Incident X-ray Energy Research Articles

Soft X-ray spectroscopy with transition-edge sensors at Stanford Synchrotron Radiation Lightsource beamline 10-1.

We present results obtained with a new soft X-ray spectrometer based on transition-edge sensors (TESs) composed of Mo/Cu bilayers coupled to bismuth absorbers. This spectrometer simultaneously provides excellent energy resolution, high detection efficiency, and broadband spectral coverage. The new spectrometer is optimized for incident X-ray energies below 2 keV. Each pixel serves as both a highly sensitive calorimeter and an X-ray absorber with near unity quantum efficiency. We have commissioned this 240-pixel TES spectrometer at the Stanford Synchrotron Radiation Lightsource beamline 10-1 (BL 10-1) and used it to probe the local electronic structure of sample materials with unprecedented sensitivity in the soft X-ray regime. As mounted, the TES spectrometer has a maximum detection solid angle of 2 × 10-3 sr. The energy resolution of all pixels combined is 1.5 eV full width at half maximum at 500 eV. We describe the performance of the TES spectrometer in terms of its energy resolution and count-rate capability and demonstrate its utility as a high throughput detector for synchrotron-based X-ray spectroscopy. Results from initial X-ray emission spectroscopy and resonant inelastic X-ray scattering experiments obtained with the spectrometer are presented.

Experimental setup combining in situ hard X-ray photoelectron spectroscopy and real-time surface X-ray diffraction for characterizing atomic and electronic structure evolution during complex oxide heterostructure growth.

We describe the next-generation system for in situ characterization of a complex oxide thin film and heterostructure growth by pulsed laser deposition (PLD) using synchrotron hard X-rays. The system consists of a PLD chamber mounted on a diffractometer allowing both real-time surface X-ray diffraction (SXRD) and in situ hard X-ray photoelectron spectroscopy (HAXPES). HAXPES is performed in the incident X-ray energy range from 4 to 12 keV using a Scienta EW4000 electron energy analyzer mounted on the PLD chamber fixed parallel with the surface normal. In addition to the standard application mode of HAXPES for disentangling surface from bulk properties, the increased penetration depth of high energy photoelectrons is used for investigation of the electronic structure changes through thin films grown deliberately as variable thickness capping layers. Such heterostructures represent model systems for investigating a variety of critical thickness and dead layer phenomena observed at complex oxide interfaces. In this new mode of operation, in situ HAXPES is used to determine the electronic structure associated with unique structural features identified by real-time SXRD during thin film growth. The system is configured for using both laboratory excitation sources off-line and on-line operation at beamline 33-ID-D at the Advanced Photon Source. We illustrate the performance of the system by preliminary scattering and spectroscopic data on oxygen vacancy ordering induced perovskite-to-brownmillerite reversible phase transformation in La2/3Sr1/3MnO3 films capped with oxygen deficient SrTiO3-δ (100) layers of varying thickness.

The potential benefits of using higher X-ray energies for macromolecular crystallography

Using X-ray energies higher than those normally used (5-15 keV) for macromolecular X-ray crystallography (MX) at synchrotron sources can theoretically increase the achievable signal as a function of dose and reduce the rate of radiation damage. In practice, a major stumbling block to the use of higher X-ray energy has been the reduced quantum efficiency of silicon detectors as the X-ray energy increases, but hybrid photon-counting CdTe detectors are optimized for higher X-ray energies, and their performance has been steadily improving. Here the potential advantages of using higher incident beam energy together with a CdTe detector for MX are explored, with a particular focus on the advantages that higher beam energies may have for MX experiments with microbeams or microcrystals. Monte Carlo simulations are presented here which for the first time include the efficiency responses of some available X-ray detectors, as well as the possible escape of photoelectrons from the sample and their entry from surrounding material. The results reveal a `sweet spot' at an incident X-ray energy of 26 keV, and show a greater than factor of two improvement in diffraction efficiency at this energy when using microbeams and microcrystals of 5 µm or less.

