron to the created outer valence vacancy, thereby creating a core-excited state. The strong resonant coupling, giving rise to Rabi oscillations involving a core transition, results in a modification of the resonant Auger-electron spectral line profile. If the valence photoelectron remains unobserved, the system of the residual ion undergoing the resonant Auger decay can be treated by an open quantum system approach. The resonant Auger-electron spectral line shape is shown to be determined by an analog of the reduced density matrix that depends on two time arguments. The equations of motion of this reduced density matrix are derived and numerical results are presented, in support of the recent experimental verification [E. Kanter et al., Phys. Rev. Lett. 107, 233001 (2011)] of this nonlinear x-ray optical effect.

Development of a multilayer mirror for high-intensity monochromatic x-ray using lab-based x-ray source

A parabolic, multilayer x-ray mirror, which can be used with a general lab-based x-ray source, was designed and fabricated. A glass substrate for the mirror was fabricated. Its surface was determined by following the rotation of a parabolic curve and was polished precisely. On the substrate surface, six W/Al bilayers were deposited to form the multilayer mirror. The effects of the mirror on x-ray images were investigated based on the calculated modulation transfer function (MTF) and image intensity values. Higher MTF and intensity values of an x-ray image were obtained using the mirror.

Study and optimization of negative polarity rod pinch diode as flash radiography source at 4.5 MV

The negative polarity rod pinch diode (NPRPD) is a potential millimeter spot size radiography source for high voltage generators (4 to 8 MV) [Cooperstein et al., “Considerations of rod-pinch diode operation in negative polarity for radiography,” in Proceedings of the 14th IEEE Pulsed Power Conference, 2003, pp. 975–978]. The NPRPD consists of a small diameter (few mm) cylindrical anode extending from the front end of the vacuum cell through a thin annular cathode, held by a central conductor. The polarity has been inverted when compared to the original rod pinch diode [Cooperstein et al., “Theoretical modeling and experimental characterization of a rod-pinch diode,” Phys. Plasmas 8(10), 4618–4636 (2001)] in order to take advantage from the maximal x-ray emission toward the anode holder at such a voltage [Swanekamp et al., “Evaluation of self-magnetically pinched diodes up to 10 MV as high resolution flash X-ray sources,” IEEE Trans. Plasma Sci. 32(5), 2004–2016 (2004). We have studied this diode at 4.5 MV, driven by the ASTERIX generator [Raboisson et al., “ASTERIX, a high intensity X-ray generator,” in Proceedings of the 7th IEEE Pulsed Power Conference, 1989, pp. 567–570.]. This generator, made up of a capacitor bank and a Blumlein line, was initially designed to test the behavior of electronic devices under irradiation. In our experiments, the vacuum diode has been modified in order to set up flash a radiographic diode [Etchessahar et al., “Negative polarity rod pinch diode experiments on the ASTERIX generator,” in Conference Records–Abstracts, 37th IEEE International Conference on Plasma Science, 2010]. The experiments and numerical simulations presented here allowed the observation and analysis of various physical phenomena associated with the diode operation. Also, the influence of several experimental parameters, such as cathode and anode diameters, materials and surface states, was examined. In order to achieve the most comprehensive characterization of the diode, both optical and x-ray diagnostics were used, including high speed multi-image ICCD (intensified CCD) cameras, streak camera, dosimeters, spot size measurements, and pinhole cameras. A set of new results have been obtained through this study. The plasma emission from the anode and cathode surfaces and its expansion appear to be critical for the diode functioning. Also, for the first time, potential sources of diode instability were identified. Finally, an optimal and stable diode configuration was found with the following parameters: 52 rad at 1 m (in Al) and 2.2 mm spot size.

Plasma-driven Z-pinch X-ray loading and momentum coupling in meteorite and planetary materials

AbstractX-ray momentum coupling coefficients, CM, were determined by measuring stress waveforms in planetary materials subjected to impulsive radiation loading from the Sandia National Laboratories Z-machine. Velocity interferometry (VISAR) diagnostics provided equation-of-state data. Targets were iron and stone meteorites, magnesium-rich olivine (dunite) solid and powder (~5–300 μm), and Si, Al, and Fe calibration targets. Samples were ~1-mm thick and, except for Si, backed by LiF single-crystal windows. X-ray spectra combined thermal radiation (blackbody 170–237 eV) and line emissions from pinch materials (Cu, Ni, Al, or stainless steel). Target fluences of 0.4–1.7 kJ/cm2 at intensities of 43–260GW/cm2 produced plasma pressures of 2.6–12.4 GPa. The short (~5 ns) drive pulses gave rise to attenuating stress waves in the samples. The attenuating wave impulse is constant, allowing accurate CM measurements from rear-surface motion. CM was 1.9 − 3.1 × 10−5 s/m for stony meteorites, 2.7 and 0.5 × 10−5 s/m for solid and powdered dunite, 0.8 − 1.4 × 10−5 s/m for iron meteorites, and 0.3, 1.8, and 2.7 × 10−5 s/m respectively for Si, Fe, and Al calibration targets. Results are consistent with geometric scaling from recent laser hohlraum measurements. CTH hydrocode modeling of X-ray coupling to porous silica corroborated experimental measurements and supported extrapolations to other materials. CTH-modeled CM for porous materials was low and consistent with experimental results. Analytic modeling (BBAY) of X-ray radiation-induced momentum coupling to selected materials was also performed, often producing higher CM values than experimental results. Reasons for the higher values include neglect of solid ejecta mechanisms, turbulent mixing of heterogeneous phases, variances in heats of melt/vaporization, sample inhomogeneities, wave interactions at the sample/window boundary, and finite sample/window sizes. The measurements validate application of CM to (inhomogeneous) planetary materials from high-intensity soft X-ray radiation.

Autocovariance Structures for Radial Averages in Small Angle X-Ray Scattering Experiments.

Small-angle X-ray scattering (SAXS) is a technique for obtaining low-resolution structural information about biological macromolecules, by exposing a dilute solution to a high-intensity X-ray beam and capturing the resulting scattering pattern on a two-dimensional detector. The two-dimensional pattern is reduced to a one-dimensional curve through radial averaging; that is, by averaging across annuli on the detector plane. Subsequent analysis of structure relies on these one-dimensional data. This paper reviews the technique of SAXS and investigates autocorrelation structure in the detector plane and in the radial averages. Across a range of experimental conditions and molecular types, spatial autocorrelation in the detector plane is present and is well-described by a stationary kernel convolution model. The corresponding autocorrelation structure for the radial averages is non-stationary. Implications of the autocorrelation structure for inference about macromolecular structure are discussed.