Advances in the measurements of the mass attenuation coefficients

The knowledge of atomic fundamental parameters, such as mass attenuation coefficients or fluorescence yields with low uncertainties, is of decisive importance in elemental quantification involving X-ray fluorescence analysis techniques. Several databases providing the mass attenuation coefficients are accessible and frequently used within a large community of users. These compilations are most often in good agreement for photon energies in the hard X-ray ranges. However, they significantly differ for low photon energies and around the absorption edges of the elements. Mass attenuation coefficients of several elements were determined experimentally in the photon energy range from 100 eV to 35 keV by using monochromatized radiation at the SOLEIL synchrotron (France). The application of high-accuracy experimental techniques resulted in low uncertainty mass attenuation coefficients. The results are compared with tabulated data.     

Measurement of the X‐ray mass attenuation coefficients of silver in the 5–20 keV range

The X‐ray mass attenuation coefficients of silver were measured in the energy range 5–20 keV with an accuracy of 0.01–0.2% on a relative scale down to 5.3 keV, and of 0.09–1.22% on an absolute scale to 5.0 keV. This analysis confirms that with careful choice of foil thickness and careful correction for systematics, especially including harmonic contents at lower energies, the X‐ray attenuation of high‐Z elements can be measured with high accuracy even at low X‐ray energies (<6 keV). This is the first high‐accuracy measurement of X‐ray mass attenuation coefficients of silver in the low energy range, indicating the possibility of obtaining high‐accuracy X‐ray absorption fine structure down to the L₁ edge (3.8 keV) of silver. Comparison of results reported here with an earlier data set optimized for higher energies confirms accuracy to within one standard error of each data set collected and analysed using the principles of the X‐ray extended‐range technique (XERT). Comparison with theory shows a slow divergence towards lower energies in this region away from absorption edges. The methodology developed can be used for the XAFS analysis of compounds and solutions to investigate structural features, bonding and coordination chemistry.     

Influence of different mass absorption coefficient datasets on PIXE yields

Tabulated mass absorption coefficient data from the XCOM, Chantler and ion beam analysis (IBA) Handbook are compared, and differences are shown against X‐ray energy and target atomic number. Over the X‐ray energy range 1–40 keV systematic differences of several tens of percent are noted between these datasets, particularly for the earlier IBA Handbook dataset. The influence of these different mass absorption coefficients on the X‐ray yields for thick target proton‐induced X‐ray emission (PIXE) are investigated and compared as a function of X‐ray energy and target atomic number. For contemporary experimental PIXE users trying to work quantitatively over a broad range of elements and X‐ray energies, differences between PIXE results obtained using these three separate mass attenuation coefficient datasets can be larger than the typical accuracy limits of ±3% to ±5%. There are systematic differences in the mass attenuation coefficients of 5–10% between the XCOM and Chantler, while the differences for the IBA Handbook dataset can be larger (up to 40% and greater) at high X‐ray energies. At this time, we recommend the dataset of Chantler as it is more recent, and the synchrotron experimental results seem to favour it over the older XCOM and IBA Handbook data. Copyright © 2013 John Wiley & Sons, Ltd.     

Improved Rayleigh to Compton scattering ratio curves for mass attenuation coefficients determination for X‐ray fluorescence analysis

Nondestructive assays are essentials for certain types of sample materials, and, among those, the X‐ray fluorescence technique enables the determinations of stable elements, and there is an increasing effort on the development of equipment to suit the various needs. Nevertheless, a great difficulty on the analysis of unknown materials' composition is to account for self‐absorption of the fluorescence photons that must be considered in the elemental concentration calculation. The correlation between the Rayleigh to Compton scattering ratio to the mass attenuation coefficient has proved to follow a single polynomial function for the first 20 elements of the periodical table with a correlation factor higher than of 0.998 for the sixth order function. The Rayleigh to Compton scattering ratios for pure elements and the 22.16 keV photons, the main energy from an X‐ray tube with silver anode, were determined with the MCNP6 Monte Carlo computer code. Two scattering angles were considered. Reference samples were measured, and the calculated results were compared to the literature values of the mass attenuation coefficient for some known samples and showed to be within 20% for de 90° scattering angle. Only Lucite was slightly above 20%. Curve fit coefficients are also presented for the 7.11‐ and 17.40‐keV photon energies.     

