Recent experimental studies of photon–atom scattering in the X-ray energy region

We present a review of our recent measurements of large angle elastic and inelastic scattering differential cross-sections in the photon energy range 14–88 keV (momentum transfer ranging 1.135 to 6.310 Å⁻¹ covering large number of elements in the atomic region Z=1–92, with special emphasis on the elements having K/Li shell/subshell binding energy in the vicinity of the incident photon energy. These measurements were performed using energy dispersive X-ray fluorescence (EDXRF) setup involving radioisotope as a photon source and a solid-state photon detector arranged in an annular geometry. The measured scattering differential cross-sections were compared with the theoretical values based on the state-of-art relativistic second order S-matrix calculations and those based on the form factor approximations in order to check their reliability. The KL and KM resonant Raman scattering (RRS) at 59.54 keV incident photon energy was also investigated for some heavy elements.     

Laboratory and synchrotron radiation total-reflection X-ray fluorescence: new perspectives in detection limits and data analysis

Having established detection limits for transition elements exceeding current requirements of the semiconductor industry, our recent efforts at the Stanford Synchrotron Radiation Laboratory (SSRL) have focused on the improvement of the detection sensitivity for light elements such as Al. Data analysis is particularly challenging for Al, due to the presence of the neighboring Si signal from the substrate. Detection limits can be significantly improved by tuning the excitation energy below the Si–K absorption edge. For conventional TXRF systems this can be done by using a W–Mα fluorescence line (1.78 keV) for excitation. At a synchrotron radiation facility energy tunability is available. However, in both cases this results in a substantial increase in background due to resonant X-ray Raman scattering. This scattering dominates the background under the Al Kα fluorescence line, and consequently limits the achievable sensitivity for the detection of Al surface contaminants. In particular, we find that for a precise determination of the achievable sensitivity, the specific shape of the continuous Raman background must be taken into account in the data analysis. The data deconvolution presented here opens a new perspective for conventional TXRF systems to mitigate this background limitation. This results in a minimum detection limit of 2.4×10⁹ atoms/cm² for Al. Based on these results it will also be demonstrated that by improving the detector resolution, the minimum detection limit can be improved significantly. For a detector resolution of 15 eV as predicted for novel superconducting tunnel junction detectors, an improvement in minimum detection limit of approximately a factor of 3 can be estimated.     

Simulation of X-ray and gamma-ray scatterings in light matrices

In the Monte Carlo simulation introduced the histories of the photons are recorded regardless of the number of scatterings. The only limiting factor is the energy threshold. The simulation is applied to XFF analysis of light matrices in the energy range of 20–60 keV. The effect of polarization is also taken into consideration. The simulation can be used for describing the spectrum background and fordetermining the enhancement of characteristic lines owing to scatterings.     

Black-body approximation to the continuous X-ray spectra

The shape of the continuous X-ray spectrum is approximated by the Planck’s distribution law for black-body radiation. The energy distribution of continuum X-rays produced by a bombardment of 30 kV electron beam onto a gold target is reproduced by the black-body radiation at a temperature of 10⁷ K. The continuous spectrum measured when the incident electron energy varies in the range 30, 20, 10, and 5 kV, and for different target elements such as gold, iron, and carbon, are again reproduced in their absolute intensity scale. Consequently, it is speculated that the Stefan–Boltzmann's law as well as the Wien's displacement law also hold for the continuum X-ray radiation.     

Near K-edge measurement of the X-ray attenuation coefficient of heavy elements using a tuneable X-ray source based on an electron LINAC

The X-ray attenuation coefficients of bismuth and of uranium were measured in the regions of 40–240 and 70–240 keV, respectively, using a tuneable hard X-ray source based on the linear electron accelerator at the University of Ghent. Results were compared with the semi-empirical values of Storm and Israel and to the theoretical values of Berger and Hubbell. We also propose a simple function for the attenuation coefficient in the vicinity of the K-edge for uranium and in an extended range of energy for bismuth. The set-up of the source at Ghent is described and the future improvements are explained.     

