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Ion beam analysis of synthetic cylindrite produced by CVT
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 249 (2006) 478–481 www.elsevier.com/locate/nimb
Ion beam analysis of synthetic cylindrite produced by CVT F. Menzel a b
a,*
, R. Kaden b, D. Spemann a, K. Bente b, T. Butz
a
Nuclear Solid State Physics, Faculty of Physics and Geosciences, University of Leipzig, Linne´str. 5, 04103 Leipzig, Germany Institute of Mineralogy, Crystallography and Materials Science, Faculty of Chemistry and Mineralogy, University of Leipzig, Scharnhorststr. 20, 04275 Leipzig, Germany Available online 27 April 2006
Abstract The mineral cylindrite belonging to the sulfosalts has a composition of FeSn4Pb3Sb2S14. The physical interest is based upon the detection of ferromagnetism in the natural mineral and its semiconducting properties, e.g. the narrow band gap of 0.7 eV. The aim of the study was to determine whether it is possible to synthesise cylindrite by CVT and to gain more information about the transport mechanisms of the CVT procedure. For this purpose, synthetic cylindrite samples produced by CVT were investigated by ion beam analysis at the LIPSION laboratory in order to determine the composition of the samples. The cylindrite samples were synthesised from source material with stoichiometric as well as slightly modified compositions and subsequently investigated by RBS and PIXE using 1.2 MeV and 2.25 MeV protons. The results show that the different transport and deposition properties of the main components of the cylindrite influence the composition and the homogeneity of the synthesised material. Furthermore, it was found that in general the sulfur content is lower and that the composition of the metals differs compared to natural mineral. 2006 Elsevier B.V. All rights reserved. PACS: 07.78.+s; 29.30.Kv; 91.60.Ed; 81.10.Bk Keywords: Ion microprobe; PIXE; RBS; Cylindrite; CVT
1. Introduction The mineral cylindrite belongs to the sulfosalts and got its name due to the extraordinary property to form small wrapped cylinders. It has a nominal composition of FeSn4Pb3Sb2S14 [1], but material with slightly different composition also shows typical cylindrite properties. This material is also interesting because of its ferromagnetic [2] and semiconducting properties, e.g. its narrow band gap of 0.7 eV [3]. In this work, synthetic cylindrite samples produced by chemical vapour transport (CVT) [4] using iodine as transport gas were investigated by ion beam analysis using the proton microbeam at the LIPSION laboratory [5]. In order to gain more information about the transport mechanisms
*
Corresponding author. Tel.: +49 341 9732 707; fax: +49 341 9732 708. E-mail address: [email protected] (F. Menzel).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.03.035
of the CVT procedure, and the dependency between the composition of the source material and the synthesised cylindrite, the elemental composition of the synthetic cylindrite samples was determined. 2. Sample preparation The synthetic cylindrite samples were produced by CVT. For this purpose, natural cylindrite or different stoichiometrically weighted samples of the pure main elements or their sulfides were used as source material. Iodine was used as transport gas (I content 0.3–1 mg/ml). The evacuated (80 kPa) and sealed quartz ampoule with the source and transport materials was heated for 3 weeks in a two-zone oven. The area where the source material was located was heated to 660 C while the deposition zone was held at a temperature of 620–630 C. The following reaction between the metallic components (Me), sulfur and iodine forms the basis of the CVT process: MeS + I2 M MeI2 + S.
