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X-ray fluorescence analysis of cultural artefacts — Applications to the Czech heritage
Radiation Physics and Chemistry 95 (2014) 381–384
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
X-ray fluorescence analysis of cultural artefacts — Applications to the Czech heritage ˇ ´k T. Trojek, L. Musı´lek n, T. Cecha ´ 7, 115 19 Praha 1, Czech Republic Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering, Bˇrehova
H I G H L I G H T S c c c
X-ray fluorescence analysis as a non-destructive tool for investigating various artefacts. Improving instrumentation and experimental techniques for analysis. Extending the measurable range to lighter elements and decreasing the detection limits.
a r t i c l e i n f o
abstract
Article history: Received 30 September 2012 Accepted 24 January 2013 Available online 31 January 2013
X-ray florescence analysis is an excellent non-destructive tool for analysing the elemental composition of materials in a wide range of works of art. The Department of Dosimetry and Application of Ionising Radiation at CTU-FNSPE has used radionuclide or X-ray tube excited energy dispersive X-ray fluorescence for many kinds of artefacts, including frescos, paintings, manuscripts, metal sculptures and other objects, ceramics, jewellery, various archaeological finds, etc. The method used is more or less ‘‘traditional’’, i.e., semiconductor spectrometry of excited X-rays, with some optional choices—capillary optics for collimation of exciting beams and two-dimensional scanning. The ‘‘hardware’’ complex is supplemented by techniques for estimating the depth distribution of measured elements, for suppressing surface effects, for in situ non-contact measurements, etc. Extending the measurable range to lighter elements and decreasing the detection limits is one of the achievements that has been attained by improving the instrumentation and techniques that are used. This paper gives a brief review of works carried out at the Department of Dosimetry and Application of Ionising Radiation at CTU-FNSPE. & 2013 Elsevier Ltd. All rights reserved.
Keywords: X-ray fluorescence analysis Cultural heritage Improving instrumentation Extending detection limits
1. Introduction X-ray florescence analysis (XRFA) is an excellent nondestructive tool for analysing the elemental composition of materials in a wide range of works of art, and can serve for identifying these materials (e.g., Ferreti, 2000; Milazzo, 2004). The information that is obtained can be used for determining the origin of an artefact, for identifying the production technology, for informing restorers and preservers, recognising imitations and counterfeits, etc. The whole line of work needs deep collaboration among radiation physicists, who carry out the measurements and calculate content of the various elements, and restorers and art historians, who use and interpret the results (e.g., Karydas et al., 2005).
n
Corresponding author. Tel.: þ420 2243 58247. E-mail address: musilek@fjfi.cvut.cz (L. Musı´lek).
0969-806X/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2013.01.035
The Department of Dosimetry and Application of Ionising Radiation (DDAIR) of CTU-FNSPE has used radionuclide or X-ray tube excited energy dispersive X-ray fluorescence for many kinds of works of art, including frescos, paintings, manuscripts, metal sculptures and other metal objects, ceramics, jewellery, various archaeological finds, etc. Collaboration with institutions traditionally associated with the humanities has therefore been necessary. These institutions supply artefacts for measurements and introduce us to problems that are to be solved. At the output end of the whole process, they must interpret the results in the language of archaeology or cultural history. This paper gives a brief review of works carried out at DDAIR. The method used for measurement is more or less traditional, i.e., semiconductor spectrometry of X-rays excited by radionuclide sources or X-ray tubes, with some optional choices — capillary optics for collimation of exciting beams and twodimensional scanning. The key problem with XRFA is the low depth range of the measurements, depending moreover on the energy of the exciting radiation and the atomic number Z of the
382
T. Trojek et al. / Radiation Physics and Chemistry 95 (2014) 381–384
detected element. The ‘‘hardware’’ complex is therefore supplemented by techniques for estimating the depth distribution of the measured elements, for suppressing surface effects, for in situ non-contact measurements, etc. Extending the measurable range to lighter elements (the third period of the periodic system) and decreasing the detection limits to units of ppm are important achievements attained by improving the instrumentation and techniques that are used. The variety of possible applications can thus be continuously extended.
