[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

Was scanned with 120 angular steps of 3 and 100

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was scanned with 120 angular steps of 3 and 100 translation steps for each angle. Fitting and background subtraction for the X-ray characteristic lines as well as for the Compton and Rayleigh signal were performed using the Axil program. All distributions were reconstructed on 100 × 100 square grids of 3 µ m pixel size. The distribution of µ ( E 0 ) was reconstructed from transmission tomogra- phy using the filtered back-projection algorithm and is presented in Fig. 7.33a. The distributions of p Compt and p Ray have been reconstructed from the Comp- ton and Rayleigh signals using the SART algorithm, modified for absorption correction (Fig. 7.33b and c). As expected from the Compton, Rayleigh and photo-absorption cross- sections, respectively proportional to Z , Z 2 and Z 4 , the relative contrast between the pure silica capillary and the ferrous silicate grain increases from the Compton to the transmission reconstructions. Iron and manganese contributed significantly to the fluorescent signal and their reconstructions with/without self-absorption corrections are presented in Fig. 7.34a and 7.34b (Fe) and 12a and b(Mn). Self-absorption effects are clearly visible in the image without self- absorption corrections, as an underestimation of the gray-level inside the sample. The average densities of iron and manganese, calculated from the distributions shown in Fig. 7.34b and Fig. 7.35b are, respectively ρ Fe = 1 . 45 ± 0 . 07 g cm 3 and 0 . 058 ± 0 . 017 g cm 3 . The average total density, evalu- ated from the distributions of Fe, Mn and unmeasured fluorescences is 3 . 89 ± 0 . 3 g cm 3 while the corresponding weight fractions of iron ρ Fe tot respec- tively manganese ρ Mn tot are approximately 37 . 3 ± 3 wt% and 1 . 5 ± 0 . 5 wt%. All these results were compared to an EPMA study [82] of thin sections of the same meteorite and discrepancies between the Fe estimations and the average
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Methodological Developments and Applications 497 (a) (b) Fig. 7.34. Fe reconstructions (a) without self-absorption, (b) with self-absorption corrections bulk chemical composition of the NWA817 olivine grains appear on the order of 1 wt%. For Mn, the discrepancies are of about 0.5 wt% and they appear largely due to the Mn/Fe overlap, already discussed in Fig. 7.29. Improvements on the collection statistics, achievable by using a multiele- ment solid state detector, and by reducing the acquisition electronics overhead time and alignment time through automatic alignment procedures are direct ways of significantly improving our accuracy. To conclude, a new technique has been developed that combines the in- formation from transmission, fluorescence and scattering tomography. The studied reconstruction methods applied to submillimeter silicate grains are efficient to determine in the volume of the sample, the average densities and the weight fractions of the major and detectable elements with precisions better than 10 wt%. Analysis of the minor elements however, remains semi- quantitative.
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