F. Menzel et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 478–481
The reaction from left to right is an endothermic reaction forming sulfur gas and the gaseous intermediate compounds of metals and iodine. At the cooler area of the ampoule, the exothermic return reaction takes place in which the intermediate compounds dissociate and the metals react again with sulfur. The compounds consisting of sulfur and the metallic elements are deposited on a small quartz rod. For some samples non-stoichiometric source material was used in order to test the effects on the synthesised cylindrite. For example, sulfur excess results in a change from divalent to teravalent Sn which may then be preferentially incorporated in the synthetic material instead of Sb. Therefore, for some CVT experiments, source material containing a sulfur surplus of 0.1 mg/ml was used in order to test whether the sulfur excess leads to a shift of the ratio between Sn and the Sb content in favour of Sn. Another sample was synthesised from source material with a composition of FeSn4.7Pb3.5Sb1.5S14 in order to find out whether the modified composition of the source material is reflected in the produced sample. For the IBA investigation material was removed from the quartz rod in the crystallisation area as well as from the source area in the quartz ampoule and placed on a sample holder. The material can further be distinguished by its shape into long and short plates and into porous samples. 3. Experimental setup PIXE and RBS measurements were carried out by using a 2.25 MeV proton microbeam focused to 1 lm. Additionally, for the analysis of the spherical structures located on the surface of some synthetic cylindrite samples, 1.2 MeV protons were used in order to avoid disturbing influences from the cylindrite material below. The PIXE signals were detected with an EG&G Ortec HPGe IGLET-X detector with an energy resolution of 148 eV @ 5.9 keV while the RBS measurements were carried out using a Canberra annular PIPS-detector with
479
an active area of 300 mm2, a solid angle of 75 msr and an energy resolution of 20 keV. For every measurement, a charge between 0.1 lC and 0.5 lC was collected at a beam current of 100 pA, which is sufficient for the RBS setup and the high content of heavy elements in the investigated samples. The elemental analysis was performed using RUMP [6] with the non-Rutherford cross sections of sulfur [7] and GeoPIXE II [8] with the following procedure. At first, the element concentration was roughly estimated by PIXE. Afterwards the charge and the thickness of the sample was determined by RBS (see Fig. 1). In the third step, the PIXE spectra were analysed with the correct charge and sample thickness and the element maps were generated (see Fig. 2). For this purpose, the yield calculation for PIXE analysis was repeated in an iterative procedure until the difference between the element concentrations between two steps was <0.1%. In order to avoid inaccuracies due to thickness inhomogeneities PIXE spectra were extracted from homogeneous sample areas prior to analysis. In the PIXE spectra, a strong overlap of S–K and Pb–M lines occurs which may cause misfits of the spectra and results in wrong element concentrations. Therefore, the PIXE results were confirmed by fitting the RBS spectra using the element concentrations obtained from PIXE. In total, the analysis error for PIXE is 10% for sulfur and 5% for the metallic components of the cylindrite samples. 4. Results The results of the elemental analysis of the synthesised cylindrite samples are summarised in Table 1. In general, the ratio Me/S between metals and sulfur of the synthesised cylindrite is larger than that of the source material. The only exception is the sample taken from the crystallisation area which was produced by the FeSn4.7Pb3.5Sb1.5S14 source material. Here the ratios are equal. Furthermore, the content of iron and antimony incorporated in the
Fig. 1. (a) RBS spectrum of a thin cylindrite sample from which the thickness of the sample and the applied charge were determined. (b) RBS spectrum of a thick cylindrite sample. The dotted fit represents the element composition at the surface while the dashed one represents the average element composition obtained from PIXE.
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F. Menzel et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 478–481
Fig. 2. Element maps (203 lm · 202 lm) of an inhomogeneous cylindrite sample from the source area synthesised from stoichiometric source material. The small spots in the Sn–L map are caused by a Ca contamination. The Ca–K lines overlap with the Sn–L lines.
synthetic samples is lower by 25% and 10%, respectively, while the tin content is higher by 12% than in the natural cylindrite. The comparison between the sample synthesised from stoichiometric source material and source material with sulfur surplus shows that sulfur surplus does not lead to a significant shift of the Sn/Sb ratio in favour of tin. In the composition of the sample synthesised from the non-stoichiometric source material, the different composition of the source material is partly reflected. As supposed, the Sn content seems to be higher and the Sb content is lower in comparison to the other samples, while the Pb content shows no difference. However, contrary to the Sb content, the difference in the Sn content is not significant considering the analysis errors. The detected iodine concentration is below the detection limit (MDL: 300 lg/g), i.e. iodine from the CVT transport process is not incorporated into synthetic cylindrite.