Discrepancies in atomic shell and fluorescent X‐ray energies in the Evaluated Photon Data Library EPDL97

There are significant differences between the atomic orbital energies listed in the evaluated photon data library EPDL97 and values published elsewhere. In particular, comparisons with the values adopted by the National Institute of Standards and Technology (NIST) show discrepancies up to several hundred electron volts. Although the uncertainties in the EPDL97 atomic orbital energies were recognised by the original authors, the library has subsequently been widely adopted as a primary source of photon transport and atomic relaxation data. We compare experimentally measured X‐ray fluorescence spectra with fits using the EPDL97 and NIST line energies. Our results strongly favour the NIST energies for K‐shell and L‐shell fluorescent X‐rays and show that the EPDL97 atomic orbital energy values should not be used for applications, such as X‐ray fluorescence, where atomic relaxation phenomena are important. Copyright © 2012 John Wiley & Sons, Ltd.     

Optimization of L‐shell X‐ray fluorescence detection of lead in bone phantoms using synchrotron radiation

Closely related toxicity and retention mechanisms of lead (Pb) in the human body involve the bone tissues where Pb can accumulate and reside on a time scale ranging from years to tens of years. In vivo measurements of bone Pb can, therefore, play an important role in a comprehensive health risk assessment of Pb exposure. In vivo L‐shell X‐ray fluorescence (LXRF) measurement of bone Pb was first demonstrated over 4 decades ago. Implementation of the method, however, encountered challenges associated with low sensitivity and calibration procedure. In this study, the LXRF measurement was optimized by varying the incident photon energy and the excitation‐detection geometry. The Canadian Light Source synchrotron radiation was used to compare 2 different excitation‐detection geometries of 90^° and 135^° using 3 different X‐ray photon energies: 15.8, 16.6, and 17.5 keV. These energies optimized excitation of the L₃ subshell of Pb and simulated the most intense K‐shell emissions of zirconium, niobium, and molybdenum, respectively. Five rectangular plaster‐of‐Paris bone phantoms with Pb concentrations of 0, 7, 17, 26, and 34 μg/g, and one rectangular 3.1‐mm‐thick resin phantom mimicked the X‐ray attenuation properties of human bone and soft tissue, respectively. Optimal LXRF detection was obtained by the 15.8‐keV energy and the 90^° and 135^° geometries for the bare bone and the bone and soft tissue phantoms, respectively.     

Characterization of the Metrology beamline at the SOLEIL synchrotron and application to the determination of mass attenuation coefficients of Ag and Sn in the range 3.5 ≤ E ≤ 28 keV

This work presents the new Metrology beamline at the SOLEIL synchrotron facility and a first attempt to quantitative measurements of mass attenuation coefficients for Ag and Sn performed on the hard X‐ray branch. We first describe the beamline itself and the characterization performed of the unfocused monochromatic beam running mode. We performed a first experimental measurement of mass attenuation coefficients in the range 3.5 ≤ E ≤ 28 keV and we also derived the K‐absorption and L‐absorption jump ratios. The results are compared with theoretical values as well as with other experimental data and agree well with previous published values. Copyright © 2011 John Wiley & Sons, Ltd.     

Synchrotron‐based spectroscopy of X‐ray channeling through hollow capillary microchannels inside glass plates

Here, soft X‐ray synchrotron radiation transmitted through microchannel plates is studied experimentally. Fine structures of reflection and XANES Si L‐edge spectra detected on the exit of silicon glass microcapillary structures under conditions of total X‐ray reflection are presented and analyzed. The phenomenon of the interaction of channeling radiation with unoccupied electronic states and propagation of X‐ray fluorescence excited in the microchannels is revealed. Investigations of the interaction of monochromatic radiation with the inner‐shell capillary surface and propagation of fluorescence radiation through hollow glass capillary waveguides contribute to the development of novel X‐ray focusing devices in the future.     