Observing pre-edgeK-shell resonances in Kr, Xe, andXeF2

The $1s\phantom{\rule{4pt}{0ex}}\ensuremath{\rightarrow}\phantom{\rule{4pt}{0ex}}np$ Rydberg transitions below the $1s$ ionization thresholds of Kr and Xe are obscured in x-ray absorption spectra due to core-hole lifetime broadening. However, the $np$ spectator electrons associated with those resonances can affect the core-hole decay spectra. We report on ion charge-state distributions of Kr and Xe measured in coincidence with ${KL}_{2,3}$ ($K{\ensuremath{\alpha}}_{1,2}$), ${KM}_{2,3}$ ($K{\ensuremath{\beta}}_{1,3}$), and ${\mathrm{KN}}_{2,3}$ ($K{\ensuremath{\beta}}_{2}$) x-ray fluorescence as the incident x-ray energy is scanned through pre-edge resonances and ionization thresholds. The coincidence measurements select vacancy cascades that begin with a radiative transition that transfers $1s$ holes to the $2p$, $3p$, and $4p$ shells followed by emission of Auger electrons. We observe shifts of ion yields from higher to lower charge states that we attribute to $np$ spectator electrons. For the special case of ${\mathrm{Kr}}^{1+}$ that is produced in coincidence with $K{\ensuremath{\beta}}_{2}$ x rays, the ion yield decreases in the pre-edge region. This is attributed to production of neutral, metastable Kr $4{p}^{\ensuremath{-}1}np$ states that reduce the ${\mathrm{Kr}}^{1+}$ yield. Model fits to the x-ray absorption spectra are presented to show the lifetime broadened pre-edge resonances and ionization thresholds. High-level relativistic coupled-cluster calculations that treat relativistic, electron correlation, and wave function relaxation effects on the same footing obtain agreement with the experimental $1s$ ionization energies of Kr and Xe to $<2$ eV. The Xe $K$-edge x-ray absorption spectrum and ion charge-state distributions of ${\mathrm{XeF}}_{2}$ were also recorded. Excitation of the $7{\ensuremath{\sigma}}_{u}$ lowest unoccupied molecular orbital (LUMO) in ${\mathrm{XeF}}_{2}$ is observed in the pre-edge region. Our ab initio calculations find that the $6{p}_{z}$ Rydberg state is strongly perturbed by the presence of the $7{\ensuremath{\sigma}}_{u}$ LUMO. A fit to the measured LUMO, $6{p}_{x}$, $6{p}_{y}$, $6{p}_{z}$ Rydberg states, and ionization threshold is guided by the relativistic coupled-cluster calculations. The F ligands modify the valence electron charge distribution and result in a $\ensuremath{\sim}2.3$ eV chemical shift of the Xe $1s$ ionization energy relative to atomic Xe. Xe $1{s}^{\ensuremath{-}1}$ core-hole decay in ${\mathrm{XeF}}_{2}$ results in ionization of the F ligands and energetic fragmentation into atomic ions. Ion charge-state spectra of ${\mathrm{XeF}}_{2}$ were recorded in coincidence with x-ray fluorescence for excitation on the LUMO resonance and above the Xe $1s$ ionization threshold. For the ion time of flight spectra recorded on the LUMO resonance, the ${\mathrm{F}}^{q+}$ ($q=1$--4) peaks are split into two peaks along the linear polarization direction of the incident x-ray beam. This effect is attributed to spatial alignment of ${\mathrm{XeF}}_{2}$ molecules by resonant x-ray absorption, and the peak splittings are used to measure the F ion fragmentation energies following $K{\ensuremath{\alpha}}_{1,2}$, $K{\ensuremath{\beta}}_{1,3}$, and $K{\ensuremath{\beta}}_{2}$ x-ray fluorescence. We observe variations of the F ion charge state yields and fragmentation energies for the three fluorescence pathways that leave the molecule in different outer-shell hole states.