Additionally, information about elemental depth profiles of thick samples can be obtained from the RBS spectra. For example, the dotted fit in Fig. 1(b) represents the element composition at the surface while the dashed one represents the average element composition obtained from PIXE. The discrepancy at lower backscattering energies between the RBS spectrum and the fits indicates an increasing concentration of the heavier elements (Sn, Sb, Pb) from the surface into the bulk of the cylindrite samples. This can be explained by the different transport and deposition rates of the elements during CVT, where SnI4 has the lowest melting and boiling temperature followed by SbI3, SnI2, PbI2 and FeI2. On some samples of synthetic cylindrite, small spherical structures were found on the surface as shown in Fig. 3. They have a diameter of 3 lm up to 12 lm and consist mainly of S and Fe. In order to determine their element concentration, single spheres were analysed (see Fig. 3(c)
Table 1 Elemental concentrations of the main components of cylindrite obtained from PIXE for the investigated samples Source material
Samples
Fe
Sn
Pb
Sb
S
Me/S
Source Me/S
Stoichiometric
Crystallisation area Source area, long plates Source area
0.72 0.87 0.71
4.43 4.30 4.52
3.03 2.97 2.84
1.81 1.86 1.93
12.53 11.76 9.14
0.80 0.85 1.09
0.714 0.714 0.714
Sulfur surplus
Crystallisation area, long plates Crystallisation area, short plates Source area
0.68 0.82 0.72
4.58 4.33 4.63
3.08 3.13 2.79
1.66 1.72 1.87
11.43 13.06 9.39
0.87 0.77 1.07
0.714 0.714 0.714
FeSn4.7Pb3.5Sb1.5S14
Crystallisation area, short plates Source area
0.67 0.79
4.78 4.57
3.07 3.08
1.48 1.56
13.29 11.48
0.75 0.87
0.764 0.764
The values are normalised on ten metal atoms within the formula-unit. Me/S is the ratio between the sum of the metallic components and sulfur. The row ‘‘source Me/S’’ shows this ratio for the source material.
F. Menzel et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 478–481
481
Fig. 3. PIXE maps obtained from cylindrite samples using 1.2 MeV protons. The spherical structures at the surface consist mainly of S and Fe. (a) and (b) (274 lm · 274 lm) show the surface structure and an accumulation of spheres (bright spots). (c) In order to determine their elemental composition single spheres were analysed (17 lm · 17 lm). A region of interest from which a PIXE spectrum of the sphere was extracted is marked. (d) (274 lm · 274 lm) and (e) (17 lm · 17 lm) illustrate the absence of Sn in the spherical structures. The element maps of Sb and Pb look similar.
and (e)). A sulfur to iron ratio between 1.75 and 3 depending on the sphere under investigation was calculated from PIXE results.
The transport material iodine is not incorporated in the samples. Acknowledgement
5. Conclusion This study shows that synthetic cylindrite can be produced by CVT. However, the elemental composition of the synthesised samples deviates from the source material. In general, the sulfur content is lower than in the natural mineral. Furthermore, the ratio between the metallic components differs from the nominal stoichiometry, e.g. the Fe and Sb content is lower while the Sn content is higher. Sulfur surplus was not found to influence the composition of the synthesised cylindrite. The physical properties of the synthetic cylindrite will be investigated in the future in order to check whether the slightly different composition of the material compared to the natural cylindrite influences these properties. RBS measurements on thick samples show an increase of the concentration of the heavier elements (Sn, Sb, Pb) from the surface to the bulk.
This work was supported by the Deutsche Forschungsgemeinschaft (DFG) under the research unit grant FOR 522. References [1] [2] [3] [4] [5]
[6] [7] [8]
E. Makovicky, N. Jb. Mineral. Monatsh. 6 (1974) 235. P.A. Salyer, L.W. ter Haar, J. Appl. Phys. 87 (9) (2000) 6025. S.I. Boldish, W.B. White, Am. Mineral. 83 (1998) 865. K. Bente, G. Wagner, M. Heuer, S. Schorr, G.N. Kryukova, Th. Do¨ring, D. Oppermann, R. Kaden, in: Proceedings Viennano’05, 2005. T. Butz, R.-H. Flagmeyer, J. Heitmann, D.N. Jamieson, G.J.F. Legge, D. Lehmann, U. Reibetanz, T. Reinert, A. Saint, D. Spemann, R. Szymanski, W. Tro¨ger, J. Vogt, J. Zhu, J. Nucl. Instr. and Meth. B 161–163 (2000) 323. L.R. Doolittle, Nucl. Instr. and Meth. B 9 (1985) 344. E. Rauhala, M. Luomaja¨rvi, Nucl. Instr. and Meth. B 33 (1988) 628. C.G. Ryan, Nucl. Instr. and Meth. B 181 (2001) 170.