Measuring partial fluorescence yield using filtered detectors

Typically, X‐ray absorption near‐edge structure measurements aim to probe the linear attenuation coefficient. These measurements are often carried out using partial fluorescence yield techniques that rely on detectors having photon energy discrimination improving the sensitivity and the signal‐to‐background ratio of the measured spectra. However, measuring the partial fluorescence yield in the soft X‐ray regime with reasonable efficiency requires solid‐state detectors, which have limitations due to the inherent dead‐time while measuring. Alternatively, many of the available detectors that are not energy dispersive do not suffer from photon count rate limitations. A filter placed in front of one of these detectors will make the energy‐dependent efficiency non‐linear, thereby changing the responsivity of the detector. It is shown that using an array of filtered X‐ray detectors is a viable method for measuring soft X‐ray partial fluorescence yield spectra without dead‐time. The feasibility of this technique is further demonstrated using α‐Fe₂O₃ as an example and it is shown that this detector technology could vastly improve the photon collection efficiency at synchrotrons and that these detectors will allow experiments to be completed with a much lower photon flux reducing X‐ray‐induced damage.     

Choice of X‐ray mass attenuation coefficients for PIXE analysis of silicate minerals and rocks

The accuracy of each of three mass attenuation coefficient databases was assessed using particle‐induced X‐ray emission measurements performed on well‐characterized, homogeneous silicate glass and mineral standards. In a fundamental parameters computation, the absolute efficiency constants of the light elements Mg, Al and Si, found within each standard, were determined. These were compared with the efficiency constants deduced from separate particle‐induced X‐ray emission measurements performed on pure targets of the same three elements. In this comparison, a 7–9% discrepancy was found when using the XCOM database for the computation, but this was reduced to 2–5% when FFAST coefficients were substituted. Further improvement was achieved when a hybrid database was adopted. This ‘Mixed’ database consisted of primarily XCOM coefficients with FFAST values inserted for the light element (Z = 11,…, 14) attenuation of photons having energy less than their K edge and for oxygen in the 1–2 keV range. The average efficiency constant discrepancies were reduced to ?0.5–2%. Copyright © 2015 John Wiley & Sons, Ltd.     

Role of the mass attenuation coefficient database in standardization of a silicon drift X‐ray detector for PIXE analysis

A simple, inexpensive procedure is described for calibrating a silicon drift detector‐based PIXE system for the analysis of geological and other “thick‐target” materials. It rests on the use of single element, chemical compound, and National Institute for Standards and Technology multielement standards. Much less effort has been focussed on the impact of mass attenuation coefficients on PIXE's analytical accuracy than on ionization cross sections and fluorescence yields. The calibrated system enables us to investigate the effects of inserting different mass attenuation coefficient datasets into the GUPIX database. For the K X‐rays of light elements, accuracy is significantly improved by replacing the formerly used XCOM self‐attenuation coefficients by the corresponding FFAST values; this effect decreases rapidly with increasing atomic number. Similar improvement was found for L X‐rays of medium‐Z, but not for high‐Z atoms.     

Upgrade of beamline BL08B at Taiwan Light Source from a photon‐BPM to a double‐grating SGM beamline

During the last 20 years, beamline BL08B has been upgraded step by step from a photon beam‐position monitor (BPM) to a testing beamline and a single‐grating beamline that enables experiments to record X‐ray photo‐emission spectra (XPS) and X‐ray absorption spectra (XAS) for research in solar physics, organic semiconductor materials and spinel oxides, with soft X‐ray photon energies in the range 300–1000 eV. Demands for photon energy to extend to the extreme ultraviolet region for applications in nano‐fabrication and topological thin films are increasing. The basic spherical‐grating monochromator beamline was again upgraded by adding a second grating that delivers photons of energy from 80 to 420 eV. Four end‐stations were designed for experiments with XPS, XAS, interstellar photoprocess systems (IPS) and extreme‐ultraviolet lithography (EUVL) in the scheduled beam time. The data from these experiments show a large count rate in core levels probed and excellent statistics on background normalization in the L‐edge adsorption spectrum.     

