Structural and chemical modifications of microsamples induced during PIXE analysis

Nuclear Instruments and Methods in Physics Research B 109/110 (1996) 192-196

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Beam Interactions with Materials & Atoms

ELSEVIER

Structural and chemical modifications of microsamples induced during PIXE analysis a*

a

Mischa Maetz " , Peter Arndt a, Ansgar Greshake b, Elmar K. J e s s b e r g e r , Wolfgang K16ck b, Kurt Traxel ° "Max-Planck-lnstitat f iir Kernphysik, Heidelberg, Germany blnstitut fiir Planetologie, Universitiit Miinster, Miinster, Germany ~Physikalisches lnstitut der Universitiit Heidelberg, Heidelberg, Germany

Abstract Mass losses caused by proton bombardment in the course of PIXE analyses have been frequently reported in the case of biological samples but only very little information on the damage of rock targets is available. With the Heidelberg Proton Microprobe we scanned seven ~ 100 txm fragments of a sheet silicate using various proton currents and doses that - after the PIXE analysis - were analysed with transmission electron microscopy (TEM). The effects of the proton bombardment range from intact lattice to a totally amorphous structure. Our results demonstrate the necessity to limit the proton current and dose to below certain values that have to be experimentally determined for a given sheet silicte in order to maintain intactness. Time resolved PIXE measurements of the alkali (Na, K) contents of feldspar and alkali glass fragments do not result in volatilisation losses which often are observed in SEM and EMPA studies.

1. Introduction With a proton microprobe characteristic X-rays are excited by the interaction of protons with target atoms. At the same time, however, the protons may alter the composition and structure of the sample. This has often been observed and is discussed in the context of mass loss or heating of biological targets during PIXE analysis [1]. On the other hand, for rock targets there is only sparse information on such alterations during PIXE analysis. However, loss of volatile elements during electron microprobe analyses is a well known phenomenon. Especially the alkali signals under routine conditions are not stable (Fig. 1). Element loss by heating through protons in cases can be n e g l e c t e d - if the considered elements are refractory and e.g. the sample is large and rapidly dissipating the energy. But more volatile elements may be lost to a considerable degree in the case of single grains in the size range 5-500 p~m mounted on e.g. thin foils [2]. The effects on the structure can also often be ignored if the analysis is aimed exclusively towards the chemical composition of the sample. Sometimes, however, the PIXE measurement is followed by mineralogical analyses using e.g. TEM. Then it must be guaranteed that the proton bombardment does * Corresponding author. Address: MPI fiir Kernphysik, P.O. Box 103980, D-69029 Heidelberg, Germany. Tel. +49 6221 516210, fax +49 6221 516540, e-mail: [email protected].

not unduly alter the original crystal structure. Thus we set out to study the effects of the proton bombardment - with varying currents and d o s e s - during PIXE analysis on the composition and structure of small (~100txm sized) individual fragments of selected samples. The samples were (a) fragments of the layer lattice silicate saponite to study structural effects and (b) fragments of natural feldspar and of synthetic alkali glass to study possible chemical effects. The size of the fragments was shown to be comparable to that of micrometeorites and interplanetary dust particles which we routinely analyse [3]. Also in sample preparation we followed the procedures implemented for these studies [3]. The experimental conditions are summarized in Table 1.

2. Structural modifications by PIXE analyses Saponite war selected for the study of structural effects because of its rather fragile lattice structure and its high water content that makes saponite sensitive to elevated temperatures: on heating, the interlayer water is mostly lost between 100 and 250°C [4]. The chemical composition of our saponite as obtained by electron microprobe is given in Table 2. By difference it contains 18.2 wt.% water. Fig. 2 is a TEM photograph of the intact saponite that clearly exhibits the identifying lattice distances of 1.1 nm while in Fig. 3 the typical " w a v y " structure is evident. Saponite fragments in the size range 50-100 ~ m were

0168-583X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved SSDI 0168-583X(95)00904-3