Nuclear Instruments and Methods in Physics Research B 249 (2006) 478–481 www.elsevier.com/locate/nimb
Ion beam analysis of synthetic cylindrite produced by CVT F. Menzel a b
a,*
, R. Kaden b, D. Spemann a, K. Bente b, T. Butz
a
Nuclear Solid State Physics, Faculty of Physics and Geosciences, University of Leipzig, Linne´str. 5, 04103 Leipzig, Germany Institute of Mineralogy, Crystallography and Materials Science, Faculty of Chemistry and Mineralogy, University of Leipzig, Scharnhorststr. 20, 04275 Leipzig, Germany Available online 27 April 2006
Abstract The mineral cylindrite belonging to the sulfosalts has a composition of FeSn4Pb3Sb2S14. The physical interest is based upon the detection of ferromagnetism in the natural mineral and its semiconducting properties, e.g. the narrow band gap of 0.7 eV. The aim of the study was to determine whether it is possible to synthesise cylindrite by CVT and to gain more information about the transport mechanisms of the CVT procedure. For this purpose, synthetic cylindrite samples produced by CVT were investigated by ion beam analysis at the LIPSION laboratory in order to determine the composition of the samples. The cylindrite samples were synthesised from source material with stoichiometric as well as slightly modified compositions and subsequently investigated by RBS and PIXE using 1.2 MeV and 2.25 MeV protons. The results show that the different transport and deposition properties of the main components of the cylindrite influence the composition and the homogeneity of the synthesised material. Furthermore, it was found that in general the sulfur content is lower and that the composition of the metals differs compared to natural mineral. 2006 Elsevier B.V. All rights reserved. PACS: 07.78.+s; 29.30.Kv; 91.60.Ed; 81.10.Bk Keywords: Ion microprobe; PIXE; RBS; Cylindrite; CVT
1. Introduction The mineral cylindrite belongs to the sulfosalts and got its name due to the extraordinary property to form small wrapped cylinders. It has a nominal composition of FeSn4Pb3Sb2S14 [1], but material with slightly different composition also shows typical cylindrite properties. This material is also interesting because of its ferromagnetic [2] and semiconducting properties, e.g. its narrow band gap of 0.7 eV [3]. In this work, synthetic cylindrite samples produced by chemical vapour transport (CVT) [4] using iodine as transport gas were investigated by ion beam analysis using the proton microbeam at the LIPSION laboratory [5]. In order to gain more information about the transport mechanisms
*
Corresponding author. Tel.: +49 341 9732 707; fax: +49 341 9732 708. E-mail address: [email protected] (F. Menzel).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.03.035
of the CVT procedure, and the dependency between the composition of the source material and the synthesised cylindrite, the elemental composition of the synthetic cylindrite samples was determined. 2. Sample preparation The synthetic cylindrite samples were produced by CVT. For this purpose, natural cylindrite or different stoichiometrically weighted samples of the pure main elements or their sulfides were used as source material. Iodine was used as transport gas (I content 0.3–1 mg/ml). The evacuated (80 kPa) and sealed quartz ampoule with the source and transport materials was heated for 3 weeks in a two-zone oven. The area where the source material was located was heated to 660 C while the deposition zone was held at a temperature of 620–630 C. The following reaction between the metallic components (Me), sulfur and iodine forms the basis of the CVT process: MeS + I2 M MeI2 + S.