Extended Line Spread Function of TES Microcalorimeters With Au/Bi Absorbers

Microcalorimeters have the potential to provide line shapes well described by a single Gaussian broadening term of few-eV width. This attribute makes the detectors especially well suited for x-ray astrophysics observations; however, low-level non-Gaussian broadening terms are expected and must be characterized. These terms depend on the composition of the x-ray absorber, the detailed x-ray absorption physics, the device thermalization processes, and the incident x-ray energy. Here we present the first measurements targeted at understanding the extended line-spread function (LSF) of x-ray microcalorimeter pixels under development for the X-ray Integral Field Unit on the Athena X-ray Observatory. These pixels are composed of Mo/Au transition-edge sensors with overhanging electroplated Au/Bi absorbers. We have measured the line shapes using monochromatic x-ray sources with <; 1-eV width at several x-ray energies (0.85, 0.93, 1.25, 1.5, 5.4 and 8.0 keV) across the instrument bandpass (0.3-12 keV) and have modeled the line profiles. These results are compared to the extended LSF of the Hitomi microcalorimeter pixels that used HgTe absorbers.

The potential benefits of using higher X-ray energies for macromolecular crystallography

Using X-ray energies higher than those normally used (5-15 keV) for macromolecular X-ray crystallography (MX) at synchrotron sources can theoretically increase the achievable signal as a function of dose and reduce the rate of radiation damage. In practice, a major stumbling block to the use of higher X-ray energy has been the reduced quantum efficiency of silicon detectors as the X-ray energy increases, but hybrid photon-counting CdTe detectors are optimized for higher X-ray energies, and their performance has been steadily improving. Here the potential advantages of using higher incident beam energy together with a CdTe detector for MX are explored, with a particular focus on the advantages that higher beam energies may have for MX experiments with microbeams or microcrystals. Monte Carlo simulations are presented here which for the first time include the efficiency responses of some available X-ray detectors, as well as the possible escape of photoelectrons from the sample and their entry from surrounding material. The results reveal a `sweet spot' at an incident X-ray energy of 26 keV, and show a greater than factor of two improvement in diffraction efficiency at this energy when using microbeams and microcrystals of 5 µm or less.

Hard X-ray spectroscopic methods using emitted X-ray to understand charge compensation in positive electrode materials for lithium-ion batteries

We examined the applicabilities of hard X-ray spectroscopic methods to understand the charge-compensation (CC) behaviors of Ni in a LiNiO2-based positive electrode (PE) material for lithium-ion batteries: Ni K pre-edge high energy resolution fluorescence detection X-ray absorption spectroscopy (XAS) and Ni 1s2p resonant X-ray emission spectroscopy (RXES). Both methods exhibited sequential changes in spectral features with the change in the state of charge of the PE material, similarly to Ni L2,3-edge XAS. This elucidates that these hard X-ray spectroscopic methods can become alternatives to soft X-ray L2,3-edge XAS with the additional ability to measure reactive samples kept in a sealed pouch. Moreover, Ni 1s2p RXES can separate the electronic transition to the unoccupied Ni orbital from those to hybridized orbitals with neighboring atoms by tuning the incident X-ray energy. In this sense, it can provide spectral information reflecting the unoccupied Ni 3d state, which could be a good oxidation-state descriptor. These methods can be done in a usual hard X-ray beamline having an appropriate crystal analyzer. The ability to offer information on the unoccupied 3d state with hard X-ray and the ubiquitous feature of these methods would prompt in situ/operando studies on CC in PE materials for rational materials development.

