- Email: [email protected]
Microanalysis of ceramics with PIXE and LA-ICP-MS
Nuclear Instruments and Methods in Physics Research B 189 (2002) 378–381 www.elsevier.com/locate/nimb
Microanalysis of ceramics with PIXE and LA-ICP-MS J.D. Robertson b
a,b,*
, H. Neff a, B. Higgins
a
a Missouri University Research Reactor, Columbia, MO 65211, USA Department of Chemistry, University of Missouri, Columbia, MO 65211, USA
Abstract The provenance postulate states that artifact raw material sources can be determined by chemical characterization as long as between-source chemical differences exceed within-source variation. For ceramics, successful differentiation of sources usually requires the measurement of a large suite of elements. While laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) can provide concentration data for nearly every element whose concentration is at or above 100 ng/g, it requires the use of ‘‘matrix matched’’ standards for quantitative analysis. In this work, we report on the use of micro-PIXE to develop standards for the microanalysis of ceramics by LA-ICP-MS. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 82.80.E; 82.80.K; 79.20.D Keywords: Archaeometry; Ceramics; LA-ICP-MS; Micro-PIXE
1. Introduction The use of inductively coupled plasma mass spectrometry (ICP-MS) to provide highly precise and sensitive elemental and isotopic analyses has revolutionized the analytical scene in archaeometry over the past decade [1–3]. Most applications have, however, required that the samples be digested prior to analysis. An alternative sampleintroduction technique that, like micro-PIXE, offers the advantage of spatially resolved instrumental analyses is laser ablation (LA). In LA-ICPMS, a pulsed laser ablates a small portion of a solid sample and the ablated plume is swept with
* Corresponding author. Tel.: +1-573-8822240; fax: +1-5738822754. E-mail address: [email protected] (J.D. Robertson).
carrier gas into the ICP torch. This technique can provide rapid, multi-elemental and isotopic analyses with sub lg/g sensitivities at a spatial resolution of 5 lm. A key challenge in the application of LA-ICP-MS to archaeometry is, however, the development of accurate quantification procedures, especially for highly heterogeneous materials like clays and ceramics. In LA-ICP-MS, signal intensities depend strongly on the sample matrix and instrumental drift. It is, therefore, important that the calibration standard is as similar to the sample as possible [4]. For instrumental drift, internal standards can be added through a dual sample introduction system in which the ablated aliquot is simultaneously calibrated against an aerosol from an aqueous standard. While this procedure provides a continuous, real-time correction for instrument drift, the presence of oxides and hydroxides from the ‘wet’ plasma create numerous interferences [5].
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 1 0 9 3 - X
J.D. Robertson et al. / Nucl. Instr. and Meth. in Phys. Res. B 189 (2002) 378–381
Alternatively, if one or more elements can be determined (or assumed independently), then these can serve as quasi-internal standards to correct for instrumental drift between the analysis of the samples and standards. For example, rhyolitic obsidian has relatively consistent silica concentrations and we have found that ratios of count rates to the silicon count rate yields normalized values that can be calibrated with external standards so as to yield concentrations that are in good agreement with neutron activation analysis (NAA) and photon-induced X-ray fluorescence measurements of the obsidian. One distinct advantage of LA-ICP-MS over bulk techniques is the potential to perform spot analysis of spatially segregated components in artifact fabrics. In ceramics, separate analyses can be obtained for individual temper grains and/or for areas of clay matrix that contain no temper grains. Moreover, because each pass of the laser only ablates microns of material, the technique can also be used to probe the composition of slipped and pigmented surfaces independent of the bulk ceramic. However, in contrast to materials like obsidian, the internal heterogeneity of clays and the diversity of materials used in ceramic manufacture preclude assuming a value for any single component such as aluminum or silicon. As a result, application of LA-ICP-MS to ceramic analysis will require the development of quantification procedures that can account for both instrumental drift and large variations in the sample matrix at the microscopic level. In this work we report on our initial laser ablation studies of ceramics and the potential for using PIXE as a non-destructive method for generating a suite of ‘‘microstandards’’ for LA-ICP-MS.