Analysis of low energy X-ray and excitation spectra of Ba 35+

We present a theoretical analysis of the low energy X-ray spectra of the Ba³⁵⁺ + e ^- system, in the energy region below 2 keV, for comparison with future experiments. In a recent study carried out at the National Institute of Standards and Technology, scandium-like Ba³⁵⁺ ions were created, trapped and excited using an Electron Beam Ion Trap and an electron beam with kinetic energy of 2.35 keV. The theory agrees well with experiment for X-ray energies in the region 2–5 keV. The experiment also gives evidence suggestive of a large cross section for X-ray production in the region below 2.0 keV. We include the contribution to the X-ray spectra of collisional excitation followed by fluorescence decay, radiative cascade processes, M-shell dielectronic recombination, Bremsstrahlung, and a new off-shell, quasi-resonant continuum dielectronic recombination process. We find that excitation fluorescence is the dominant process, with smaller contributions from the other radiative cascade processes as well as a continuous Bremsstrahlung background.     

Determination of rare earth elements and thorium in britholite ore by energy dispersive X-ray fluorescence spectrometry

A radioisotope-excited X-ray fluorescence technique is applied for the determination of thorium and rare earth elements in britholite ore from Canada. An annular source of⁵⁷Co is employed for excitation of characteristic K X-rays of thorium and rare earth elements. The peak ratios of lanthanides were used to remove the difficulties because of overlapping lines at the 33–50 keV energy region. Matrix effects are examined also in detail and compensated for infinitely thick powdered ore sample. Results obtained in the analysis agree well with recommended values of Canada.     

Analysis of low Z elements on Si wafer surfaces with undulator radiation induced total reflection X-ray fluorescence at the PTB beamline at BESSY II

Synchrotron radiation induced TXRF allows the nondestructive investigation of low Z contaminations on Si wafer surfaces at trace levels required by the semiconductor industry. The PTB (Physikalisch Technische Bundesanstalt) U180 undulator beamline at BESSY II, equipped with a plane grating monochromator ensuring an energy resolving power E/ΔE between 500 and 5000, can be operated either in wiggler mode for photon energies up to 1.7 keV to excite Al, Mg and Na efficiently, or in undulator mode, i.e. using one of the first odd U180 harmonics, to obtain intensive low energy radiation below 0.7 keV to excite carbon, nitrogen and oxygen. The specific feature of the beamline is its high spectral purity that allows for fundamental investigations. The TXRF wafer chamber of the Atominstitut was used for the experiments with a sidelooking Si(Li) detector with the wafer arranged vertically to take advantage of the linear polarization for background reduction. The energy dependence of the resonant Raman scattering, which is a limiter for the determination of Al at ultra trace levels excited with energies just below the Si absorption edge was studied as well as the influence of the incidence angle on the Raman peak. Droplet samples containing boron were measured and the detection limit of 3 ng determined. A single Carbon layer (5 nm) and a C–Ni–C multilayer sample on a Si wafer were characterized and it was shown that the thickness and density of these layers could be determined.     

Enhancement of electron-induced X-ray intensity for single particles under grazing-exit conditions

Grazing-exit electron probe microanalysis (GE-EPMA) was performed for single Al₂O₃ and atmospheric particles, deposited on a flat Si substrate coated by gold, by using an aperture (1 mm in diameter) in front of an energy-dispersive X-ray detector. Silicon Kα X-rays from the Si substrate were strongly observed at an exit angle of ∼45°. However, they disappeared at grazing-exit angles about 0° and only the X-rays from particles were detected. Furthermore, Al Kα and O Kα intensities from single Al₂O₃ particle were enhanced approximately three- and sixfold at the grazing-exit angles (∼1°), respectively, in comparison with those at large angle (∼7°). The background intensities at the energy of Al Kα and O Kα almost monotonously decreased with decreasing exit angle. As a result, the intensity ratios of Al Kα and O Kα X-rays to the background intensities were enhanced five- and sixfold, respectively. This enhancement is considered to be caused by the interference effect of both directly detected X-rays and reflected X-rays on the flat substrate. The similar results are also obtained for Al Kα, Si Kα, K Kα and Ca Kα emitted from single atmospheric particle. The significance of the matrix effect in the particle is also pointed out.     

