Nuclear microscopy of inhomogeneous thick samples

Nuclear Instruments North-Holland

and Methods

in Physics

Research

353

B54 (1991) 353-362

Nuclear microscopy of inhomogeneous thick samples G.W. Grime a, F. Watt a, A.R. Duval b and M. Menu b p University of Oxford Department of Nuclear Physics, Oxford OX1 3RH, UK b Laboratoire

de Recherche des M&es

de France, Louvre Museum,

75041, Paris, France

The long range of MeV protons in matter means that PIXE analysis of inhomogenous thick samples is subject to uncertainty if the elemental composition changes with depth. Samples which are particularly affected by this include rock and mineral samples and material of archaeological interest, such as pigments. In this study, elemental mapping of lead-tin yellow pigment grains using electron probe microanalysis (EPMA) is compared with PIXE using microfocused 2 and 3 MeV protons. It is found that, although well-defined grains can be seen using EPMA, which is sensitive only to a thin surface layer, the structure is lost using PIXE, because of Pb or Sn X-rays induced from deep within the sample. Simultaneous mapping using backscattered particles (BS) allows the grains to be seen clearly, and by taking different energy slices from the BS energy spectra the grains can be mapped at different depths below the surface. It is concluded that simultaneous BS analysis is essential when carrying out PIXE analysis of thick inhomogeneous samples to confirm that the sample is uniform at the point where the spectrum is taken. This is illustrated using a point analysis of a thin lead-tin grain overlying a region of organic material.

1. Introduction Nuclear microscopy using MeV light ions focused to submicron dimensions is capable of determining the distribution of elements within a sample with high sensitivity and speed [l]. Using a variety of ion-solid interactions (most commonly PIXE, nuclear elastic backscattering and nuclear reactions), elements throughout the periodic table can be mapped and their concentrations at specific points determined. One potential drawback of this technique is the relatively long penetration depth of MeV ions, which means that, if the composition of the sample is not constant with depth, the concentrations determined from the measurement will not be accurate. This problem is most acute in PIXE analysis of samples where the matrix absorbs the emitted X-rays strongly, and the assumption of sample homogeneity is essential to the accuracy of the results. Such samples include rocks and minerals with zone structure or inclusions and materials of archaeological interest such as ceramics and pigments. In this paper we compare the analysis of lead-tin yellow pigment using energy dispersed electron probe microanalysis (EPMA) with nuclear microscopy using PIXE and RBS.

2. Lead-tin

yellows

Painting layers are made up of grains of pigment ranging in size from 1 to 10 urn suspended in an organic 0168-583X/91/.$03.50

0 1991 - Elsevier

Science Publishers

binder such as glue or oil. Lead-tin yellow is a synthetic pigment which was used in easel paintings from the 14th up to the 18th century. Two varieties were identified by Kuhn [2] in 1968. Until recently they were distinguished solely by their crystal structure; PbSnO, with a tetragonal structure is called variety 1 because it is more commonly encountered. PbSnO, or Pb(Sn,Si)O,, with a cubic structure, is variety 2. It is now known that these two varieties can be distinguished by the presence of silicon in variety 2, and EPMA on cross sections can be used for this purpose. Furthermore, images obtained with the scanning electron microscope give additional information; grains of variety 1 are homogeneous in composition while grains of variety 2 contain inclusions of Pb-Sn in a Pb-Si matrix. [3]. In this research approximately 100 samples of lead-tin yellow taken from paintings carried out mainly in Italy before the Renaissance were analysed. It was found that, at least on the Italian peninsula, lead-tin yellow used as a pigment in the 14th and early 15th centuries corresponds to variety 2, which was replaced by variety 1 in the second quarter of the 15th century. The technique most commonly used for analysing individual pigment grains is EPMA. Although this has a spatial resolution capable of distinguishing small grains, the limits of detection for trace elements are high, and many elements of.interest are not detectable. In order to evaluate the potential of exploiting the higher sensitivity of nuclear microscopy to carry out the analysis of the pigment grains, a sample of lead-tin yellow was analysed both by EPMA at the Laboratoire de Recherche

B.V. (North-Holland)