2. Experimental The LA-ICP-MS measurements are made using a 213 nm laser ablation system (Merchantek) and a VG Axiom High-Resolution ICP-MS. For this initial work, the ablation was performed at a spot size of 100 lm, a laser power of 60%, and a laser pulse frequency of 20 Hz. The carrier gas, which flows through the ablation cell into the ICP torch,
379
is a mixture of Ar (0.95 L/m) and N2 (0.3 L/m). During the course of this work we found that a much more stable signal could be achieved by passing the carrier gas through a bead-impact spray chamber prior to the ICP torch. Our hypothesis is that the spray chamber averages out the ablation signal and removes the large particles from the ablation plume that tend to make the plasma less stable (briefly cool the plasma). Details of the system used to obtain the PIXE results described in this paper are given in [6]. Briefly, a dual-energy, external-beam irradiation is performed in order to obtain information for both the low- and high-Z elements in the matrix. For 10 min of the 15-min irradiation, the detector is placed 3.5 cm from the target, a 1900 lm-thick polypropylene filter is put in place and the sample is irradiated with a 2.1 MeV proton beam. For the remaining 5 min of the analysis, the filter is removed, the detector is moved 7.5 cm from the target, and the sample is irradiated with a 1.6 MeV proton beam. At each energy, a relative measure of current is made by measuring the number of protons that backscatter from the Kapton exit window in the target chamber. The two spectra are then combined and the data is analyzed with a modified version of the G U P I X software package [7]. The system is calibrated with a series of thin film gravimetric standards from MicroMatter Inc.
3. Results and discussion The LA-ICP-MS quantification procedure that we have employed thus far is to normalize the signal (count rates) for each element from a spot analysis so that they sum to a single standard value (e.g. 1 million) for all standards and unknowns. A regression of the normalized counts on the elemental concentration in the standards then yields a calibration equation that can be used to calculate concentrations in the unknowns. The basic assumption of this approach is that the 43 elements being measured represent all of the material in the ceramic, other than oxygen, that is ablated in the analysis. Incorporation of corrections for isotopic abundance and for differences in resolving power (transmission through the instrument) had little
380
J.D. Robertson et al. / Nucl. Instr. and Meth. in Phys. Res. B 189 (2002) 378–381
Fig. 1. Average (n ¼ 23) ratio of LA-ICP-MS to NAA measurements for Ohio Red clay. The dashed lines are 1 standard deviation.
effect on the calculated concentrations. The results of using this approach for the analysis of Ohio Red clay, a quality control standard in activation analysis of ceramics [8], are shown in Fig. 1. Averages of multiple runs of two glass standards, NIST-SRM612 and Glass Buttes obsidian, were used to calculate the average (n ¼ 23) concentrations shown. It is surprising that, given the significant difference between the clay matrix of the ‘‘unknown’’ and the glass matrix of the standards, that the ratio of the LA-ICP-MS measurements to the NAA concentrations hovers between 0.75 and 1.25, on average, for most elements. Key future measurements will be aimed at determining how well the normalization procedure performs with better-matched standards. The one-standard deviation ranges for some of the elements in Fig. 1 are substantial (>70%). Because the relative standard deviations of the silicon-normalized signals for 43 elements from the NIST-SRM612 standard over the course of one month are much smaller (the average RSD of 15 measurements was 17%), we attribute the large deviations in the Ohio Red clay to sample heterogeneity at the microscopic level. This large variation demonstrates that the standards used to test the accuracy of LA-ICP-MS of ceramics and clays will have to be analyzed with a microprobe of similar dimensions. In contrast to LA-ICP-MS, the matrix effects associated with PIXE analysis are well understood and the technique has been successfully applied to the analysis of ceramics (see e.g. [9–12]). As a result, PIXE can provide accurate analytical results in widely varying matrices without the use of