2. Development of instrumentation In the course of time, five generations of XRFA instruments have been developed at DDAIR. Some commercial components have been used in constructing the instruments, but the whole concept has been of our own design, as our requirements have differed from those of compact commercial analysers for in situ measurements. The advantages of this approach are as follows:
Flexibility — our instruments allow more changes of measure
ment techniques, geometrical arrangement and evaluation than commercial instruments. Non-contact measurement — the distance from the surface of measured objects can be adjusted in the range of millimetres to centimetres. Price — commercial instruments cost considerably more than home-made equipment. In addition, the radiation sources and semiconductor detectors of our instruments can be taken out and used for other purposes. Educational value — being a university department, we need to familiarise students with various measurement methods through experiments. Commercial instruments are usually ‘‘black boxes’’, and do not allow a deeper view into how the measurement and evaluation are done.
The first analyser (marked as No. 1 in Table 1), assembled at the end of the 1990s, consisted of radionuclide sources (55Fe, 238 Pu, and in rare cases 241Am were used) and a liquid nitrogen cooled Si(Li) X-ray detector (effective diameter 6 mm, thickness 5 mm, and 25.4 mm beryllium window). The beam of the exciting radiation was collimated to an area approximately 9–15 mm in diameter. A small Si-PIN diode with a Peltier cooler replaced the Si(Li) detector in the second version of the analyser (No. 2). A ring radionuclide source was used and the collimator was removed to minimise the sources of parasitic radiation. The resulting compact analyser was advantageous for measurements in situ, but only for Table 1 Summary of basic parameters of the XRFA devices at DDAIR FNSPE. Number of device
Analysed area [mm2]
DL for calcium [ppm]
DL for zinc [ppm]
DL for lead [ppm]
1a 2b 3c 3d 4
180 1400 0.7 60 Up from 4 10 4 20
1500 1000 719 151 58
58 21 78 13 16
67 26 169 26 40
2
9
5 a
18
With a 238Pu radiation source (1.11 GBq) and collimator diameter 10 mm. With a 238Pu radiation source (1.11 GBq). c With the narrowest collimator. d With the widest collimator. b
large homogeneous areas, due to the large irradiated spot on the measured object. At about the same time, about 10 years ago, a third analyser (No. 3) was constructed with a small X-ray tube (molybdenum anode, 30 kV, 100 mA). The much higher flux of the exciting photons in comparison with radionuclide sources allowed narrow collimation of the beam with satisfactory excitation effectiveness. The smallest used area was 0.7 cm2. However, with increasing popularity of the method among Czech art conservers and historians, new tasks have appeared, requiring even higher space resolution (e.g., manuscripts). A fourth apparatus (No. 4) was therefore built with a more powerful X-ray tube (50 kV, 1 mA) and with capillary X-ray optics, focusing the beam on a diameter of less than 20 mm at a distance of 4 mm from the end of the capillary. The size of the spot can easily be increased by changing the distance of the measured object from the focus. Neither radionuclide sources (except 55Fe) nor X-ray tubes with a molybdenum anode are optimal for exciting the lower Z elements (approximately with Z o20). The fifth, and latest apparatus (No. 5) uses a small MINI-X self-contained X-ray tube system, integrated with the power supply and the control electronics, with a gold transmission target (L-lines of Au are significantly present in the spectrum). It can be operated at voltages from 10 to 40 kV and at currents from 5 to 200 mA (power not exceeding 4 W). The silicon drift detector with a thin Be window (12.7 mm) has the optimum detection range from 1 to 40 keV. This means that K-lines starting from sodium can be detected. This combination of a low-energy X-ray source and detector extends the possibilities of our laboratory for measurements in the direction towards lighter elements. The key parameters of all devices are summarised in Table 1. The detection limits in the table (DL) are values for which analytical signals differ significantly from the background. They are determined in such a way that the measured signal is larger than three times the square root of the background. They are determined for the reference NIST 1412 multicomponent glass, and the measurement time is 10 min. The values can differ somewhat for different materials, due to the influence of the different composition of their matrices. For example, they increase for some metal alloys due to the presence of high Z elements, and they decrease (sometimes even by one order of magnitude) for elements in light matrices (samples of organic origin). For comparison, the oldest device (No. 1) and the newest device (No. 5) are shown in Fig. 1.