Cascade Mi (i = 1–5) subshell X‐ray emission at incident photon energies across the Lj (j = 1–3) subshell absorption edges of 66Dy

The total M shell and the M_k (k = ξ, αβ, γ, m) X‐ray production cross sections for ₆₆Dy have been measured at incident photon energies across its L_j (j = 1–3) subshell absorption edge energies, ranging 7.8–9.2 keV. This study aims to investigate the evolution of the probability for cascade decay of L_j subshell vacancies as the tunable incident energy ionizes progressively different ₆₆Dy L_j subshells. The experimental X‐ray production cross sections have been compared with theoretical ones calculated using the nonrelativistic Hartree–Fock–Slater (HFS) model‐based photoionization cross sections; three sets of the X‐ray emission rates, fluorescence and Coster–Kronig yield based on the nonrelativistic Hartree–Slater (NRHS) model, Dirac–Hartree–Slater (DHS) model and Dirac–Fock (DF) model; the L_j (j = 1–3) subshell to the M_i (i = 1–5) subshell vacancy transfer probabilities evaluated in the present work. Presently measured total M shell and the M_αβ X‐ray production cross sections are found to be significantly lower than the theoretical ones evaluated using physical parameters based on the relativistic Dirac–Fock/Dirac–Hartree–Slater model calculations, whereas a much better agreement is observed with respect to the NRHS model‐based calculations; however, the measured X‐ray production cross sections are still systematically lower than the NRHS values.     

Si PIN X‐ray photon counter

A Si PIN detector for visible light detection, instead of a Geiger‐Müller tube, is applied to X‐ray photon counting. We counted radiation from a checking source of a Geiger‐Müller counter with a Si PIN counter and with a Geiger‐Müller counter. White X‐ray of energy up to 20 keV emitted from a pyroelectric X‐ray emitter was also counted, and the Si PIN X‐ray counter showed a similar curve of count rate versus source distance in both measurements. Pulse counting was performed by spectroscopy circuits. An audio digitizer with computer software for signal processing was also used to simplify the photon counter. A plot of count rate versus time was obtained with this setup. With simple pulse counting circuits, Si PIN X‐ray counters have advantages such as compact structure, low cost and easy application. Copyright © 2011 John Wiley & Sons, Ltd.     

A scanning transmission X‐ray microscope at the Pohang Light Source

A scanning transmission X‐ray microscope is operational at the 10A beamline at the Pohang Light Source. The 10A beamline provides soft X‐rays in the photon energy range 100–2000 eV using an elliptically polarized undulator. The practically usable photon energy range of the scanning transmission X‐ray microscopy (STXM) setup is from ～150 to ～1600 eV. With a zone plate of 25 nm outermost zone width, the diffraction‐limited space resolution, ～30 nm, is achieved in the photon energy range up to ～850 eV. In transmission mode for thin samples, STXM provides the element, chemical state and magnetic moment specific distributions, based on absorption spectroscopy. A soft X‐ray fluorescence measurement setup has been implemented in order to provide the elemental distribution of thicker samples as well as chemical state information with a space resolution of ～50 nm. A ptychography setup has been implemented in order to improve the space resolution down to 10 nm. Hardware setups and application activities of the STXM are presented.     