‘‘well-matched’’ standards. For example, the PIXE results presented in Tables 1 and 2 for pressed pellets of two very different matrices, NISTSRM1633a Coal Fly Ash and NIST-SRM1577 Bovine Liver, were acquired with the same calibration curve. The calibration curve for these thick-target analyses was obtained by the analysis of thin film gravimetric standards. These results demonstrate that micro-PIXE could be used to Table 1 Results of the PIXE analysis of NBS SRM-1633a Coal Fly Ash Elementa
PIXEb
Certifiedc
% Difference
Mg (%) Al (%) Si (%) K (%) Ca (%) Ti (%) V Cr Mn Fe (%) Ni Cu Zn Ga Ge As Sr Zr Rb Pb
0:51 0:18 14:6 0:9 22:6 1:5 2:01 0:14 1:16 0:08 0:85 0:05 296 69 186 17 200 12 9:4 0:1 130 15 117 13 225 21 58 9 24 7 130 32 855 83 300 49 131 13 72 12
0.46 14.3 22.8 1.88 1.11 (0.82) 297 196 179 9.4 127 118 220 (56) (34) 145 830 (330) 131 72.4
10.9 2.1 0.9 6.9 4.5 3.6 0.3 5.1 11.7 0.0 2.4 0.8 2.3 3.6 29.4 10.3 3.0 9.1 0.0 0.6
a
In lg/g unless otherwise noted. Average and standard deviation of the analysis of three samples. c Values given in brackets are recommended or consensus value. b
J.D. Robertson et al. / Nucl. Instr. and Meth. in Phys. Res. B 189 (2002) 378–381 Table 2 Results of the PIXE analysis of NBS-SRM1577 Bovine Liver Elementa
PIXEb
Certifiedc
% Difference
Na (%) P (%) S (%) Cl (%) K (%) Ca Mn Fe Cu Zn Se Br Rb
0:27 0:03 1:17 0:08 0:72 0:05 0:27 0:02 1:00 0:07 164 15 10:6 0:5 264 15 191 12 141 9 1:2 0:3 8:2 1:1 16:6 1:4
0.243 (1.13) (0.79) (0.27) 0.97 124 10.3 268 193 130 1.1 (9.1) 18.3
11.1 3.4 8.9 0.0 3.1 32 2.9 1.5 1.0 8.5 9.1 9.9 9.2
a
In lg/g unless otherwise noted. Average and standard deviation of the analysis of seven samples. c Values given in brackets are recommended or consensus value.
381
ergies and filters or in which two detectors are used to record the full PIXE spectrum simultaneously. In the future, we plan to perform a series of microPIXE analyses of ceramics and clays at the University of Guelph proton microprobe to develop a set of standards for our LA-ICP-MS work. Acknowledgements The National Science Foundation (SBR9802366) and the University of Missouri provided funding for this work. References
b
accurately quantify microanalysis of ceramics and clays where, for example, the concentration of Ca can vary from 100 lg/g to 10 wt.% from one spot to another. It should be stressed, however, that accurate micro-PIXE analysis of ceramic and clay materials to be used as standards will require the measurement of both the low- and high-Z elements at each spot. Using again the example of Ca variations, the results for Fe in NIST-SRM1413 High Alumina Sand change by nearly 10% if Ca is left out of the matrix in the PIXE analysis. This analytical bias can be readily overcome with a complete micro-PIXE analysis in which the same spot is analyzed multiple times with different en-
[1] D.J. Kennett, H. Neff, M.D. Glascock, A.Z. Mason, The SAA Archaeol. Record 1 (2001) 22. [2] L.M. Mallory-Greenough, J.D. Greenough, J.V. Owen, J. Archaeol. Sci. 25 (1998) 85. [3] A.M. Pollard, C. Heron, Archaeological Chemistry, The Royal Society of Chemistry, Cambridge, UK, 1996. [4] A.J. Campbell, M. Humayun, Anal. Chem. 71 (1999) 939. [5] S.F. Durrant, J. Anal. Atom Spectrom. 14 (1999) 1385. [6] L.J. Blanchard, J.D. Robertson, S. Srikantapura, B.K. Parekh, J. Radioanal. Nucl. Chem. 221 (1997) 23. [7] J.A. Maxwell, W.J. Teesdale, J.L. Campbell, Nucl. Instr. and Meth. B 94 (1994) 172. [8] J.W. Cogswell, H. Neff, M.D. Glascock, Am. Antiquity 63 (1998) 63. [9] C.P. Swann, C.H. Nelson, Nucl. Instr. and Meth. B 161– 163 (2000) 694. [10] E. Aloupi, A.G. Karydas, T. Paradellis, X-Ray Spectrom. 29 (2000) 18. [11] C.P. Swann, S. Caspi, J. Carlson, Nucl. Instr. and Meth. B 150 (1999) 571. [12] G. Lagarde, I. Brissaud, J. Cailleret, P. Fontes, C. Heitz, Nucl. Instr. and Meth. 229 (1984) 45.