3. Techniques for refining and extending what can be measured Investigations of various artefacts are a multi-faced problem, and no simple and routine approach can be formed. Analyses of large homogeneous areas of pigments on frescos are very different from analyses of tiny lines of pigments on manuscripts, metal statues differ from ceramics, etc. It has therefore been necessary to supplement the variety of instruments with a variety of methods for measuring and processing the output signal. Above all, computer simulations have become a very useful tool for testing and optimising measurements (see, e.g., Trojek and ˇ ´ k, 2007). Though some analytical approaches and formulas Cecha have been published, they cannot include all the complicated boundary conditions, e.g. the dimensions of the radiation source, the production of secondary characteristic radiation, the influence of scattered radiation, the shape and the inhomogeneities of a measured object, surface roughness, etc. Monte Carlo calculation is the
T. Trojek et al. / Radiation Physics and Chemistry 95 (2014) 381–384
Fig. 1. The first apparatus (above) and the newest apparatus (below) used at FNSPE for XRFA of cultural heritage.
most versatile tool for describing various configurations of experiments, providing results that correspond best with the measurement results. In particular, the MCNP code has proved very useful for calculations of results allowing the geometrical configuration of the devices to be optimised, testing new ways of quantitative evaluation, and studying effects that are not pronounced in experiments. Needless to say, agreement between the results of a simulation and the results of an experiment depends significantly on the quality of the input data for the calculations, e.g., the spectra of exciting radiation, the shape and roughness of the investigated surface, the composition of the materials, etc. Simulations of various influences can determine the extent to which a change in some parameter reflects in a change in the particular peak area that serves for quantifying the concentration of the corresponding element. Our device No. 4 (with capillary X-ray optics) is designed preferentially for 2D measurements, i.e., for scanning the surfaces of measured objects and determining inhomogeneities in composition. It is therefore equipped with an automated positioning system, which scans the area of interest with steps down to 10 mm. Scanning a large object with small steps is, of course, a lengthy process, and some optimisation is needed that takes into consideration the minimum sufficient resolution. A further step leading to a deeper view into the analysed materials is 3D scanning. The depth distribution of elements is often not uniform. XRFA can estimate this distribution only in thin layers of materials, e.g., when various layers of pigments are superimposed on a painting. There are two complicated and expensive methods for mapping the depth distribution — XRF tomography and confocal XRF (see, e.g. ASTM, 1992; Fiorini et al.,
383
2001, or more recently Silversmit et al., 2009; Tsuji and Nakano, 2007). Both need a strong radiation source, ideally synchrotron radiation. Our laboratory has concentrated on simpler methods that do not enable the depth distribution to be mapped exactly, but provide information that depth inhomogeneities exist, and in some cases enable the characteristic of the lower material layer to be determined. Three techniques have been investigated for this purpose (Trojek et al., 2007, 2008). The first is based on changing the angle of the measuring head of the analyser to the surface of measured object. This involves changing the angle of penetration of the narrow exciting radiation beam into the material and changing the corresponding angle of emission of excited radiation to the detector. Simple geometry considerations show the change in the thickness of the layer from which the signal is collected, and therefore the reason, why the contribution of the signals from layers at different depths differs. Depth inhomogeneity changes the ratio of the peaks corresponding to various elements in dependence on the depth distribution of the elements. The technique can be improved by using two detectors positioned in such a way that the angles of detection of excited radiation are significantly different. The third technique is based on the different energy and thus also the different absorption of Ka and Kb lines, La and Lb lines or Ka and La lines of excited radiation. Their ratio thus depends on the depth at which the element is located. Lines with lower energy are usually more suppressed if their source is deeper (Trojek et al., 2010). Surface effects can influence the result of measurements (Trojek, 2011). One effect is curvature of the surface. Changing the angle of impact of the excitation beam on the surface changes the output signal. This influence has been intensively investigated. This fact can be used for scanning the structure of the surface of objects with a homogeneous composition. Fig. 2 shows an example of the reconstruction of the surface relief on a 1 Euro coin from an XRFA scan by a narrow X-ray beam. This is an interesting by-product of techniques designed primarily for analyses.