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M. Maetz et al. / Nucl. blstr, and Meth. in Phys. Res. B 109/110 (1996) 192-196

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Table 2 Composition in wt.% of the saponite as analysed with the electron microprobe. By difference it contains 18.2 wt.% H20

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time [sec] Fig. 1. SEM analyses of two glasses. Time resolved measurements-time resolution 20 s-of the alkali content (top: Na. bottom: K) of two different synthetic glasses analysed with standardless SEM. The alkali/Si ratio is normalized to the initial ratio after the first 20 s measurement. Losses of up to 90% are evident.

g l u e d with a small drop of pure silicon oil on a 7.5 txm

Kapton foil and coated with carbon. Seven fragments were individually bombarded with 2.0 MeV protons with different currents and doses (Fig. 4) by scanning the proton beam over the whole sample. The proton current was monitored at regular intervals with a Faraday cup. The effective scanning area - identified as a decoloration spot was determined after the bombardment with an optical

microscope. After the PIXE analyses the fragments were embedded in epoxy, cut with an ultra-microtome and analysed by TEM. Fig. 5 is a TEM photograph of a fragment that has been exposed to 1 . 4 p A / t x m 2 for one hour. It is loaded with small holes inside the 'crystals' probably due to explosive vaporisation of water. Thus, the temperature raised during the proton bombardment to more than 100°C. The saponite lattice symmetry is no longer present in high magnification TEM and the structure is completely amorphous. In Fig. 4 we summarize the results: below the dashed curve the layer lattice structure remains intact while above that values saponite becomes amorphous. The damage does not only depend on the current density that increases the local temperature, but also on the total irradiation dose which is probably related to the formation of vacancies or bubbles [5,6]. Our results clearly demonstrate that systematic experiments to study the effects of the proton bombardment on the structure of the sample under investigation are required if PIXE studies are to be followed by mineralogical, e.g. TEM, analyses. Certainly, the preparation techniques and the scanning frequency of such test experiments have to mimic those of the real study.

3. Chemical effects of PIXE analyses As has been demonstrated in Fig. 1, alkali metals are prone to be lost from a sample during analysis with the

Table 1 Summary of the experimental conditions as applied in the present study

Particle size (&m) Energy (MeV) Absorber Current Particle mounting Analysing mode

Saponite

Feldspar

KBSi308

NaBSi308

50-100 txm 2.0 MeV various 7.5 Ixm Kapton silicon oil scanning mode

100 ixm 2.3 MeV no 10 pA 7.5 txm Kapton silicon oil scanning mode ~nd fixed beam

90 txm 2.3 MeV Be (46 ixm) 4 pA 7.5 I~m Kapton

30 Ixm 2.3 MeV no 4 pA 7.5 &m Kapton

scanning mode

scanning mode

II. EXPERIMENTAL

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M. Maetz et al. / Nucl. Instr. and Meth. in Phys. Res. B 109/110 (1996) 192-196

Fig. 2. High magnification TEM photograph of an intact saponite. The lattice distance is 1.1 nm.

Fig. 3. TEM photograph (magnification: ×11500) of an intact saponite showing its typical wavy structure. The dark and the white areas are cutting artefacts: the dark areas are due to overlapping material and the white areas are due to tearing parts of the sample during cutting. Some gas bubbles resulting from the embedding process are also visible.