Microanalysis of ceramics with PIXE and LA-ICP-MS J.D. Robertson b
a,b,*
, H. Neff a, B. Higgins
a
a Missouri University Research Reactor, Columbia, MO 65211, USA Department of Chemistry, University of Missouri, Columbia, MO 65211, USA
Abstract The provenance postulate states that artifact raw material sources can be determined by chemical characterization as long as between-source chemical differences exceed within-source variation. For ceramics, successful differentiation of sources usually requires the measurement of a large suite of elements. While laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) can provide concentration data for nearly every element whose concentration is at or above 100 ng/g, it requires the use of ‘‘matrix matched’’ standards for quantitative analysis. In this work, we report on the use of micro-PIXE to develop standards for the microanalysis of ceramics by LA-ICP-MS. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 82.80.E; 82.80.K; 79.20.D Keywords: Archaeometry; Ceramics; LA-ICP-MS; Micro-PIXE
1. Introduction The use of inductively coupled plasma mass spectrometry (ICP-MS) to provide highly precise and sensitive elemental and isotopic analyses has revolutionized the analytical scene in archaeometry over the past decade [1–3]. Most applications have, however, required that the samples be digested prior to analysis. An alternative sampleintroduction technique that, like micro-PIXE, offers the advantage of spatially resolved instrumental analyses is laser ablation (LA). In LA-ICPMS, a pulsed laser ablates a small portion of a solid sample and the ablated plume is swept with
* Corresponding author. Tel.: +1-573-8822240; fax: +1-5738822754. E-mail address: [email protected] (J.D. Robertson).
carrier gas into the ICP torch. This technique can provide rapid, multi-elemental and isotopic analyses with sub lg/g sensitivities at a spatial resolution of 5 lm. A key challenge in the application of LA-ICP-MS to archaeometry is, however, the development of accurate quantification procedures, especially for highly heterogeneous materials like clays and ceramics. In LA-ICP-MS, signal intensities depend strongly on the sample matrix and instrumental drift. It is, therefore, important that the calibration standard is as similar to the sample as possible [4]. For instrumental drift, internal standards can be added through a dual sample introduction system in which the ablated aliquot is simultaneously calibrated against an aerosol from an aqueous standard. While this procedure provides a continuous, real-time correction for instrument drift, the presence of oxides and hydroxides from the ‘wet’ plasma create numerous interferences [5].
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 1 0 9 3 - X
J.D. Robertson et al. / Nucl. Instr. and Meth. in Phys. Res. B 189 (2002) 378–381
Alternatively, if one or more elements can be determined (or assumed independently), then these can serve as quasi-internal standards to correct for instrumental drift between the analysis of the samples and standards. For example, rhyolitic obsidian has relatively consistent silica concentrations and we have found that ratios of count rates to the silicon count rate yields normalized values that can be calibrated with external standards so as to yield concentrations that are in good agreement with neutron activation analysis (NAA) and photon-induced X-ray fluorescence measurements of the obsidian. One distinct advantage of LA-ICP-MS over bulk techniques is the potential to perform spot analysis of spatially segregated components in artifact fabrics. In ceramics, separate analyses can be obtained for individual temper grains and/or for areas of clay matrix that contain no temper grains. Moreover, because each pass of the laser only ablates microns of material, the technique can also be used to probe the composition of slipped and pigmented surfaces independent of the bulk ceramic. However, in contrast to materials like obsidian, the internal heterogeneity of clays and the diversity of materials used in ceramic manufacture preclude assuming a value for any single component such as aluminum or silicon. As a result, application of LA-ICP-MS to ceramic analysis will require the development of quantification procedures that can account for both instrumental drift and large variations in the sample matrix at the microscopic level. In this work we report on our initial laser ablation studies of ceramics and the potential for using PIXE as a non-destructive method for generating a suite of ‘‘microstandards’’ for LA-ICP-MS.