4. Examples of problems that are being investigated Analyses of inorganic pigments in wall paintings, illuminated manuscripts, glazes on ceramics, etc., are very rewarding. As wall paintings usually consist of large areas of homogeneous materials,
Fig. 2. Photograph (above) and reconstruction from the XRFA scan (below) of the relief of a 1 Euro coin.
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T. Trojek et al. / Radiation Physics and Chemistry 95 (2014) 381–384
it is not a problem to use devices with radionuclide sources (our Nos. 1 and 2), which are able to differentiate various pigments ˇ ´ k et al., 2001). Pigments and even close to their boundaries (Cecha inks in manuscripts with tiny lines and miniature paintings are at ˇ ´ k et al., 2010), and need a the opposite extreme (Cecha very narrow beam of exciting radiation (our devices Nos. 3 and 4). A narrow beam is also needed for 2D and 3D analyses of artefacts. Scanning can also detect migration of elements due to corrosion of inks and changes caused by past interventions of restorers. XRFA has also found a wide field of use in investigations of archaeological finds, especially ceramics and metallic objects. A good example of the possibilities and the sensitivity of these methods is analysis of the bone of the phalanx with deposited corrosion products from the ring, which has been completely desintegrated during the past centuries and has not preserved ˇ ´ k et al., 2007). (Cecha This method can be used more or less routinely, and a combination of devices with various parameters can be used in investigations of a wide range of problems that cannot be described here, due to lack of space. Finally, it can be mentioned that we have investigated some unique artefacts of Czech cultural history. The helmet of Czech patron St. Wenceslas can serve as an example. Its decoration was formed by cladding silver foil to an iron basis, but there were some theories that the silver was gilded. Estimates of the depth distribution of the gold in the silver from the ratio of the La and Lb peaks did not confirm these theories, as the ratio corresponded to the admixture of gold in the volume and there was no increased concentration of gold at the surface.
Acknowledgement The authors would like to express their thanks to all colleagues, both from DDAIR and from many collaborating institutions, who have contributed to the extensive works briefly reviewed here. Last but not least, we appreciate the financial support from
Research Plan no. MSM 6840770040 of the Ministry of Education, Youth and Sports of the Czech Republic. References ASTM, 1992. Standard guide for computed tomography (CT) imaging. ASTM designation E 1441—92a. In: 1992 Annual Book of ASTM Standards, Section 3 Metals Test Methods and Analytical Procedures. ASTM, Philadelphia, pp. 690–713. ˇ ´ k, T., et al., 2001. Analysis of fresco paintings by X-ray fluorescence method. Cecha Radiat. Phys. Chem. 61, 717–719. ˇ ´ k, T., et al., 2007. X-ray fluorescence in investigations of archaeological finds. Cecha Nucl. Instrum. Methods Phys. Res. B 263, 54–57. ˇ ´ k, T., et al., 2010. Application of X-ray fluorescence in investigations of Cecha Bohemian historical manuscripts. Appl. Radiat. Isot. 68, 875–878. Ferreti, M., 2000. X-ray fluorescence applications for the study and conservation of cultural heritage. In: Creagh, D.C., Bradley, D.A. (Eds.), Radiation in Art and Archeometry. Elsevier, Amsterdam, pp. 285–296. Fiorini, C., Longoni, A., Bjeoumikhov, A., 2001. A new detection system with polycapillary conic collimator for high-localized analysis of X-ray fluorescence emission. IEEE Trans. Nucl. Sci. 48, 268–271. Karydas, A.G., et al., 2005. Importance of in-situ EDXRF measurements in the preservation and conservation of material culture. In: Uda, M., Demortier, G., Nakai, I. (Eds.), X-rays for Archaeology. Springer, Dordrecht, pp. 27–53. Milazzo, M., 2004. Quantitative XRF analysis in archaeometric applications In: Martini, M., Milazzo, M., Piacentini, M. (Eds.), Physics Methods in Archaeometry. IOS