Cross-section determination for one- and two-photon absorption of cobalt at hard-x-ray energies

For incident x-ray energies below the core level ionization threshold, one-photon absorption (OPA) leads to off-resonant x-ray excitations that are mediated by a virtual intermediate electronic state. In the linear regime of x-ray interaction, this intermediate state may decay radiatively and the maximum energy of the emitted photon is given by the initial and final atomic state as well as the energy of the incident photon. In the nonlinear excitation regime, however, this intermediate state may be depleted by a second x-ray absorption process as described in the sequential two-photon absorption (TPA) mechanism. Since the same intermediate state is involved in the OPA and TPA processes, the cross-section values for OPA and TPA are interconnected. We report on the determination of cross-section values for off-resonant excitation conditions, and show how the data can be used for estimating the TPA cross section in the sequential absorption mechanism. The determined off-resonant cross sections for cobalt exhibit a Lorentz-like energy-dependent behavior allowing for an energy-dependent description of the OPA and TPA cross sections with semiempirical models. The dependence of TPA cross sections on atomic number is discussed and different scaling laws are investigated. The effect of the lifetimes of a virtual intermediate state is also studied in relation to the incidence x-ray energy.

(Invited) X-Ray Microscopy of Operating Electrochemical Energy Storage Systems

Hard X-ray transmission X-ray microscopy (TXM) is an ideal tool for in situ and operando studies of functional materials and materials synthesis routes. The high energy X-rays provides relatively relaxed restrictions on in situ environments enabling high resolution 2D microscopy and tomography (3D microscopy)1 across a large range of pressures and temperatures and in varying gas or liquid environments. The full field geometry of TXM allows imaging at the sub-second time scale, allowing relevant dynamics to be captured during; for example, battery cycling,2,3 catalysis reactions,4 electrochemical synthesis,5 and corrosion.6 Moreover, by tuning the incident X-ray energy to specific absorption edges, TXM can capture elemental and chemical (spectro-microscopy) changes at 30 nm resolution within a few minutes (Figure). Li-ion batteries promise the high specific capacity required to replace the internal combustion engine with a number of possible earth abundant electrode materials; however, setbacks such as capacity fading hinder the full capability of these rechargeable batteries. In the search for better electrode materials, high resolution X-ray microscopy during typical battery operation is vital in understand and overcoming the failure mechanisms of these materials. I will discuss the use of X-ray microscopy including spectro-microscopy and nano-tomography to track electrochemical and morphological changes in the electrode material in real time during typical battery operation. N. Weker, N. Liu, S. Misra, J. C. Andrews, Y. Cui and M. F. Toney, Energy Environ. Sci. 2014, 7, 2771-2777.Nelson Weker, Y. Li, R. Shanmugam, W. Lai and W. C. Chueh, ChemElectroChem 2015, 2, 1576-1581.Nelson Weker, A. M. Wise, K. Lim, B. Shyam and M. F. Toney, Electrochim. Acta 2017, 247, 977-982.D. Gonzalez-Jimenez, K. Cats, T. Davidian, M. Ruitenbeek, F. Meirer, Y. Liu, J. Nelson, J. C. Andrews, P. Pianetta, F. M. F. de Groot and B. M. Weckhuysen, Angew. Chem. Int. Ed. 2012, 51, 11986-11990.E. R. Tay, A. E. Goode, J. Nelson Weker, A. A. Cruickshank, S. Heutz, A. E. Porter, M. P. Ryan and M. F. Toney, Nanoscale 2016, 8, 1849-1853.A. Koronfel, A. E. Goode, J. Nelson Weker, S. E. R. Tay, C. A. Stitt, T. A. Simoes, J. F. W. Mosselmans, P. Quinn, R. Brydson, A. Hart, Michael F. Toney, A. E. Porter and M. P. Ryan accepted to npj Mater. Degrad., 2018, DOI: 10.1038/s41529-018-0029-2. Figure 1

Energy-modulated x-ray fluorescence and luminescence emissions from therapeutic nanoparticles