F. Menzel et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 478–481
The reaction from left to right is an endothermic reaction forming sulfur gas and the gaseous intermediate compounds of metals and iodine. At the cooler area of the ampoule, the exothermic return reaction takes place in which the intermediate compounds dissociate and the metals react again with sulfur. The compounds consisting of sulfur and the metallic elements are deposited on a small quartz rod. For some samples non-stoichiometric source material was used in order to test the effects on the synthesised cylindrite. For example, sulfur excess results in a change from divalent to teravalent Sn which may then be preferentially incorporated in the synthetic material instead of Sb. Therefore, for some CVT experiments, source material containing a sulfur surplus of 0.1 mg/ml was used in order to test whether the sulfur excess leads to a shift of the ratio between Sn and the Sb content in favour of Sn. Another sample was synthesised from source material with a composition of FeSn4.7Pb3.5Sb1.5S14 in order to find out whether the modified composition of the source material is reflected in the produced sample. For the IBA investigation material was removed from the quartz rod in the crystallisation area as well as from the source area in the quartz ampoule and placed on a sample holder. The material can further be distinguished by its shape into long and short plates and into porous samples. 3. Experimental setup PIXE and RBS measurements were carried out by using a 2.25 MeV proton microbeam focused to 1 lm. Additionally, for the analysis of the spherical structures located on the surface of some synthetic cylindrite samples, 1.2 MeV protons were used in order to avoid disturbing influences from the cylindrite material below. The PIXE signals were detected with an EG&G Ortec HPGe IGLET-X detector with an energy resolution of 148 eV @ 5.9 keV while the RBS measurements were carried out using a Canberra annular PIPS-detector with
479
an active area of 300 mm2, a solid angle of 75 msr and an energy resolution of 20 keV. For every measurement, a charge between 0.1 lC and 0.5 lC was collected at a beam current of 100 pA, which is sufficient for the RBS setup and the high content of heavy elements in the investigated samples. The elemental analysis was performed using RUMP [6] with the non-Rutherford cross sections of sulfur [7] and GeoPIXE II [8] with the following procedure. At first, the element concentration was roughly estimated by PIXE. Afterwards the charge and the thickness of the sample was determined by RBS (see Fig. 1). In the third step, the PIXE spectra were analysed with the correct charge and sample thickness and the element maps were generated (see Fig. 2). For this purpose, the yield calculation for PIXE analysis was repeated in an iterative procedure until the difference between the element concentrations between two steps was <0.1%. In order to avoid inaccuracies due to thickness inhomogeneities PIXE spectra were extracted from homogeneous sample areas prior to analysis. In the PIXE spectra, a strong overlap of S–K and Pb–M lines occurs which may cause misfits of the spectra and results in wrong element concentrations. Therefore, the PIXE results were confirmed by fitting the RBS spectra using the element concentrations obtained from PIXE. In total, the analysis error for PIXE is 10% for sulfur and 5% for the metallic components of the cylindrite samples. 4. Results The results of the elemental analysis of the synthesised cylindrite samples are summarised in Table 1. In general, the ratio Me/S between metals and sulfur of the synthesised cylindrite is larger than that of the source material. The only exception is the sample taken from the crystallisation area which was produced by the FeSn4.7Pb3.5Sb1.5S14 source material. Here the ratios are equal. Furthermore, the content of iron and antimony incorporated in the
Fig. 1. (a) RBS spectrum of a thin cylindrite sample from which the thickness of the sample and the applied charge were determined. (b) RBS spectrum of a thick cylindrite sample. The dotted fit represents the element composition at the surface while the dashed one represents the average element composition obtained from PIXE.
480
F. Menzel et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 478–481
Fig. 2. Element maps (203 lm · 202 lm) of an inhomogeneous cylindrite sample from the source area synthesised from stoichiometric source material. The small spots in the Sn–L map are caused by a Ca contamination. The Ca–K lines overlap with the Sn–L lines.
synthetic samples is lower by 25% and 10%, respectively, while the tin content is higher by 12% than in the natural cylindrite. The comparison between the sample synthesised from stoichiometric source material and source material with sulfur surplus shows that sulfur surplus does not lead to a significant shift of the Sn/Sb ratio in favour of tin. In the composition of the sample synthesised from the non-stoichiometric source material, the different composition of the source material is partly reflected. As supposed, the Sn content seems to be higher and the Sb content is lower in comparison to the other samples, while the Pb content shows no difference. However, contrary to the Sb content, the difference in the Sn content is not significant considering the analysis errors. The detected iodine concentration is below the detection limit (MDL: 300 lg/g), i.e. iodine from the CVT transport process is not incorporated into synthetic cylindrite.