IX. ARTS/ARCHAEOLOGY

354

G. W. Grime et al. / Nuclear microscgopy of inhomogeneous thick samples

des Mu&es de France (LRMF) in Paris and using the scanning proton microprobe facility (SPM) at Oxford. It was found that the inhomogeneous nature of the sample leads to difficulties in interpreting the results from SPM analysis. The sample was prepared by removing a small fragment of pigment from the painting. This fragment was embedded in a resin block and polished normal to the paint surface, so that the different paint layers are

visible. The sample was coated with a thin evaporated carbon layer to avoid charging effects. Fig. la shows a scanning electron micrograph of a flake of paint from a painting by Zanino di Pietro (ca. 1410) showing the different layers. Fig. lb shows an enlargement of part of the lead-tin yellow layer indicated in fig. la. The grain structure of the pigment.can be clearly seen. EPMA elemental maps of Pb and Sn were accumulated using the Lo X-ray lines of Pb and

Fig. 1. Zanino di Pietro, Polyptic, Venice ca. 1410. (a) Cross section of pigment layers. @EM with backscattered electrons.) (b) Cross sectiIon of lead-tin yellow pigment (region indicated in (a)). The arrow indicates the small Pb/Sn grain selected for analysis. (SEM with backscattered electrons.) (c) Pb Lcr, (d) Sn La EPMA maps of the large grain in (b); scan size 15 X 15 pm.

G. W. Grime et al. / Nuclear microscopy of inhomogeneous thick samples

Fig. 1. (continued),

Sn. These are shown in figs. lc and Id. The maps show a region of 15 X 15 pm cont~ing one large grain of lead oxide and smaller grains of tin oxides. The compositional analysis determined by EPMA for the point indicated in fig. lb is given in table 1. No other elements were detected.

3. PIXE mapping The same sample was analysed using the Oxford scanning proton microprobe [4] using both PIXE and RBS with protons of 2 and 3 MeV. The beam current used was 150-200 pA with a spatial resolution of 0.5 pm. The X-ray detector was a Link Systems Si(Li) detector of 80 mm2 active area and 140 eV resolution at 5.9 keV. The detector was located 17 mm from the sample giving a solid angle of 277 msr. A filter of 250 pm beryllium was fitted to reduce Si and Pb Mtx pileup effects in the region of the spectrum between Ca and Fe. The presence of the filter meant that Si could not be determined accurately in this experiment and Si maps were not recorded. Backscattered protons were detected using three surface barrier detectors with a total surface

Table 1 Composition of Pb/Sn grain determined by EPMA (electron energy: 20 keV, detector: EGG-Ortec Si-Li with 7.5 pm Be window) Hement

wt.% oxide

wt.% element

SiO, PbO SnOz

13 72 15

6 66 11

area of 700 mm’ mounted at angles ranging from 140° to 160 o to the incident beam. Elemental maps of lead and tin were obtained using the Pb M and La X-ray lines and the Sn La and Ka lines using protons of both 2 and 3 MeV. The elemental maps are shown in fig. 2. These cover the same region as fig. lb, but it will be seen that little of the structure seen in the EPMA maps is visible. The lack of structure is due to the long penetration depth of the protons which means that underlying grains are sampled by the beam and contribute an X-ray signal depending on the absorption of the X-rays in the sample. The penetration depths of 20 keV electrons and 2 and 3 MeV protons in lead are given in table 2 together with the depths from which 90% of the proton-induced X-rays are generated. (The values for protons were calculated using the program GUYLD [5] and the electron penetration depth was obtained from ref. [6].) The figures show that for PIXE the sampling depth is limited only by X-ray transmission (varying from 0.9 pm for Sn

Table 2 Penetration depth and 90% production depth of Pb and Sn X-rays for electrons, 2 MeV protons and 3 MeV protons in lead

Penetration depth [pm] Depth for 90% yield of Sn Lcu Sn Ka Pb M PbLa

Electrons 20 keV

2 MeV

Protons 3 MeV

1.3

21

49

0.9 10 I.5 8

0.9 16 1.5 10

IX. ARTS/ARCHAEOLOGY

356

G. W. Grime et al. / Nuclear microscopy of inhomogeneous thick samples

Fig. 2. PIXE elemental maps of the region shown in fig. lb. Pb M (Y,Pb La, Sn La, Sn Ka at 2 MeV. Scan size 40 x 40 pm. La to 16 pm for Sn Ka), whereas for EPMA sampling depth is limited by electron penetration < 1.3 pm.

the to

4. RBS mapping In order to obtain information on the depth structure of the sample, the energy of the backscattered

protons was measured simultaneously with the PIXE signal. Because the sample is predominantly lead, the RBS spectrum (shown in fig. 3 for 2 MeV protons) is relatively simple, and the region of the spectrum between the lead surface energy and the oxygen edge is due predominantly to scattering from lead at different depths within the sample. (A depth scale for lead has been added to fig. 3.) By gating at different energies on the RBS spectra, maps of lead at different depths can

G. W. Grime et al. / Nuclear microscopy of inhomogeneous thick samples

Fig. 2. (continued)

PIXE elemental maps of the region shown in fig. lb. Pb Ma, Pb Lq Sn La, Sn Ka at 3 MeV. Scan size 40x40 pm.