2. Experimental The LA-ICP-MS measurements are made using a 213 nm laser ablation system (Merchantek) and a VG Axiom High-Resolution ICP-MS. For this initial work, the ablation was performed at a spot size of 100 lm, a laser power of 60%, and a laser pulse frequency of 20 Hz. The carrier gas, which flows through the ablation cell into the ICP torch,
379
is a mixture of Ar (0.95 L/m) and N2 (0.3 L/m). During the course of this work we found that a much more stable signal could be achieved by passing the carrier gas through a bead-impact spray chamber prior to the ICP torch. Our hypothesis is that the spray chamber averages out the ablation signal and removes the large particles from the ablation plume that tend to make the plasma less stable (briefly cool the plasma). Details of the system used to obtain the PIXE results described in this paper are given in [6]. Briefly, a dual-energy, external-beam irradiation is performed in order to obtain information for both the low- and high-Z elements in the matrix. For 10 min of the 15-min irradiation, the detector is placed 3.5 cm from the target, a 1900 lm-thick polypropylene filter is put in place and the sample is irradiated with a 2.1 MeV proton beam. For the remaining 5 min of the analysis, the filter is removed, the detector is moved 7.5 cm from the target, and the sample is irradiated with a 1.6 MeV proton beam. At each energy, a relative measure of current is made by measuring the number of protons that backscatter from the Kapton exit window in the target chamber. The two spectra are then combined and the data is analyzed with a modified version of the G U P I X software package [7]. The system is calibrated with a series of thin film gravimetric standards from MicroMatter Inc.
3. Results and discussion The LA-ICP-MS quantification procedure that we have employed thus far is to normalize the signal (count rates) for each element from a spot analysis so that they sum to a single standard value (e.g. 1 million) for all standards and unknowns. A regression of the normalized counts on the elemental concentration in the standards then yields a calibration equation that can be used to calculate concentrations in the unknowns. The basic assumption of this approach is that the 43 elements being measured represent all of the material in the ceramic, other than oxygen, that is ablated in the analysis. Incorporation of corrections for isotopic abundance and for differences in resolving power (transmission through the instrument) had little
380
J.D. Robertson et al. / Nucl. Instr. and Meth. in Phys. Res. B 189 (2002) 378–381
Fig. 1. Average (n ¼ 23) ratio of LA-ICP-MS to NAA measurements for Ohio Red clay. The dashed lines are 1 standard deviation.
effect on the calculated concentrations. The results of using this approach for the analysis of Ohio Red clay, a quality control standard in activation analysis of ceramics [8], are shown in Fig. 1. Averages of multiple runs of two glass standards, NIST-SRM612 and Glass Buttes obsidian, were used to calculate the average (n ¼ 23) concentrations shown. It is surprising that, given the significant difference between the clay matrix of the ‘‘unknown’’ and the glass matrix of the standards, that the ratio of the LA-ICP-MS measurements to the NAA concentrations hovers between 0.75 and 1.25, on average, for most elements. Key future measurements will be aimed at determining how well the normalization procedure performs with better-matched standards. The one-standard deviation ranges for some of the elements in Fig. 1 are substantial (>70%). Because the relative standard deviations of the silicon-normalized signals for 43 elements from the NIST-SRM612 standard over the course of one month are much smaller (the average RSD of 15 measurements was 17%), we attribute the large deviations in the Ohio Red clay to sample heterogeneity at the microscopic level. This large variation demonstrates that the standards used to test the accuracy of LA-ICP-MS of ceramics and clays will have to be analyzed with a microprobe of similar dimensions. In contrast to LA-ICP-MS, the matrix effects associated with PIXE analysis are well understood and the technique has been successfully applied to the analysis of ceramics (see e.g. [9–12]). As a result, PIXE can provide accurate analytical results in widely varying matrices without the use of
