62 (2007) 817–823www.elsevier.com/locate/sab

Spectrochimica Acta Part B

Development of a micro-X-ray fluorescence system based on polycapillaryX-ray optics for non-destructive analysis of archaeological objects

Lin Cheng ⁎, Xunliang Ding, Zhiguo Liu, Qiuli Pan, Xuelian Chu

The Key Laboratory of Beam Technology and Material Modification of Ministry of Education, Institute of Low Energy Nuclear Physics,Beijing Normal University, Beijing Radiation Center, Beijing, 100875, China

Received 18 November 2006; accepted 13 June 2007Available online 28 June 2007

Abstract

A new micro-X-ray fluorescence (micro-XRF) system based on rotating anode X-ray generator and polycapillary X-ray optics has been set upin XOL Lab, BNU, China, in order to be used for analysis of archaeological objects. The polycapillary X-ray optics used here can focus theprimary X-ray beam down to tens of micrometers in diameter that allows for non-destructive and local analysis of sub-mm samples with minor/trace level sensitivity. The analytical characteristics and potential of this micro-XRF system in archaeological research are discussed. Somedescribed uses of this instrument include studying Chinese ancient porcelain.© 2007 Elsevier B.V. All rights reserved.

PACS: 32.30. R j; 82.80.Ej; 91.65.NdKeywords: Micro-XRF; Polycapillary optics; Archaeological objects

1. Introduction

Non-destructive analysis of archaeological objects by micro-X-ray fluorescence (micro-XRF) spectrometry is an advanta-geous multi-element technique that has rapidly been developedduring the past few years [1–4]. Nowadays, the application ofmicro-X-ray fluorescence has substantially increased thepotential and possibility of two-dimensional mapping andthree-dimensional imaging. The micro-XRF technique hasproved to be a valuable tool, since the reduction of beamsizes to the sub-mm scale has significantly increased theanalytical spatial resolution and allows the study of small detailsand/or remains on the artifacts, and it has been considered as anideal method for analysis of archaeological objects by Lahanieret al. [5]. Ancient Chinese ceramics are very famous andvaluable in the worlds. However, many unsolved archaeologicalquestions, such as the sources of the cobalt blue pigment used inChinese ancient blue and white porcelain in different dynastiesare still puzzling archaeologists, and in recent years, more andmore fakes of ancient porcelains have come out. Those

⁎ Corresponding author. Tel.: +86 10 6220 7417; fax: +86 10 6220 8258.E-mail address: chenglin@bnu.edu.cn (L. Cheng).

0584-8547/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.sab.2007.06.010

questions can probably be solved by the analysis of the minoror trace elements of ancient porcelain. Therefore, it is necessaryto develop micro-XRF techniques in China in order to identifythe provenances, date and distinguish precious ancient porcelainware from the fakes.

The Synchrotron Radiation (SR) microprobe features asignificantly higher sensitivity and lower minimal detectedlimits (MDL) than that of any other micro-XRF, especially, it canbe combined with micro-ray diffraction and micro-X-rayabsorption spectroscopy. A major disadvantage associatedwith state-of-art devices is it is not compatible with the needsof in situ analysis for archaeology. Rotating anode X-raygenerators can generate higher intensities of X-rays than that ofthe common X-ray tube.

To the same X-ray source, the focusing X-ray intensity of thecommercial available polycapillary is 100 times stronger thanthat of the monocapillary [6]. More high intensity by focusingthe X-ray used to excite archaeological samples can help thedetector to collect X-ray fluorescence emitted from samples in ashorter time. It is important in archaeology to finish analyzingmany samples or elemental mappings in a few days, even in afew hours. Micro-XRF instruments consisting of a rotatinganode X-ray generator combined with polycapillary optics is an

Fig. 1. Schematic drawing of the components of micro-XRF system.