Press, Amsterdam, pp. 227–249. Silversmit, G.B., et al., 2009. X-ray fluorescence nanotomography on cometary matter from comet 81P/Wild2 returned by stardust. Anal. Chem. 81, 6107–6112. ˇ ´ k, T., 2007. Use of MCNP code in energy dispersive X-ray Trojek, T., Cecha fluorescence. Nucl. Instrum. Methods Phys. Res. B 263, 72–75. ˇ ´ k, T., Musı´lek, L., 2007. Techniques for depth heterogeneity Trojek, T, Cecha identification in X-ray fluorescence. Nucl. Instrum. Methods Phys. Res. B 263, 76–78. ˇ ´ k, T., Musı´lek, L., 2008. Ka/Kb ratios of fluorescence X-rays as an Trojek, T, Cecha information source on the depth distribution of iron in a low Z matrix. Anal. Sci. 24, 851–854. ˇ ´ k, T., Musı´lek, L., 2010. Recognition of pigment layers in illumiTrojek, T, Cecha nated manuscripts by means of Ka/Kb and La/Lb ratios of characteristic X-rays. Appl. Radiat. Isot. 68, 871–874. Trojek, T., 2011. Reduction of surface effects and relief reconstruction in X-ray fluorescence microanalysis of metallic objects. J. Anal. A. Spectrom. 26, 1253–1257. Tsuji, K., Nakano, K., 2007. Development of confocal 3D micro-XRF spectrometer with dual Cr–Mo excitation. X-ray Spectrom. 36 (3), 146–149.
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
X-ray fluorescence analysis of cultural artefacts — Applications to the Czech heritage ˇ ´k T. Trojek, L. Musı´lek n, T. Cecha ´ 7, 115 19 Praha 1, Czech Republic Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering, Bˇrehova
H I G H L I G H T S c c c
X-ray fluorescence analysis as a non-destructive tool for investigating various artefacts. Improving instrumentation and experimental techniques for analysis. Extending the measurable range to lighter elements and decreasing the detection limits.
a r t i c l e i n f o
abstract
Article history: Received 30 September 2012 Accepted 24 January 2013 Available online 31 January 2013
X-ray florescence analysis is an excellent non-destructive tool for analysing the elemental composition of materials in a wide range of works of art. The Department of Dosimetry and Application of Ionising Radiation at CTU-FNSPE has used radionuclide or X-ray tube excited energy dispersive X-ray fluorescence for many kinds of artefacts, including frescos, paintings, manuscripts, metal sculptures and other objects, ceramics, jewellery, various archaeological finds, etc. The method used is more or less ‘‘traditional’’, i.e., semiconductor spectrometry of excited X-rays, with some optional choices—capillary optics for collimation of exciting beams and two-dimensional scanning. The ‘‘hardware’’ complex is supplemented by techniques for estimating the depth distribution of measured elements, for suppressing surface effects, for in situ non-contact measurements, etc. Extending the measurable range to lighter elements and decreasing the detection limits is one of the achievements that has been attained by improving the instrumentation and techniques that are used. This paper gives a brief review of works carried out at the Department of Dosimetry and Application of Ionising Radiation at CTU-FNSPE. & 2013 Elsevier Ltd. All rights reserved.
Keywords: X-ray fluorescence analysis Cultural heritage Improving instrumentation Extending detection limits
1. Introduction X-ray florescence analysis (XRFA) is an excellent nondestructive tool for analysing the elemental composition of materials in a wide range of works of art, and can serve for identifying these materials (e.g., Ferreti, 2000; Milazzo, 2004). The information that is obtained can be used for determining the origin of an artefact, for identifying the production technology, for informing restorers and preservers, recognising imitations and counterfeits, etc. The whole line of work needs deep collaboration among radiation physicists, who carry out the measurements and calculate content of the various elements, and restorers and art historians, who use and interpret the results (e.g., Karydas et al., 2005).
n
Corresponding author. Tel.: þ420 2243 58247. E-mail address: musilek@fjfi.cvut.cz (L. Musı´lek).