In this study, we have investigated the possibility of modulating x-ray fluorescence (XF) and x-ray luminescence (XL) emissions from therapeutic nanoparticles (NPs) by fine-tuning the energy of incident x-rays from benchtop x-ray sources. We have carried out detailed experimental studies to determine the strength of XF and XL emissions from Y2O3:Eu3+ and LaF3:Tb3+ NPs being irradiated with x-rays from benchtop x-ray sources operated with different tube-voltages and coupled to various filter configurations. These studies demonstrated that low-energy x-rays with average energy at around 10–15 keV are the most efficient to stimulate XL emission from the Y2O3:Eu3+ and LaF3:Tb3+ NPs. The efficiency falls quickly when x-ray energies go above or below the optimum energy range. As one would expect, x-rays with average energy just above the corresponding absorption edge of the target metal would be the most efficient in inducing XF emission. In this study, we have also demonstrated that one could fine-tune the incident x-ray energy to modulate the XL and XF emissions, such as (a) selectively inducing either XL or XF emission from the same type of NPs, (b) inducing preferential XL activation of Y2O3:Eu3+ over LaF3:Eu3+ or controlling the ratio of XL activation of these two types of NPs, and (c) introducing preferential XF emission from one type of NPs over the other. As a potential application, one could optimize the energy-characteristics of the incident x-rays to facilitate multiplexed combinatorial delivery of photodynamic therapy (X-PDT), where different agents could be administrated and then selectively activated in user-defined spatial and temporal patterns to fulfill combinatorial therapeutic effects. The understanding gained through this study could prove critical for enhancing the therapeutic delivery in X-PDT, and for attaining high-quality x-ray fluorescence computed tomography (XFCT) and x-ray luminescence computed tomography (XLCT) images while minimizing the x-ray dose to the sample.

Simulated performances of pixelated CsI(Tl) scintillation screens with different micro-column shapes and array structures in X-ray imaging

The performances of the pixelated CsI(Tl) scintillation screens based on oxidized silicon micro-pore array templates with different CsI(Tl) micro-column shapes and array structures in X-ray imaging were simulated using the Geant4 Monte Carlo simulation code. The shapes of the micro-columns include square, hexagonal and circular, and the array structures include square and hexagonal arrangements. The pitch size of the pixelated CsI(Tl) scintillation screens was set to 4 µm, and the incident X-ray energy was set to 20 keV. The ratios of the number of scintillation photons that propagate along the CsI(Tl) micro-columns to the total number of scintillation photons of the micro-columns gradually decrease with the increase in total reflection time on the lateral surfaces of the micro-columns. However, these ratios are closely related to the shapes of the micro-columns and the incident positions of X-ray on the cross-sections of the micro-columns, especially for the circular micro-column. The sequence of bottom light outputs stimulated by a uniform flood field of X-ray from high to low corresponds to the circular, square and hexagonal CsI(Tl) micro-columns with the same cross-section areas. In addition, all spatial resolutions in terms of modulation transfer functions (MTFs) for the pixelated CsI(Tl) scintillation screens with square and hexagonal array structures are over 100 lp/mm. However, the resolution for the pixelated screen with the hexagonal array structure is approximately 8.5% higher than that for the screen with the square array structure. Moreover, the former screen has a higher detective quantum efficiency (DQE) than the latter screen at the same thickness. The pixelated CsI(Tl) scintillation screen with circular micro-column and hexagonal array structure in X-ray imaging has superior performance compared to other pixelated screens in this work.

A new synchrotron rapid-scanning X-ray fluorescence (SRS-XRF) imaging station at SSRL beamline 6-2.

This paper describes a new large-range rapid-scan X-ray fluorescence (XRF) imaging station at beamline 6-2 at the Stanford Synchrotron Radiation Lightsource at SLAC National Accelerator Laboratory. This station uses a continuous rapid-scan system with a scan range of 1000 × 600 mm and a load capacity of up to 25 kg, capable of 25-100 µm resolution elemental XRF mapping and X-ray absorption spectroscopy (XAS) of a wide range of objects. XRF is measured using a four-element Hitachi Vortex ME4 silicon drift detector coupled to a Quantum Detectors Xspress3 multi-channel analyzer system. A custom system allows the X-ray spot size to be changed quickly and easily via pinholes ranging from 25 to 100 µm, and the use of a poly-capillary or axially symmetric achromatic optic may achieve a <10 µm resolution in the future. The instrument is located at wiggler beamline 6-2 which has an energy range of 2.1-17 keV, creating K emission for elements up to strontium, and L or M emission for all other elements. XAS can also be performed at selected sample positions within the same experiment, allowing for a more detailed chemical characterization of the elements of interest. Furthermore, sparse excitation energy XRF imaging can be performed over a wide range of incident X-ray energies. User friendliness has been emphasized in all stages of the experiment, including versatile sample mounts, He purged chambers for low-Z analyses, and intuitive visualization hardware and software. The station provides analysis capabilities for a wide range of materials and research fields including biological, chemical, environmental and materials science, paleontology, geology and cultural heritage.