Submicron hard X-ray fluorescence imaging of synthetic elements

Synchrotron-based X-ray fluorescence microscopy (XFM) using hard X-rays focused into sub-micron spots is a powerful technique for elemental quantification and mapping, as well as microspectroscopic measurements such as μ-XANES (X-ray absorption near edge structure). We have used XFM to image and simultaneously quantify the transuranic element plutonium at the L₃ or L₂-edge as well as Th and lighter biologically essential elements in individual rat pheochromocytoma (PC12) cells after exposure to the long-lived plutonium isotope ²⁴²Pu. Elemental maps demonstrate that plutonium localizes principally in the cytoplasm of the cells and avoids the cell nucleus, which is marked by the highest concentrations of phosphorus and zinc, under the conditions of our experiments. The minimum detection limit under typical acquisition conditions with an incident X-ray energy of 18 keV for an average 202 μm² cell is 1.4 fg Pu or 2.9 × 10⁻²⁰ moles Pu μm⁻², which is similar to the detection limit of K-edge XFM of transition metals at 10 keV. Copper electron microscopy grids were used to avoid interference from gold X-ray emissions, but traces of strontium present in naturally occurring calcium can still interfere with plutonium detection using its L_α X-ray emission.     

Depth profile and microscopic structure of gold-implanted aluminum using X-ray spectroscopies

Ion-implanted gold in polycrystalline aluminum was studied with energies ranging from 100 keV to 500 keV and a dose density of 1 × 10¹⁶ ions/cm². The centroid depths of the implantation profiles were determined by Angle-Resolved Self-Ratio X-ray Fluorescence Spectrometry using synchrotron radiation of the electron storage ring ELSA at the University of Bonn. A linear correlation between the implantation energy and the centroid depth of the profile was found. Comparing these results to TRIM calculations a range enhancement of the experimental data up to 10% is observed. X-ray Absorption Near Edge Structure (XANES) spectroscopy was used at the Au-L_III edge to investigate surface modification due to the implantation process on a microscopic scale. For all energies the implantation leads to nanocrystalline precipitates of the intermetallic compound AuAl₂.     

Hf, Ta, W and Re experimental L X-ray fluorescence cross-sections at 12, 13 and 14 keV with synchrotron radiation

Experimental determinations of L X-ray fluorescence cross-sections for elements with 72≤Z≤75 at 12, 13 and 14 keV were carried out with synchrotron radiation. The experimental set-up provided a linearly polarized monoenergetic photon beam producing a very low background, which improved the signal-to-noise ratio and reduced the experimental uncertainties. Results of the experimental cross-sections obtained for the L lines were grouped considering the transitions scheme, the energies of the emission lines and the detector's resolution. The data obtained were compared with theoretical ones using two different data tables. A very good agreement was found between our experimental values and the theoretical values calculated using Puri's data (X-Ray Spectrom. 22 (1993) 358–361).     

Information depth determination for hard X‐ray photoelectron spectroscopy up to 15 keV photoelectron kinetic energy

In the present work, we have determined the information depth in a solid for hard X‐ray photoelectron spectroscopy (HAXPES) up to a photoelectron kinetic energy of 15 keV. For that, we have followed the evolution of the photoemission signal from different core levels of a gold overlayer grown in situ on a polycrystalline copper substrate as a function of the photoelectron kinetic energy. We demonstrate that in the case of gold, an information depth of 57 nm can be achieved by detecting photoelectrons with 15‐keV kinetic energy. The photoemission signal produced at this depth corresponds to 0.2% of the signal coming from a semi‐infinite solid bulk. Such a high sensitivity can only be reached with the combination of a third‐generation synchrotron radiation beam with a high‐transmission electron analyzer. Copyright © 2008 John Wiley & Sons, Ltd.     

L X-ray production cross sections, average L shell fluorescence yield and intensity ratios in heavy elements

L X-ray fluorescence cross sections, and intensity ratios were measured for elements in the 70￡Z￡92 atomic range at the excitation energy 59.5 keV using a Si(Li) detector. Furthermore, L X-ray fluorescence cross sections and intensity ratios were calculated for elements in the same range. The average L shell fluorescence yields were derived using experimental L X-ray fluorescence cross sections and theoretical photoionization cross sections. The obtained results were compared with other experimental and theoretical values. This revised version was published online in August 2006 with corrections to the Cover Date.     

