Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..326b&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 326
Other
Interplanetary Dust Particles, Pixe, Proton Microprobe
Scientific paper
Excitation of characteristic X-rays in a sample by 2-4 MeV protons (Proton Induced X-ray Emission, PIXE) enables the non- destructive detection of elements with Z >= 12 with high sensitivity [1]. In conjunction with a focused proton beam provided by the proton microprobe such measurements are possible with a spatial resolution in the micrometer range [2]. In the last two years a new proton microprobe was developed and constructed in Heidelberg. We will report on some of its properties and present the first high resolution measurements of Interplanetary Dust Particles (IDPs). The critical elements of a proton microprobe are the object collimator and the demagnifying magnetic lens. Our collimator is composed of highly polished, water-cooled crossed slits. The lens is a magnetic quadrupole doublet, manufactured very precisely to avoid parasitic lens errors. To minimize necessary adjustments, the two quadrupoles are connected rigidly with each other, which makes it necessary, however, to deposit the whole lens system inside the vacuum tube. An electrostatic beam deflection system allows the generation of two dimensional secondary electron images from the sample. Before the first PIXE measurements were carried out, the optical characteristics of the new microprobe were measured. A special beam diagnosis program was developed, which allows an easy determination of the beam shape both in X and Y directions. The measurements are in very good agreement with ray tracing calculations. The most important result is the beam focus available, which is less 1 x 1 micrometer^2. In PIXE spectra of IDPs nearly all elements between Mg and Br are detectable. This leads to interferences between X-ray lines of neighboring elements. A correct quantitative analysis of such spectra is possible only if the response function of the used X- ray Si(Li) detector is known [3]. We developed a special absorber method for the determination of the exact line shapes of our Si(Li)- detector, leading to much better fits than those achieved with the assumption of simple gaussian lines. To calculate the elemental abundances one has to know the matrix of the sample as well as its shape. For IDPs it is incorrect to assume a homogeneous plane sample, a better approximation is a homogeneous sphere. An appropriate program for the calculation of the yield of proton induced X-rays in spheres of arbitrary composition was developed. First PIXE measurements with the new microprobe were carried out at three chondritic IDPs. Beside bulk analyses, which show the well-known [4,5] almost C1-abundances of the detected elements between Mg and Br, line scans with a resolution of better than 2 micrometers were performed. In Fig. 1 the results for a line scan taken over the mid-plane of the spherical shaped particle L2005 B11 (diameter 10 micrometers) are depicted. The normalized K- alpha-line contents of 16 elements (filled circles) are shown together with the theoretical results (dotted lines) for a homogeneous distribution. While some elements like Mg, Al, Si, Mn, or Fe are distributed homogeneously, others like Ca, Cu, Co, or Ni exhibit irregular distributions. It is interesting, that the latter elements also show the strongest deviations from C1 abundances in the bulk analyses. References: [1] Johansson S. A. E. and Johansson T. B. (1976) Nucl. Instr. Meth., 137, 473-516. [2] Traxel K. (1990) Nucl. Instr. Meth., B50, 177-188. [3] Campbell J. L. et al. (1987) X- ray Spectrom., 16, 195-201. [4] Flynn G. J. and Sutton S. R. (1991) Meteoritics, 26, 334. [5] Jessberger E. K. et al. (1992) EPSL, 112, 91-99. Fig. 1, which appears here in the hard copy, shows a line scan of IDP L2005 B11.
Bohsung Joerg
Jessberger Elmar K.
Traxel Kurt
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