Mechanism of Li3PS4 Liquid Phase Synthesis By Total X-Ray Scattering Measurement Coupled to Pair Distribution Function Analysis

Introduction All-solid-state batteries are one of the most promising candidates for next generation batteries because of their safety property and high power density1. The sulfur solid electrolyte is prepared conventionally by hand-mixing in the all-solid-state batteries. The ball-milling method is not suitable for large-scale preparation. Therefore it is important to develop the large-scale synthesis method of solid electrolyte. Recently, some research groups have reported on the synthesis of Li3PS4 electrolyte via liquid phase by dissolving the starting sulfide materials such as Li2S and P2S5 in organic solvents2. However, the formation mechanism has not been clearly understood. In this study, we evaluate a local structure change of Li3PS4 (LPS) in the liquid phase synthesis process by total X-ray scattering measurement coupled with pair distribution function (PDF) analysis to clarify the formation mechanism. Experimental Li3PS4 was prepared by liquid-phase shaking method as previously reported2. Li2S (Sigma-Aldrich), P2S5(Sigma-Aldrich), dehydrated ethyl propionate (Sigma-Aldrich) and zirconia balls were added in centrifugation tube and shaken under Ar atmosphere from 0 min to 360 min at 1500 rpm. The tube was centrifuged at 10000 rpm for 5 min. The supernatant was decanted and the residual solvent was evacuated at room temperature. Then the precursor was annealed at 170 ºC in low pressure for 2 h. The high-energy X-ray diffraction experiments were carried out at the beamline BL08W using a two-dimensional detector in SPring-8, Japan. The incident X-ray energy was 115 keV. The diffraction patterns of the samples and an empty tube were measured in the transmission geometry. The obtained data of each sample was corrected for background, absorption correction, multiple scattering and inelastic scattering. Results and discussion Figure 1 shows that reduced pair distribution functions, G(r), obtained from a Fourier transformation of total structure factors for each liquid phase sample. In the initial stage of the synthesis process, peaks were attributed to the starting materials of P2S5 and Li2S. In the middle stage of the synthesis process, although the intensity of the peak from Li2S decrease, intensity of the first peak from P-S bond of PS4 tetrahedral anion increased. In the end stage of the synthesis process, two peaks, which were observed in Li3PS4 obtained by subsequent annealing treatment, appeared around 3.3 Å, 4.0 Å. These results indicate that the locally ordered structure of Li2S is broken while the locally ordered structure at short range is grown in the middle stage of the synthesis process. In the end stage of the synthesis process, locally ordered structure at middle range of Li3PS4 forms with keeping locally ordered structure at the short range. Acknowledgement This research was financially supported by the Japan Science and Technology Agency (JST), Advanced Low Carbon Technology Research and Development Program (ALCA), Specially Promoted Research for Innovative Next Generation Batteries (SPRING) Project. Figure 1