These are shown in fig. 4, where four be obtained. difl Ferent depth slices at 2 and 3 MeV are shown. The three-dimensional grain structure is now clearly ble, and the three grains visible in the EPMA maps seen to have depths in the range of 3-5 urn. The

closest similarity to the EPMA Pb map is obtained with the 0.4-1.2 pm depth slice, confirming the rang ;e of electron penetration given in table 2. The longest r ange of depth analysis is obtained with 3 MeV protons, and in the 5.3-9 Pm slice all three grains have disappe; ared. IX. ARTS/ARCHAEOLOGY

G. W. Grime et al. / Nuclear microscopy of inhomogeneous thick samples

Proton energy (keV) Fig. 3. Energy spectrum of 3 MeV protons backscattered from the paint layer for the scanned area shown in the 3 MeV maps of fig. 2. Markers indicate the surface energy of Pb and 0 and a depth scale for lead has been added. Also shown are the energy windows (1 to 4) used to generate the 3 MeV maps in fig. 4. A similar spectrum was obtained for 2 MeV protons.

The material surrounding the grains is the organic binding medium, which does not contribute any signal in the energy range defined for these maps.

5. Analysis of the lead-tin grain

Table 3 Composition of Pb/Sn layer determined by BS simulation at 3 MeV. Chemical composition: PbSn,.,Si,O,,,; thickness of the layer: 1 mg cmW2

could not be achieved, presumably because the sample structure is more complex than assumed, the results indicate that the majority of the Pb and Sn is in a layer with a composition of approximately PbSn0,2Si203,3 and thickness of 1 mg cmm2 overlying a substrate of organic material. The major and trace elemental concentrations were

Element

wt.%

Pb Sn Si 0

61 7 11 15

In order to analyse the small grain of lead-tin oxide, the proton beam was held stationary at the point indicated in fig. lb. The resulting PIXE and BS spectra obtained with 3 MeV protons are shown in fig. 5. The BS spectrum was simulated using a modified version of the program RUMP [7]. Although a perfect simulation

Composition of Pb/Sn layer determined by PIXE at 2 and 3 MeV assuming the matrix composition derived from RBS. Thick target yields calculated for a) a thin layer of 1 mg cm-’ and b) an infinitely thick sample. Values are presented as percentage by weight with minimum detectable limits in ppm in parentheses. Concentrations are normal&d to Pb = 61% in each case. Collected charge: for 2 MeV analysis Q = 0.024 PC and for 3 MeV analysis Q = 0.077 PC Element

Si Cd (La) Sn (Lo.) Ca Fe cu Zn Pb (La) Sn (Ko)

2 MeV

3 MeV

thin

thick

thin

thick

9.60 (2300) 1.10 ( 540) 3.60 ( 700) 0.239 ( 250) 0.458 ( 50) Q.061 ( 70) 0.035 ( 90) 61.0 (4500) 3.07 (3500)

31.0 (7500) 3.70 (1800) 11.1 (2100) 0.695 ( 740) 1.80 ( 50) 0.075 ( 100) 0.040 ( 100) 61.0 (4500) 3.01 (3500)

4.89 (1400) 0.722 ( 420) 2.60 ( 360) 0.046 ( 100) 0.135 ( 25) 0.061 ( 100) 0.019 ( 100) 61.0 (2700) 7.37 (1100)

24.1 (7300) 3.65 (1500) 12.0 (1700) 0.186 ( 440) 0.271 ( 60) 0.088 ( 50) 0.025 ( 150) 61.0 (2700) 6.05 ( 950)

G. W. Grime et al. / Nuclear microscopy of inhomogeneous thick samples

Fig. 4. Backscattered proton maps corresponding to lead at different depths within the sample. 2 MeV incident beam energy. Scan size 40x40 pm. derived from the PIXE spectrum with thick-target yields cafculated using PIXAN [8] for two cases: a) assuming a layer of thickness and composition derived from the BS simulation and b) assuming an infinitely thick target of the same chemical composition. It is also assumed that the trace elements are present only in the lead-tin silicate component of the sample. The results are listed in table 3 for both 2 and 3 MeV protons. Because

charge collection from the surface of the sample is not accurate due to secondary electron emission, the concentrations were normalised to Pb = 61%. It is seen that the discrepancy between the thin and thick target results increases at low X-ray energies (earlier entries in the table). This is due to the increased absorption of low-energy X-rays in the sample. The discrepancy between the 2 MeV and 3 MeV results is IX. ARTS/AR~~EOLO~Y

