Héctor Jorge Sánchez

Synchrotron radiation X-Ray fluorescence at the LNLS: beamline instrumentation and experiments

The x-ray fluorescence heamline of the Laboratorio Nacional de Luz Sincrotron (LNLS) is described. The main optical component of the beamline is a silicon (111) channel-cut monochromator, which can tune energies between 3 and 14 keV. A general description of two experimental stations is given. Beam characterization was done by measuring experimental parameters such as vertical profile and monochromatic flux. These results show that the photon flux at 8 keV in an area of 20 mm 2 is 4.2 × 10 9 photons s -1 . The possibility of achieving fine tuning of energies (high resolution) was confirmed. This paper presents some original results derived from the commissioning of the beamline, such as a comparison of detection limits in different experimental conditions, and a novel mechanical system to align capillaries together with a semi-automatic adjustment procedure. So far, there have been several proposals to perform a variety of experiments at this beamline, covering different fields, such as physics, chemistry, geology and biology.

X-ray resonant Raman scattering cross sections of Mn, Fe, Cu and Zn

X-ray fluorescence spectra present singular characteristics produced by the different scattering processes. When atoms are irradiated with incident energy lower and close to an absorption edge, scattering peaks appear due to an inelastic process known as resonant Raman scattering. It constitutes an important contribution to the background of the fluorescent line. The resonant Raman scattering must be taken into account in the determination of low concentration contaminants, especially when the elements have proximate atomic numbers. The values of the mass attenuation coefficients experimentally obtained when materials are analysed with monochromatic x-ray beams under resonant conditions differ from the theoretical values (between 5% and 10%). This difference is due, in part, to the resonant Raman scattering. Monochromatic synchrotron radiation was used to study the Raman effect on pure samples of Mn, Fe, Cu and Zn. Energy scans were carried out in different ranges of energy near the absorption edge of the target element. As the Raman peak has a non-symmetric shape, theoretical models for the differential cross section, convoluted with the instrument function, were used to determine the RRS cross section as a function of the incident energy.

Microscopic X-ray fluorescence analysis of human dental calculus using synchrotron radiation

In this work, new results of one- and two-dimensional spatial distributions of major, minor and trace elements of human dental calculus are presented. X-ray fluorescence induced by a synchrotron radiation source was used. The measurements were carried out in the X-ray microprobe station mounted at the X-ray fluorescence beamline of the Brazilian National Synchrotron Light Laboratory (LNLS). Two-dimensional distributions show that calcium, the major element in this type of sample, has a uniform pattern over the mapped area, while trace elements are non-homogeneously distributed. By means of different linear scans along the growing axis of the dental calculus, a markedly spatial correlation between Cu and Zn was found. A semi-quantitative analysis for Cu and Zn was performed by means of calibration curves using appropriate fabricated standards. The concentration values found for these metals were higher than those obtained in conventional bulk analysis. It was observed that these metals accumulated preferentially in the subgingival part of the calculus, near the tooth surface. These results indicate that the mineralization processes, which take place for both type of calculus, are essentially different.

Anisotropic angular distribution of Er L x-rays following photoionization by linearly polarized radiation

The angular distribution of the Er L emission after photoionization by linearly polarized monochromatic synchrotron radiation has been measured. A dedicated instrumentation, designed for this kind of experiment, allowed observation of even the smallest anisotropy of the line. The experimental anisotropy parameter and degree of alignment parameter show very good agreement with the predicted theoretical values. It has been found that the ion alignment with the incident beam direction is a function of the polarization of the incident beam. Evidence of non-dipole contributions was observed.