Additionally, information about elemental depth profiles of thick samples can be obtained from the RBS spectra. For example, the dotted fit in Fig. 1(b) represents the element composition at the surface while the dashed one represents the average element composition obtained from PIXE. The discrepancy at lower backscattering energies between the RBS spectrum and the fits indicates an increasing concentration of the heavier elements (Sn, Sb, Pb) from the surface into the bulk of the cylindrite samples. This can be explained by the different transport and deposition rates of the elements during CVT, where SnI4 has the lowest melting and boiling temperature followed by SbI3, SnI2, PbI2 and FeI2. On some samples of synthetic cylindrite, small spherical structures were found on the surface as shown in Fig. 3. They have a diameter of 3 lm up to 12 lm and consist mainly of S and Fe. In order to determine their element concentration, single spheres were analysed (see Fig. 3(c)
Table 1 Elemental concentrations of the main components of cylindrite obtained from PIXE for the investigated samples Source material
Samples
Fe
Sn
Pb
Sb
S
Me/S
Source Me/S
Stoichiometric
Crystallisation area Source area, long plates Source area
0.72 0.87 0.71
4.43 4.30 4.52
3.03 2.97 2.84
1.81 1.86 1.93
12.53 11.76 9.14
0.80 0.85 1.09
0.714 0.714 0.714
Sulfur surplus
Crystallisation area, long plates Crystallisation area, short plates Source area
0.68 0.82 0.72
4.58 4.33 4.63
3.08 3.13 2.79
1.66 1.72 1.87
11.43 13.06 9.39
0.87 0.77 1.07
0.714 0.714 0.714
FeSn4.7Pb3.5Sb1.5S14
Crystallisation area, short plates Source area
0.67 0.79
4.78 4.57
3.07 3.08
1.48 1.56
13.29 11.48
0.75 0.87
0.764 0.764
The values are normalised on ten metal atoms within the formula-unit. Me/S is the ratio between the sum of the metallic components and sulfur. The row ‘‘source Me/S’’ shows this ratio for the source material.
F. Menzel et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 478–481
481
Fig. 3. PIXE maps obtained from cylindrite samples using 1.2 MeV protons. The spherical structures at the surface consist mainly of S and Fe. (a) and (b) (274 lm · 274 lm) show the surface structure and an accumulation of spheres (bright spots). (c) In order to determine their elemental composition single spheres were analysed (17 lm · 17 lm). A region of interest from which a PIXE spectrum of the sphere was extracted is marked. (d) (274 lm · 274 lm) and (e) (17 lm · 17 lm) illustrate the absence of Sn in the spherical structures. The element maps of Sb and Pb look similar.
and (e)). A sulfur to iron ratio between 1.75 and 3 depending on the sphere under investigation was calculated from PIXE results.
The transport material iodine is not incorporated in the samples. Acknowledgement
5. Conclusion This study shows that synthetic cylindrite can be produced by CVT. However, the elemental composition of the synthesised samples deviates from the source material. In general, the sulfur content is lower than in the natural mineral. Furthermore, the ratio between the metallic components differs from the nominal stoichiometry, e.g. the Fe and Sb content is lower while the Sn content is higher. Sulfur surplus was not found to influence the composition of the synthesised cylindrite. The physical properties of the synthetic cylindrite will be investigated in the future in order to check whether the slightly different composition of the material compared to the natural cylindrite influences these properties. RBS measurements on thick samples show an increase of the concentration of the heavier elements (Sn, Sb, Pb) from the surface to the bulk.
This work was supported by the Deutsche Forschungsgemeinschaft (DFG) under the research unit grant FOR 522. References [1] [2] [3] [4] [5]
[6] [7] [8]
E. Makovicky, N. Jb. Mineral. Monatsh. 6 (1974) 235. P.A. Salyer, L.W. ter Haar, J. Appl. Phys. 87 (9) (2000) 6025. S.I. Boldish, W.B. White, Am. Mineral. 83 (1998) 865. K. Bente, G. Wagner, M. Heuer, S. Schorr, G.N. Kryukova, Th. Do¨ring, D. Oppermann, R. Kaden, in: Proceedings Viennano’05, 2005. T. Butz, R.-H. Flagmeyer, J. Heitmann, D.N. Jamieson, G.J.F. Legge, D. Lehmann, U. Reibetanz, T. Reinert, A. Saint, D. Spemann, R. Szymanski, W. Tro¨ger, J. Vogt, J. Zhu, J. Nucl. Instr. and Meth. B 161–163 (2000) 323. L.R. Doolittle, Nucl. Instr. and Meth. B 9 (1985) 344. E. Rauhala, M. Luomaja¨rvi, Nucl. Instr. and Meth. B 33 (1988) 628. C.G. Ryan, Nucl. Instr. and Meth. B 181 (2001) 170.