G. W. Grime et al. / Nuclear microscopy of inhomogeneous thick samples

Fig. 4. (continued) Backscattered proton maps corresponding to lead at different depths within the sample. 3 MeV incident beam energy. Scan size 40 X 40 pm. assumed to be due to Pb X-rays from underlying layers which are excited more efficiently at 3 MeV but are not included in the model used in the calculation. The most accurate set of results is assumed to be from the 2 MeV thin-layer calculations. In this experiment it is likely that an improvement in accuracy could have been achieved using 1 MeV protons, which have a range of

10 pm in Pb, although production cross section sensitivity. It will be seen that a detected which were not sis, such as Cd, Fe, Cu limits are in the range

the conco~tant fall in X-ray would lead to a much reduced number of trace elements were measured in the EPMA analyand Zn. Minimum detectable of 20-100 ppm. These trace

G. W. Grime et al. / Nuclear microscopy of inhomogeneous thick samples

X - ray energy

361

keV

2500.0

2250.0

2750.0

3000.0

ProtonenergykeV Fig. 5 . (a) PIXE spectrum and (b) backscattered proton energy spectrum from the point indicated in fig. lb. Incident proton energy 3 MeV, total charge 0.077 pC. PIXE detector filtered with 250 Pm Be. The BS energy spectrum shows a simulation for a 1 mg cm-* layer with composition PbSn0.,Si20j,.1 overlying a thick organic substrate with composition C,O.

elements may be of importance in the study of lead-tin yellows if they can be quantified accurately, but additional measurements, such as analysis at 1 MeV, or separate analysis of the organic binder would be required to confirm their location in the pigment.

6. Conclusion

Grains of pigment in a sample of lead-tin yellow were mapped using EPMA and PIXE. Although well defined lead-oxide maps, no structure

grains were visible in the EPMA was visible in the PIXE maps due to

the long penetration of the, protons, which produce a signal from underlying grams. The depth structure of the grams was visualised using the backscattered proton

signal. By selecting backscattered protons of different energies, maps of lead at increasing depth could be obtained and the shape of the grains could be observed. A point analysis was carried out on a region of the sample consisting of a thin lead-tin silicate grain overlying the organic matrix. Trace elements were detected which were not observed in the EPMA analysis, but these could not be quantified accurately because of uncertainties in the structure of the sample. Using a simple model of the sample composition derived from the BS data allowed the X-ray yield to be calculated and approximate trace element concentrations measured. It is concluded that PIXE analysis of thick samples should always be carried out in conjunction with BS analysis, so that the depth uniformity of the sample can IX. ARTS/ARCHAEOLOGY

362

G. W, Grime et al. / Nuclear microscopy of inhomogeneous thick samples

be confirmed. This also permits nondestructive three-dimensional mapping of elements in thick inhomogeneous SiiIX@ZS.

Acknowledgements

The authors wish to acknowledge the support of the Welcome Trust in setting up the scanning microprobe facility. They also wish to acknowledge the assistance of E. Martin of LRMF in carrying out this work,

References [l] G.W. Grime and F. Watt, Proc. ECAART Conf., Frankfurt, 1989, Nucl. Instr. and Meth. B50 (1990) 197.

[2] H. Kuhn, Studies in Conservation 13 (1969) 7. [3] E. Martin and A.R. Duval, Le jaune de plomb et detain et ses deux variet~s. Utikation en Italie aux XIVe et XVe siecles, 2eme Conf. Int. sur les essais non-destructifs, les methodes de microanalyse et les enqu&tes sur le milieu envirronant pour l’ttude et la conservation des oeuvres d’art, P&rouse, 1988. (41 G.W. Grime et al., these Proceedings (2nd Int. Conf. on Nuclear Microprobe Technology and Applications, Melbourne, Australia, 1990) Nucl. Instr. and Meth. B54 (1991) 52. [5] J.A. Maxwell, J.L. Campbell and W.J. Teesdale, Nucl. Instr. and Meth. B43 (1988) 218. [6] K. Kanaya and S. Ono, in: Electron Beam Interactions with Solids, eds. D.F. Kyser et al. (SEM Inc., Chicago, 1982) p. 69. [7] L.R. Doolittle, Nucl. Instr. and Metb. B9 (1985) 344. [8] E.J. Clayton, Australian Atomic Energy Commission Report AAEC/Ml 13 (1986).