0969-806X/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2013.01.035
The Department of Dosimetry and Application of Ionising Radiation (DDAIR) of CTU-FNSPE has used radionuclide or X-ray tube excited energy dispersive X-ray fluorescence for many kinds of works of art, including frescos, paintings, manuscripts, metal sculptures and other metal objects, ceramics, jewellery, various archaeological finds, etc. Collaboration with institutions traditionally associated with the humanities has therefore been necessary. These institutions supply artefacts for measurements and introduce us to problems that are to be solved. At the output end of the whole process, they must interpret the results in the language of archaeology or cultural history. This paper gives a brief review of works carried out at DDAIR. The method used for measurement is more or less traditional, i.e., semiconductor spectrometry of X-rays excited by radionuclide sources or X-ray tubes, with some optional choices — capillary optics for collimation of exciting beams and twodimensional scanning. The key problem with XRFA is the low depth range of the measurements, depending moreover on the energy of the exciting radiation and the atomic number Z of the
382
T. Trojek et al. / Radiation Physics and Chemistry 95 (2014) 381–384
detected element. The ‘‘hardware’’ complex is therefore supplemented by techniques for estimating the depth distribution of the measured elements, for suppressing surface effects, for in situ non-contact measurements, etc. Extending the measurable range to lighter elements (the third period of the periodic system) and decreasing the detection limits to units of ppm are important achievements attained by improving the instrumentation and techniques that are used. The variety of possible applications can thus be continuously extended.
2. Development of instrumentation In the course of time, five generations of XRFA instruments have been developed at DDAIR. Some commercial components have been used in constructing the instruments, but the whole concept has been of our own design, as our requirements have differed from those of compact commercial analysers for in situ measurements. The advantages of this approach are as follows:
Flexibility — our instruments allow more changes of measure
ment techniques, geometrical arrangement and evaluation than commercial instruments. Non-contact measurement — the distance from the surface of measured objects can be adjusted in the range of millimetres to centimetres. Price — commercial instruments cost considerably more than home-made equipment. In addition, the radiation sources and semiconductor detectors of our instruments can be taken out and used for other purposes. Educational value — being a university department, we need to familiarise students with various measurement methods through experiments. Commercial instruments are usually ‘‘black boxes’’, and do not allow a deeper view into how the measurement and evaluation are done.
The first analyser (marked as No. 1 in Table 1), assembled at the end of the 1990s, consisted of radionuclide sources (55Fe, 238 Pu, and in rare cases 241Am were used) and a liquid nitrogen cooled Si(Li) X-ray detector (effective diameter 6 mm, thickness 5 mm, and 25.4 mm beryllium window). The beam of the exciting radiation was collimated to an area approximately 9–15 mm in diameter. A small Si-PIN diode with a Peltier cooler replaced the Si(Li) detector in the second version of the analyser (No. 2). A ring radionuclide source was used and the collimator was removed to minimise the sources of parasitic radiation. The resulting compact analyser was advantageous for measurements in situ, but only for Table 1 Summary of basic parameters of the XRFA devices at DDAIR FNSPE. Number of device
Analysed area [mm2]
DL for calcium [ppm]
DL for zinc [ppm]
DL for lead [ppm]
1a 2b 3c 3d 4
180 1400 0.7 60 Up from 4 10 4 20
1500 1000 719 151 58
58 21 78 13 16
67 26 169 26 40
2
9
5 a
18