Polarized XRF Spectrometer with a 40-mW X-Ray Tube

A three-dimensional polarized X-ray fluorescence (XRF) spectrometer was assembled with a low-power X-ray tube of 40 milliwatts, and a sample of stainless steel was measured with an acrylic polarizer. Based on the Schrodinger’s wave picture of Compton scattering, the dependence of the de Broglie wavelength and the scattering angle of a recoil electron on the incident X-ray energy were calculated. When the incident X-rays were scattered at an angle of 90°, the de Broglie of the recoil electron always went away at an angle of 45° with the incident X-rays and the de Broglie wavelength varied for any wavelength of the incident X-rays, i.e. we could select the scattering angle of the recoil electron to be 45°according to the incident X-ray energy and it could then be regarded as 45° Bragg diffraction that could produce highly polarized X-rays. The polarization measurement of scattered X-rays from an acrylic and lead plates substantiated that Compton scattered X-rays from the acrylic plate had higher degree of polarization than the elastically scattered X-rays from the lead plate. Highly polarized Compton scattered X-rays from the light-element polarizer led to the background reduction and the changes of the characteristic X-ray intensity in XRF spectra measured by our laboratory-made spectrometer. Key words: polarized X-ray fluorescence spectrometer, stainless steel, acrylic polarizer, Compton scattering, background. DOI: http://dx.doi.org/10.15826/analitika.2018.22.2.010

Measured X-ray Spectra and Absorbed Dose Conversion Factors in a Water Equivalent Phantom Irradiated with Diagnostic X-rays

The diagnostic X-ray spectra in a water equivalent phantom have been measured. From these measured spectra, the absorbed dose conversion factors of water were derived. The primary X-ray spectra were also measured and the scattered X-ray spectra were calculated by subtraction. The measurements were made at the depths of 0, 5, 10, 15, and 20 cm in a 20 cm-thick phantom and at the X-ray tube voltages 60, 90, and 120 kV by using a small silicon diode detector. The radiation field size was 30×30 cm2 at the phantom surface. In the obtained spectra, the fraction of the scattered photon number is increased with the depth. The X-ray qualities of the spectra in the phantom were near the qualities of primary X-rays when the depth is 0 cm, and became near the qualities of scattered X-rays as the depth increases. The changes of the X-ray qualities due to the depth change were small; photon mean-energy changes were within 4.6 keV. The changes in the absorbed dose conversion factors were also small (within 0.68%). These conversion factors were 0.4-2.3% larger than those obtained from the effective energy of incident X-rays and only -0.3 to 0.5% larger than those obtained from the X-ray spectra calculated from the aluminum half value layer and the tube voltage of incident X-rays. This study shows experimentally that the absorbed dose in a water-like phantom can be calculated with good accuracy by using the absorbed dose conversion factor obtained from the incident X-rays.

Simultaneous X-ray diffraction, crystallography and fluorescence mapping using the Maia detector

Interactions between neighboring grains influence the macroscale behavior of polycrystalline materials, particularly their deformation behavior, damage initiation and propagation mechanisms. However, mapping all of the critical material properties normally requires that several independent measurements are performed. Here we report the first grain mapping of a polycrystalline foil using a pixelated energy-dispersive X-ray area detector, simultaneously measuring X-ray fluorescence and diffraction with the Maia detector in order to determine grain orientation and estimate lattice strain. These results demonstrate the potential of the next generation of X-ray area detectors for materials characterization. By scanning the incident X-ray energy we investigate these detectors as a complete solution for simultaneously mapping the crystallographic and chemical properties of the sample. The extension of these techniques to broadband X-ray sources is also discussed.

Detecting element specific electrons from a single cobalt nanocluster with synchrotron x-ray scanning tunneling microscopy

We use a nanofabricated scanning tunneling microscope tip as a detector to investigate local X-ray induced tunneling and electron emission from a single cobalt nanocluster on a Au(111) surface. The tip-detector is positioned a few angstroms above the nanocluster, and ramping the incident X-ray energy across the Co photoabsorption K-edge enables the detection of element specific electrons. Atomic-scale spatial dependent changes in the X-ray absorption cross section are directly measured by taking the X-ray induced current as a function of X-ray energy. From the measured sample and tip currents, element specific X-ray induced current components can be separated and thereby the corresponding yields for the X-ray induced processes of the single cobalt nanocluster can be determined. The detection of element specific synchrotron X-ray induced electrons of a single nanocluster opens an avenue for materials characterization on a one particle at-a-time basis.