Measurement of relative line intensities for L‐shell X‐rays from selected elements between Z = 68 (Er) and Z = 79 (Au)

Relative line intensities of L1, L2 and L3 sub‐shell X‐rays were measured for Er, Tm, Yb, Hf, Ta, W, Pt and Au. The L‐shell X‐ray spectra were recorded by exciting pure element samples (eight cases) and oxide samples (two cases) with approximately 17‐keV exciting radiation from a filtered X‐ray tube source, and measuring the fluorescence spectra with a silicon drift detector. The spectra were carefully fitted to determine line energies and intensities, accounting for Lorentzian line broadening, incomplete charge collection and escape effects. A Monte Carlo approach was used to calculate attenuation and detector efficiency corrections. We report up to 15 line intensity ratios for each element and compare these to Scofield's theoretical predictions and Elam's extrapolated experimental database. Our measured relative line intensities agree best with Elam's data, but overall we find significant discrepancies with previously reported results. For the element Ta, we also find significant errors in the accepted L‐shell line energies in the widely used National Institute of Standard and Technology (NIST) database. Our results highlight the need for an experimental and theoretical re‐evaluation of L‐shell intensity databases to support high‐accuracy X‐ray analysis methods such as X‐ray fluorescence and particle‐induced X‐ray emission. Copyright © 2016 John Wiley & Sons, Ltd.     

Mass attenuation coefficients of composite materials by Geant4, XCOM and experimental data: comparative study

The main goal of this present study is focused on testing the applicability of Geant4 electromagnetic models for studying mass attenuations coefficients for different types of composite materials at 59.5, 80, 356, 661.6, 1173.2 and 1332.5 keV photon energies. The simulated results of mass attenuation coefficients were compared with the experimental and theoretical XCOM data for the same samples and a good agreement has been observed. The results indicate that this process can be followed to determine the data on the attenuation of gamma rays with the several energies in different materials. The modeling for photon interaction parameters was standard for any type of composite samples. The Geant4 code can be utilized for gamma ray attenuation coefficients for the sample at different energies, which may sometimes be impractical by experiment investigation.     

Absolute determination of the X‐ray absorption coefficient of barium in the L region using a liquid absorption cell

A method of absolute measurement of X‐ray absorption coefficient is proposed, on the basis of the use of an adjustable pathlength liquid cell and a solution of the investigated element. A variational analysis of the measured data is developed to remove some sources of systematic error. A check of the exponential attenuation with thickness is used to suppress beam harmonics admixture and dark current drifts. The method is illustrated by a measurement on Ba in the L edge region, using aqueous solution of Ba nitrate. Measured data provide information on detailed energy dependence of the absorption beyond the Victoreen‐like trends reproduced in compilations. The proposed method provides a way to measure the absorption data for practically all elements and a wide range of photon energies. Copyright © 2012 John Wiley & Sons, Ltd.     

Performance of a fuel cell optimized for in situ X‐ray absorption experiments

A commercial fuel cell has been successfully modified to carry out X‐ray absorption spectroscopy (XAS) measurements under optimized in operando conditions. The design is conceived for the performance of XAS experiments in transmission mode over a wide range of X‐ray energies above 6 keV, owing to the reduced absorption of the cell. The wide angular aperture allows the collection of XAS in fluorescence mode and of X‐ray diffraction patterns when needed. Details of the design of the cell and its performances are given. The quality of the extended X‐ray absorption fine‐structure spectra under working conditions has been verified at the ESRF and ELETTRA synchrotron radiation facilities, showing that relatively fast and low‐noise transmission measurements on electrodes over a wide range of catalyst concentrations and energies are feasible.     

A method to determine the absolute harmonic content of an X‐ray beam using attenuation measurements

We present a new method for determining the absolute harmonic content of an X‐ray beam. The technique is applied to determine the harmonic content of a synchrotron beam to high‐accuracy by measuring the X‐ray attenuation of a large number of aluminium foils with thicknesses varying over several orders of magnitude. Earlier methods always determined relative quantities such as the effective harmonic content, which are dependant on experimental geometry and not transferable between detectors. We use a more fundamental and useful parameter: the harmonic‐photon percentage. Copyright © 2009 John Wiley & Sons, Ltd.