With a 238Pu radiation source (1.11 GBq) and collimator diameter 10 mm. With a 238Pu radiation source (1.11 GBq). c With the narrowest collimator. d With the widest collimator. b
large homogeneous areas, due to the large irradiated spot on the measured object. At about the same time, about 10 years ago, a third analyser (No. 3) was constructed with a small X-ray tube (molybdenum anode, 30 kV, 100 mA). The much higher flux of the exciting photons in comparison with radionuclide sources allowed narrow collimation of the beam with satisfactory excitation effectiveness. The smallest used area was 0.7 cm2. However, with increasing popularity of the method among Czech art conservers and historians, new tasks have appeared, requiring even higher space resolution (e.g., manuscripts). A fourth apparatus (No. 4) was therefore built with a more powerful X-ray tube (50 kV, 1 mA) and with capillary X-ray optics, focusing the beam on a diameter of less than 20 mm at a distance of 4 mm from the end of the capillary. The size of the spot can easily be increased by changing the distance of the measured object from the focus. Neither radionuclide sources (except 55Fe) nor X-ray tubes with a molybdenum anode are optimal for exciting the lower Z elements (approximately with Z o20). The fifth, and latest apparatus (No. 5) uses a small MINI-X self-contained X-ray tube system, integrated with the power supply and the control electronics, with a gold transmission target (L-lines of Au are significantly present in the spectrum). It can be operated at voltages from 10 to 40 kV and at currents from 5 to 200 mA (power not exceeding 4 W). The silicon drift detector with a thin Be window (12.7 mm) has the optimum detection range from 1 to 40 keV. This means that K-lines starting from sodium can be detected. This combination of a low-energy X-ray source and detector extends the possibilities of our laboratory for measurements in the direction towards lighter elements. The key parameters of all devices are summarised in Table 1. The detection limits in the table (DL) are values for which analytical signals differ significantly from the background. They are determined in such a way that the measured signal is larger than three times the square root of the background. They are determined for the reference NIST 1412 multicomponent glass, and the measurement time is 10 min. The values can differ somewhat for different materials, due to the influence of the different composition of their matrices. For example, they increase for some metal alloys due to the presence of high Z elements, and they decrease (sometimes even by one order of magnitude) for elements in light matrices (samples of organic origin). For comparison, the oldest device (No. 1) and the newest device (No. 5) are shown in Fig. 1.
3. Techniques for refining and extending what can be measured Investigations of various artefacts are a multi-faced problem, and no simple and routine approach can be formed. Analyses of large homogeneous areas of pigments on frescos are very different from analyses of tiny lines of pigments on manuscripts, metal statues differ from ceramics, etc. It has therefore been necessary to supplement the variety of instruments with a variety of methods for measuring and processing the output signal. Above all, computer simulations have become a very useful tool for testing and optimising measurements (see, e.g., Trojek and ˇ ´ k, 2007). Though some analytical approaches and formulas Cecha have been published, they cannot include all the complicated boundary conditions, e.g. the dimensions of the radiation source, the production of secondary characteristic radiation, the influence of scattered radiation, the shape and the inhomogeneities of a measured object, surface roughness, etc. Monte Carlo calculation is the
T. Trojek et al. / Radiation Physics and Chemistry 95 (2014) 381–384
Fig. 1. The first apparatus (above) and the newest apparatus (below) used at FNSPE for XRFA of cultural heritage.