SRXRF analysis with spatial resolution of dental calculus

This work presents elemental-composition studies of dental calculus by X-ray fluorescence analysis using synchrotron radiation. The intrinsic characteristics of synchrotron light allow for a semi-quantitative analysis with spatial resolution. The experiments were carried out in the high-vacuum station of the XRF beamline at the Synchrotron Light National Laboratory (Campinas, Brazil). All the measurements were performed in conventional geometryO45∞a 45∞U and the micro-collimation was attained via a pair of orthogonal slits mounted in the beamline. In this way, pixels of 50 lm 50 lm were obtained keeping a high flux of photons on the sample. Samples of human dental calculus were measured in diAerent positions along their growing axis, in order to determine variations of the compositions in the pattern of deposit. Intensity ratios of minor elements and traces were obtained, and linear profiles and surface distributions were determined. As a general summary, we can conclude that lXRF experiments with spatial resolution on dental calculus are feasible with simple collimation and adequate positioning systems, keeping a high flux of photon. These results open interesting perspectives for the future station of the line, devoted to lXRF, which will reach resolutions of the order of 10 lm. ” 2000 Elsevier Science B.V. All rights reserved.

Erbium L-subshell Coster-Kronig and fluorescence yields determination by synchrotron radiation photoionization

The method of selective photoionization of individual subshells by monochromatized synchrotron radiation has been employed to measure independently all Coster-Kronig and fluorescence yields of Er L subshells. Fitting the measured fluorescence intensities versus primary energy with the photoelectric cross sections derived the Coster-Kronig coefficients and fluorescence yields. The obtained yields were compared with tabulated data and a very good agreement among them was found.

Total reflection X-ray fluorescence analysis using plate beam-guides

Abstract This work presents a new device for total reflection X-ray fluorescence analysis based on plate beam-guides. This device allows the photons transport inside a beam-guide maintaining the total reflection regime. Due to the basic characteristics of the experimental setup the system is easily aligned and can be mounted in conventional spectrometers without difficulty; this represents an important advantage with respect to standard equipment for total reflection analysis. Experimental spectra were measured using a conventional X-ray source and a reference sample. Detection limits were obtained and these values were compared with those obtained by means of conventional X-ray fluorescence analysis for the same sample and the same irradiation system.

Determination of the oxidation state by resonant-Raman scattering spectroscopy

X-Ray fluorescence spectra present singular characteristics produced by the different scattering processes. When atoms are irradiated with incident energy lower and close to an absorption edge, scattering peaks appear due to an inelastic process known as resonant Raman scattering (RRS). In this process, the emitted photons have a continuous energy distribution with a high energy cut-off limit. This work presents results regarding the possibility of determining the oxidation state by resonant Raman scattering using an energy dispersive system. Pure samples of transition metals (Cu, Fe, Mn) and different oxides of them (CuO, Cu2O, Fe2O3, Mn2O3, MnO2) were irradiated with monochromatic synchrotron radiation below their absorption edges to inspect the RRS emissions. The spectra were analyzed with specific programs using non-conventional functions for data fitting and a FFT smoothing procedure was applied. After smoothing, the RRS residuals are studied in order to detect variation with respect to the theoretical curve. These variations are closely related with the chemical environments of the absorbing element and can provide relevant structural information of the sample. The changes existing in the RRS structure between pure elements and their oxides are clearly discriminated and suggest the possibility of structural characterization by means of resonant Raman scattering using an energy-dispersive system combined with synchrotron radiation.

Theoretical calculations of detection limits in total reflection XRF analysis

Theoretical expressions for fluorescent intensity and scattered radiation in total reflection experiments are presented. Trace elements in samples with different matrices were studied. The contributions of the different components were analyzed and a comparison with conventional x-ray fluorescence was performed. Detection limits were calculated as a function of the sample thickness in both experimental situations. Very interesting conclusions were obtained regarding the contribution of the reflected beam, the components of the background and the behavior of detection levels.

New spectrometer for grazing exit x-ray fluorescence

Among the x-ray fluorescence techniques using energy dispersive systems, the excitation in the total reflection regime is becoming widespread. This technique may be applied to surface analysis if the fluorescent radiation emitted by the sample is measured as a function of the grazing incidence angle. The main limitation in performing this sort of analysis is the need to use a monochromator, which notably reduces the incident flux of radiation. Besides, the method requires the radiated surface to be perfectly smooth and polished and to have a total length of some centimeters. A similar technique known as grazing exit x-ray fluorescence presents certain advantages and is easier to implement. By this method, fluorescent photons in terms of the grazing exit angle are detected. In this work, a spectrometer for performing surface analysis at grazing exit is described, the different experimental situations are analyzed, and some measurements performed with this device are shown.