‘‘well-matched’’ standards. For example, the PIXE results presented in Tables 1 and 2 for pressed pellets of two very different matrices, NISTSRM1633a Coal Fly Ash and NIST-SRM1577 Bovine Liver, were acquired with the same calibration curve. The calibration curve for these thick-target analyses was obtained by the analysis of thin film gravimetric standards. These results demonstrate that micro-PIXE could be used to Table 1 Results of the PIXE analysis of NBS SRM-1633a Coal Fly Ash Elementa
PIXEb
Certifiedc
% Difference
Mg (%) Al (%) Si (%) K (%) Ca (%) Ti (%) V Cr Mn Fe (%) Ni Cu Zn Ga Ge As Sr Zr Rb Pb
0:51 0:18 14:6 0:9 22:6 1:5 2:01 0:14 1:16 0:08 0:85 0:05 296 69 186 17 200 12 9:4 0:1 130 15 117 13 225 21 58 9 24 7 130 32 855 83 300 49 131 13 72 12
0.46 14.3 22.8 1.88 1.11 (0.82) 297 196 179 9.4 127 118 220 (56) (34) 145 830 (330) 131 72.4
10.9 2.1 0.9 6.9 4.5 3.6 0.3 5.1 11.7 0.0 2.4 0.8 2.3 3.6 29.4 10.3 3.0 9.1 0.0 0.6
a
In lg/g unless otherwise noted. Average and standard deviation of the analysis of three samples. c Values given in brackets are recommended or consensus value. b
J.D. Robertson et al. / Nucl. Instr. and Meth. in Phys. Res. B 189 (2002) 378–381 Table 2 Results of the PIXE analysis of NBS-SRM1577 Bovine Liver Elementa
PIXEb
Certifiedc
% Difference
Na (%) P (%) S (%) Cl (%) K (%) Ca Mn Fe Cu Zn Se Br Rb
0:27 0:03 1:17 0:08 0:72 0:05 0:27 0:02 1:00 0:07 164 15 10:6 0:5 264 15 191 12 141 9 1:2 0:3 8:2 1:1 16:6 1:4
0.243 (1.13) (0.79) (0.27) 0.97 124 10.3 268 193 130 1.1 (9.1) 18.3
11.1 3.4 8.9 0.0 3.1 32 2.9 1.5 1.0 8.5 9.1 9.9 9.2
a
In lg/g unless otherwise noted. Average and standard deviation of the analysis of seven samples. c Values given in brackets are recommended or consensus value.
381
ergies and filters or in which two detectors are used to record the full PIXE spectrum simultaneously. In the future, we plan to perform a series of microPIXE analyses of ceramics and clays at the University of Guelph proton microprobe to develop a set of standards for our LA-ICP-MS work. Acknowledgements The National Science Foundation (SBR9802366) and the University of Missouri provided funding for this work. References
b
accurately quantify microanalysis of ceramics and clays where, for example, the concentration of Ca can vary from 100 lg/g to 10 wt.% from one spot to another. It should be stressed, however, that accurate micro-PIXE analysis of ceramic and clay materials to be used as standards will require the measurement of both the low- and high-Z elements at each spot. Using again the example of Ca variations, the results for Fe in NIST-SRM1413 High Alumina Sand change by nearly 10% if Ca is left out of the matrix in the PIXE analysis. This analytical bias can be readily overcome with a complete micro-PIXE analysis in which the same spot is analyzed multiple times with different en-
[1] D.J. Kennett, H. Neff, M.D. Glascock, A.Z. Mason, The SAA Archaeol. Record 1 (2001) 22. [2] L.M. Mallory-Greenough, J.D. Greenough, J.V. Owen, J. Archaeol. Sci. 25 (1998) 85. [3] A.M. Pollard, C. Heron, Archaeological Chemistry, The Royal Society of Chemistry, Cambridge, UK, 1996. [4] A.J. Campbell, M. Humayun, Anal. Chem. 71 (1999) 939. [5] S.F. Durrant, J. Anal. Atom Spectrom. 14 (1999) 1385. [6] L.J. Blanchard, J.D. Robertson, S. Srikantapura, B.K. Parekh, J. Radioanal. Nucl. Chem. 221 (1997) 23. [7] J.A. Maxwell, W.J. Teesdale, J.L. Campbell, Nucl. Instr. and Meth. B 94 (1994) 172. [8] J.W. Cogswell, H. Neff, M.D. Glascock, Am. Antiquity 63 (1998) 63. [9] C.P. Swann, C.H. Nelson, Nucl. Instr. and Meth. B 161– 163 (2000) 694. [10] E. Aloupi, A.G. Karydas, T. Paradellis, X-Ray Spectrom. 29 (2000) 18. [11] C.P. Swann, S. Caspi, J. Carlson, Nucl. Instr. and Meth. B 150 (1999) 571. [12] G. Lagarde, I. Brissaud, J. Cailleret, P. Fontes, C. Heitz, Nucl. Instr. and Meth. 229 (1984) 45.