most versatile tool for describing various configurations of experiments, providing results that correspond best with the measurement results. In particular, the MCNP code has proved very useful for calculations of results allowing the geometrical configuration of the devices to be optimised, testing new ways of quantitative evaluation, and studying effects that are not pronounced in experiments. Needless to say, agreement between the results of a simulation and the results of an experiment depends significantly on the quality of the input data for the calculations, e.g., the spectra of exciting radiation, the shape and roughness of the investigated surface, the composition of the materials, etc. Simulations of various influences can determine the extent to which a change in some parameter reflects in a change in the particular peak area that serves for quantifying the concentration of the corresponding element. Our device No. 4 (with capillary X-ray optics) is designed preferentially for 2D measurements, i.e., for scanning the surfaces of measured objects and determining inhomogeneities in composition. It is therefore equipped with an automated positioning system, which scans the area of interest with steps down to 10 mm. Scanning a large object with small steps is, of course, a lengthy process, and some optimisation is needed that takes into consideration the minimum sufficient resolution. A further step leading to a deeper view into the analysed materials is 3D scanning. The depth distribution of elements is often not uniform. XRFA can estimate this distribution only in thin layers of materials, e.g., when various layers of pigments are superimposed on a painting. There are two complicated and expensive methods for mapping the depth distribution — XRF tomography and confocal XRF (see, e.g. ASTM, 1992; Fiorini et al.,
383
2001, or more recently Silversmit et al., 2009; Tsuji and Nakano, 2007). Both need a strong radiation source, ideally synchrotron radiation. Our laboratory has concentrated on simpler methods that do not enable the depth distribution to be mapped exactly, but provide information that depth inhomogeneities exist, and in some cases enable the characteristic of the lower material layer to be determined. Three techniques have been investigated for this purpose (Trojek et al., 2007, 2008). The first is based on changing the angle of the measuring head of the analyser to the surface of measured object. This involves changing the angle of penetration of the narrow exciting radiation beam into the material and changing the corresponding angle of emission of excited radiation to the detector. Simple geometry considerations show the change in the thickness of the layer from which the signal is collected, and therefore the reason, why the contribution of the signals from layers at different depths differs. Depth inhomogeneity changes the ratio of the peaks corresponding to various elements in dependence on the depth distribution of the elements. The technique can be improved by using two detectors positioned in such a way that the angles of detection of excited radiation are significantly different. The third technique is based on the different energy and thus also the different absorption of Ka and Kb lines, La and Lb lines or Ka and La lines of excited radiation. Their ratio thus depends on the depth at which the element is located. Lines with lower energy are usually more suppressed if their source is deeper (Trojek et al., 2010). Surface effects can influence the result of measurements (Trojek, 2011). One effect is curvature of the surface. Changing the angle of impact of the excitation beam on the surface changes the output signal. This influence has been intensively investigated. This fact can be used for scanning the structure of the surface of objects with a homogeneous composition. Fig. 2 shows an example of the reconstruction of the surface relief on a 1 Euro coin from an XRFA scan by a narrow X-ray beam. This is an interesting by-product of techniques designed primarily for analyses.
4. Examples of problems that are being investigated Analyses of inorganic pigments in wall paintings, illuminated manuscripts, glazes on ceramics, etc., are very rewarding. As wall paintings usually consist of large areas of homogeneous materials,
Fig. 2. Photograph (above) and reconstruction from the XRFA scan (below) of the relief of a 1 Euro coin.
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it is not a problem to use devices with radionuclide sources (our Nos. 1 and 2), which are able to differentiate various pigments ˇ ´ k et al., 2001). Pigments and even close to their boundaries (Cecha inks in manuscripts with tiny lines and miniature paintings are at ˇ ´ k et al., 2010), and need a the opposite extreme (Cecha very narrow beam of exciting radiation (our devices Nos. 3 and 4). A narrow beam is also needed for 2D and 3D analyses of artefacts. Scanning can also detect migration of elements due to corrosion of inks and changes caused by past interventions of restorers. XRFA has also found a wide field of use in investigations of archaeological finds, especially ceramics and metallic objects. A good example of the possibilities and the sensitivity of these methods is analysis of the bone of the phalanx with deposited corrosion products from the ring, which has been completely desintegrated during the past centuries and has not preserved ˇ ´ k et al., 2007). (Cecha This method can be used more or less routinely, and a combination of devices with various parameters can be used in investigations of a wide range of problems that cannot be described here, due to lack of space. Finally, it can be mentioned that we have investigated some unique artefacts of Czech cultural history. The helmet of Czech patron St. Wenceslas can serve as an example. Its decoration was formed by cladding silver foil to an iron basis, but there were some theories that the silver was gilded. Estimates of the depth distribution of the gold in the silver from the ratio of the La and Lb peaks did not confirm these theories, as the ratio corresponded to the admixture of gold in the volume and there was no increased concentration of gold at the surface.
Acknowledgement The authors would like to express their thanks to all colleagues, both from DDAIR and from many collaborating institutions, who have contributed to the extensive works briefly reviewed here. Last but not least, we appreciate the financial support from
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