electron microprobe. As a test if such an effect also occurs with the proton microprobe we prepared several 100p~m fragments of alkali rich samples, a natural feldspar [7] from Kakanui, New Zealand with a drop of silicon oil and two synthetic glasses, K B S i 3 0 s and NaBSi308 without any glue. Before PIXE analyses the glass fragments were analysed with SEM and the above losses as shown in Fig. 1 were encountered. With PIXE we exposed these fragments to a proton beam of 2.3MeV energy and scanned individually over the whole sample. The time resolved measurements show no indication of volatilisation (Figs. 6 - 8 ) . Both the Na and the K contents of the fragments are the same even if measured with high time resolution on the order of seconds as well as after 20 min exposure. No systematic loss trends of the element ratios are observed. The mean element ratios resulting from the high (5 s) and lower (60 s) time resolution experiments are indistinguishable within the (mostly statistical) uncertainties. The mean N a / S i ratio for the synthetic glass is 0.258-+0.009. With an uncertainty of 10% in our standardisation it is in good agreement with the stoichiometric value of 0.273. The corresponding K / S i ratio, 0.443-+0.008, is about 5% lower than the stoic!aiometric value of 0.464. That difference most probably is due to our standardisation. In addition to these experiments, we measured one feldspar fragment before and after proton bombardment with 600 pA for 1 h with a constant 5 p~m spot size and 2.3 MeV energy proton beam. Also then no alkali loss is observed, i.e. K / S i and N a / S i before and after the treatment differ by only 4%.

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M. Maetz et al. / Nucl. Instr. and Meth. in Phys. Res. B 109/110 (1996) 192-196

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current density [pA/pm2] Fig. 4. Structural modifications by PIXE analyses of saponite fragments with different proton currents and doses. The error bars indicate the variation of the current and the uncertainties of the scanned area. The circles represent the fragments that have been amorphousized, the triangles represent intact particles. For saponite we establish that doses and current densities below the dashed line maintain its intactness.

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Fig. 6. Results of PIXE analyses of a 100 ixm feldspar fragment. The time resolution is 5 s (top) and 1 min (bottom). The alkali abundance is normalized to (Si + A1). The larger uncertainties in the upper panel result from the short (5 s) integration times.

4. Discussion and conclusions

Fig. 5. TEM photograph (magnification: ×20000) of a saponite fragment that has been exposed to a current density of 1.4 pA/ 2 Izm for 1 h. It is loaded with small holes probably due to explosive vaporisation of water.

We have demonstrated that b o m b a r d m e n t o f individual particles with protons in the MeV range induces unalterable damage in sensitive minerals by both amorphisation and explosive evaporisation o f crystal water. The concomitant temperature increase amounts to at least 100°C. This effect has to be taken into account in PIXE studies if they are to be followed by structural or mineralogical analyses. In contrast to these severe structural modifications, we found no indication o f chemical changes o f the volatile element content during PIXE analyses under standard conditions. Our results even exclude lateral migration o f atoms due to recoil over the short distance o f 5 txm.

II. EXPERIMENTAL

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M. Maetz et al. / Nucl. Instr. and Meth. in Phys. Res. B 109/110 (1996) 192-196

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Fig. 8. Result of time resolved PIXE measurements of the Na content of a 90 ixm fragment of synthetic NaBSi308 glass. Time resolution is 5 s (top) and 1 min (bottom). The larger uncertainties in the upper panel result from the short (5 s) integration times. The Na abundance is normalized to Si with mean values of 0.241-+0.035 (top) and 0.259-+0.009 (bottom), respectively.

References

the Rock-Forming Minerals (Longman Group, Hong Kong, 1992) p. 373. [5] E.K. Jessberger, P. Horn and T. Kirsten, Lunar Science VI (1975) 441. [6] P. Horn, E.K. Jessberger, T. Kirsten and H. Richter, Proc. 6th Lunar Sci. Conf. (1975) 1563. [7] E.J. Jarosewich, J.A. Nelena and J.A. Norberg, Corrections in Geostandards Newsletter (2) (1980) 257.

[1] K. Themner, E Spanne and K.W. Jones, Nucl. Instr. and Meth. B 49 (1990) 52. [2] C. Antz, Diploma Thesis, Universit~it Heidelberg, Heidelberg (1988). [3] J. Bohsung, P. Arndt, E.K. Jessberger, M. Maetz, K. Traxel and A. Willianos, Planet. and Space Sci. 43 (1995) 411. [4] W.A. Deer, R.A. Howie and J. Zussman, An Introduction to