Improvement of total reflection X-ray fluorescence analysis of low Z elements on silicon wafer surfaces at the PTB monochromator beamline for undulator radiation at the electron storage ring BESSY II

Several different total reflection X-ray fluorescence (TXRF) experiments were conducted at the plane grating monochromator beamline for undulator radiation of the Physikalisch-Technische Bundesanstalt (PTB) at the electron storage ring BESSY II, which provides photon energies between 0.1 and 1.9 keV for specimen excitation. The lower limits of detection of TXRF analysis were investigated for some low Z elements such as C, N, O, Al, Mg and Na in two different detection geometries for various excitation modes. Compared to ordinary XRF geometries involving large incident angles, the background contributions in TXRF are drastically reduced by the total reflection of the incident beam at the polished surface of a flat specimen carrier such as a silicon wafer. For the sake of an application-oriented TXRF approach, droplet samples on Si wafer surfaces were prepared by Wacker Siltronic and investigated in the TXRF irradiation chamber of the Atominstitut and the ultra-high vacuum TXRF irradiation chamber of the PTB. In the latter, thin C layer depositions on Si wafers were also studied.     

Quantitative X-Ray Microanalysis of Heterogeneous Materials Using Monte Carlo Simulations

Monte Carlo simulations are used to obtain new results of x-ray microanalysis of sample types frequently encountered in practical analytical situations such as a vertical layer embedded in a homogeneous matrix and a spherical particulate deposited on a substrate. The simulations show that a 10-nm layer of boron in a steel matrix can be imaged using backscattered electrons and detected using x-ray microanalysis with a field emission scanning electron microscope even with an electron beam energy equals to 20 keV and also that these simulations can be useful to estimate the optimum acceleration voltage to perform such analyses. For a carbon spherical particulate located on the top of a gold substrate, it is shown that x-ray emission and electron backscattering are a strong function of the diameter of the particulate and also of the electron beam energy. Finally, a new method to determine the thickness of a thin film deposited on a substrate is proposed that does not require the measurement of the beam current. That technique can also be used for a spherical particulate deposited on a substrate.     

Studies on the effective atomic numbers of some human tissues in the energy region 15–100 keV

The effective atomic numbers for total photon interaction in muscle, bone, brain, heart, kidney, liver, lungs, ovaries, pancreas, spleen and tongue are evaluated using three different methods, for practical use in the energy region 15–100 keV. Muscle, brain, heart, kidney, lungs, ovaries, pancreas, spleen, tongue and water; bone and silicon; liver and oxygen are found to behave in an approximately similar manner in this energy region.     

Photon-induced M-shell X-ray production cross-sections and fluorescence yields in heavy elements at 5.96 keV

Total M-shell X-ray production cross-sections (MXRP) for selected heavy elements between Yb and U at have been measured at 5.96 keV incident photon energy using a Si(Li) detector. The average M-shell fluorescence yields have been derived, using the experimental total M X-ray production cross-sections and theoretical M-shell photoionization cross-sections. Extracted cross-sections and fluorescence yields have been compared with the literature experimental values, theoretical predictions and semiempirical fits.     

The application of long wavelength radiation in X-ray analysis

The authors used long wavelength X-ray radiation excited by low energy electrons for analytical purposes. Low energy electron excitation was achieved with an open window tube. The authors used a type that was developed in Philips laboratories. The electron source was a glow discharge from which the electrons ( 15 keV) are extracted and directed towards the anode. A photon spectrum of “Bremsstrahlung” and characteristic peaks is generated at a gold anode. In addition about half of the impinging electrons is reflected and may, thanks to the open window, be used to irradiate a specimen in spite of a partial loss of energy due to the collisions.The authors mention the following attractive characteristics: the tube is simple to operate and may easily be exchanged for a closed tube; no high vacuum is needed, a feature which, however, at the same time inhibits the determination of traces of O, N, C; about 7 cm² of the specimen, which may be an insulator, is irradiated. A discussion of applications illustrates the usefulness of the tube.