Determination of observable depth of dislocations in 4H-SiC by X-ray topography in back reflection

We performed X-ray topography using the asymmetric back-reflection by changing the incident X-ray energy (E) in order to evaluate the observable depth (tobs) of dislocations in 4H-SiC(0001) wafers with a 4° off-axis angle toward the []-direction. Since the incident angle (ω) of the X-rays satisfying the Bragg condition changed along with E, the observable length of the straight segment of the basal plane dislocation (BPD) on the X-ray topographs also changed. The tobs values were determined from the observable length of the straight BPD segment, and consequently, plots of tobs versus E, and tobs versus ω were obtained. We discussed the relationships between tobs and the absorption depth (tμ), between tobs and the extinction depth (tΛ), and between tobs and tΛ/tμ as criteria of the dislocation visibility, and we found that the plot of tobs versus tμ was described as tobs ∼ 0.75tμ with good linearity.

A method to stabilize the incident X-ray energy for anomalous diffraction measurements.

A method to calibrate and stabilize the incident X-ray energy for anomalous diffraction data collection is provided and has been successfully used at the single-crystal diffraction beamline 1W2B at the Beijing Synchrotron Radiation Facilities. Employing a feedback loop to control the movement of the double-crystal monochromator, this new method enables the incident X-ray energy to be kept within a 0.2 eV range at the inflection point of the absorption edge.

Operando Spectromicroscopy of Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are a “beyond Li-ion” technology that provides high capacity (1675 mAh g-1) while also being low cost, earth-abundant, and lightweight. Li-S batteries have a unique chemistry that achieves high capacity via chemical transformation rather than Li intercalation as in Li-ion. Elemental sulfur S8 is reduced through a series of soluble Li polysulfides (Li2Sx, 2 ≤ x ≤ 8) to a final solid discharge product Li2S, and the process is reversed upon charging. However, Li-S suffers from unrealized theoretical capacity and rapid capacity fade due to loss processes that are not well-understood. Batteries are spatially and chemically heterogeneous. Deciphering the chemical states of sulfur and the distribution of dissolved polysulfides in the anode, cathode, and especially the electrolyte during cycling is critical for realizing the potential of Li-S. We present operando characterization using sulfur K-edge X-ray absorption spectromicroscopy performed at the Stanford Synchrotron Radiation Lightsource (SSRL, BL 14-3). Synchrotron radiation provides the high energy resolution and micron-scale beam size needed to distinguish complicated polysulfide spectral features in the tender X-ray regime. From this information, we can observe a spatially and chemically varied system to gain insight into how sulfur speciation and spatial distribution contribute to capacity fade. Typically, operando X-ray characterization is conducted perpendicular to battery electrodes, providing an average of the chemistry throughout the entire stack. The cross-sectional pouch cell geometry presented here provides a spatially resolved picture of sulfur diffusion between the cathode and anode, which cannot be achieved by typical characterization methods. Cells consisting of a sulfur/carbon cathode, Li anode, and 1 M LiClO4 + 0.5 LiNO3 in 1,3-dioxolane/1,2-dimethoxyethane (DOL/DME) were discharged, and maps were collected at energies sensitive to the sulfur and Li polysulfide species. Tuning the incident X-ray energy allows for targeted observations of species such as polysulfides, sulfates, elemental sulfur, and lithium sulfide during cycling. A novel electrochemical cell with well-defined electrode geometry, electrolyte volume, and current collection has been designed at SSRL to map the battery cross-section in fluorescence mode. This cell will mitigate the challenges of using pouch cells for X-ray experiments such as water and oxygen contamination and sample-to-sample variation. It will also provide more robust, longer-term cycling that better replicates the offline electrochemistry of a typical coin or pouch cell. Because the new cell will increase reproducibility, we can alter electrode architecture and electrolyte composition to directly compare polysulfide and sulfur distributions, illuminating the direct influence that these parameters have on battery chemistry. Using this new design, changes in sulfur and polysulfide chemistry are systematically correlated to structure and performance.