Silvina P. Limandri

Effects of the carbon coating and the surface oxide layer in electron probe microanalysis.

Effects related with the attenuation and deflection suffered by an electron beam when it passes through a carbon conductive coating and an oxide film layer on the surface of bulk samples are studied by Monte Carlo simulations and energy dispersive spectroscopy with electron excitation. Analytical expressions are provided for the primary beam energy and intensity losses and for the deflection of the incident electrons in both layers, in terms of the incidence energy, the film mass thicknesses, and the atomic number of the oxidized element. From these analytical expressions, suitable corrections are proposed for the models used to describe the X-ray spectrum of the substrate, including also the contribution of the X-rays generated in the oxide and conductive films and the characteristic X-ray absorption occurring in those layers. The corrections are implemented in a software program for spectral analysis based on a routine of parameter refinement, and their influence is studied separately in experimental spectra of single-element standards measured at different excitation energies. Estimates for the layer thicknesses are also obtained from the spectral fitting procedure.

Experimental Method to Determine the Absolute Efficiency Curve of a Wavelength Dispersive Spectrometer

A method for the experimental determination of the absolute efficiency of wavelength dispersive spectrometers was developed, based on the comparison of spectra measured with a wavelength dispersive system and with an energy dispersive spectrometer. The aim of studying this parameter arises because its knowledge is necessary to perform standardless analysis. A simple analytical expression was obtained for the efficiency curve for three crystals ~TAP, PET, and LiF! of the spectrometer used, within an energy range from 0.77 to 10.83 keV. Although this expression is particular for the system used in this work, the method may be extended to other spectrometers and crystals for electron probe microanalysis and X-ray fluorescence.

EBSD spatial resolution for detecting sigma phase in steels

The spatial resolution of the electron backscatter diffraction signal is explored by Monte Carlo simulation for the sigma phase in steel at a typical instrumental set-up. In order to estimate the active volume corresponding to the diffracted electrons, the fraction of the backscattered electrons contributing to the diffraction signal was inferred by extrapolating the Kikuchi pattern contrast measured by other authors, as a function of the diffracted electron energy. In the resulting estimation, the contribution of the intrinsic incident beam size and the software capability to deconvolve patterns were included. A strong influence of the beam size on the lateral resolution was observed, resulting in 20nm for the aperture considered. For longitudinal and depth directions the resolutions obtained were 75nm and 16nm, respectively. The reliability of this last result is discussed in terms of the survey of the last large-angle deflection undergone by the backscattered electrons involved in the diffraction process. Bearing in mind the mean transversal resolution found, it was possible to detect small area grains of sigma phase by EBSD measurements, for a stabilized austenitic AISI 347 stainless steel under heat treatments, simulating post welding (40h at 600°C) and aging (284h at 484°C) effects-as usually occurring in nuclear reactor pressure vessels.

3D scanning electron microscopy applied to surface characterization of fluorosed dental enamel

The enamel surfaces of fluorotic teeth were studied by scanning electron stereomicroscopy. Different whitening treatments were applied to 25 pieces to remove stains caused by fluorosis and their surfaces were characterized by stereomicroscopy in order to obtain functional and amplitude parameters. The topographic features resulting for each treatment were determined through these parameters. The results obtained show that the 3D reconstruction achieved from the SEM stereo pairs is a valuable potential alternative for the surface characterization of this kind of samples.

Standardless semi-quantitative analysis by PIXE

A method based on the refinement of atomic and experimental parameters developed for the description of PIXE spectra is presented and applied to standardless semi-quantitative PIXE analysis. This method was implemented in the sofware PAMPA (Parameter Assessment Method for PIXE Analysis) and consists in minimizing the quadratic differences between an experimental spectrum and an analytical function proposed to describe it. The first results of PAMPA are presented for the quantification of synthetic and mineral, thin and bulk samples, and they are compared with the results obtained with a commercial software.