Table 1The MDL of micro-XRF measured by NIST SRM 610 certified referencematerials

Atomic number 19 22 25 26 27 28 29 30 37 38 82 90 92

Element K Ti Mn Fe Co Ni Cu Zn Rb Sr Pb Th UMDL (μg g−1) 97 32 14 11 14 13 12 13 9 9 27 12 17

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optimistic choice for non-destructive analysis of art andarchaeological objects in spite of the fact that it is not suitedfor in situ analysis for archaeology.

2. Experimental

The micro-XRF system is mainly comprised of a rotatinganode X-ray generator, glass polycapillary X-ray optics, Si-PINdetector, XYZθ sample stage, CCD camera and associatedelectronics (Fig. 1). The rotating anode X-ray generator (RigakuD/Max-B system) with Mo target materials is available to suitmost demands of archaeological objects, such as ancientceramics. The glass polycapillary X-ray optics (manufacturedby our research groups led by Prof. X. L. Ding, BNU) is used tofocus the X-ray to micro-size. The geometrical parameters of thepolycapillary X-ray optics are as follows. In regard to focaldistance, the distance from the X-ray source to the lens input ( f1)is 61.6 mm and the distance from the lens output to the focal spot( f2) is 18.7 mm. The length of optics (l ) is 68 mm. Relative toMo–Kα(17.4 KeV), the coefficient of power density amplifi-cation is G=1560 and transmission efficiency is η=3.8%. Theoptics captured X-rays that are emitted at a takeoff angle of 6°from the X-ray tube. The XYZθ sample stage and CCD cameraare controlled by a computer in order to ensure the measured spotis located in the focal spot of the polycapillary optics. A Peltiercooled Si-PIN diode detector equipped with PX4 system (XR-100CR, Amptek Inc, MA, USA) with resolution of 197 eV(Mn–Kα,5.9 KeV) and 7 mm2 active area, 300 μm Berylliumwindow was employed to collect the fluorescent radiationemitted by the samples. In order to allow easy and reproduciblepositioning of objects relative to the focal spot of polycapillaryoptics and Si-PIN detector, two laser pointers were mounted onthe two sides of the X-ray generator, so that the intersection pointof the two laser beams coincides with the cross-point of the X-ray beam and the detector-axis.

3. Results and discussion

3.1. Analytical characteristics of our micro-XRF system

Some analytical characteristics and applications of archaeo-logical objects of the rotating anode generator equippedwith glasspolycapillary X-ray optics used for sample irradiations werereported by Janssens et al. [7]. It could obtain elemental yieldsaround 1 counts−1 (μg cm−2)−1 level, corresponding to absolutedetection limits for thin samples in the 0.05–1×10−15 g range andto relative MDL levels of 3–10 μg g−1 for thick organic samples.In ourmicro-XRF system, themeasured elements covered fromKto U in the air and in the same time relative MDL levels (Table 1)of around 10μg g−1 for NIST (National Institute of Standards andTechnology) SRM 610 (Multi-element Glass certified referencematerial).

In order to characterize the focal X-ray beam size, a 25 μmCr–Ni alloy wire (80% Cr, 20% Ni) was scanned perpendicu-larly relative to the X-ray beam in steps of 10 μm with themeasured spot positioned on the focal plane. The X-ray tube wasoperated at 35 kV tube voltage and 10 mA tube current with1 min counting time. The net peak areas of Cr–Kα and Ni–Kαwere recorded as functions of the relative position (distance) ofthe alloy wire on the focal plane with respect to the X-ray beamposition, and the full width at half-maximum (FWHM) of thisdistribution was calculated. The focal sizes (FWHM) were67.89 μm and 66.12 μm, corresponding to the energy of Cr–Kαand Ni–Kα, respectively. Because the K-edge of Cr was situatedat a lower energy (5.98 keV) than that of Ni (8.33 keV), Cr wasexcited by primary photons of lower average energy than thoseof Ni. Since the critical angle for total reflection in glass wasapproximately θC (mrad)≈30/E (keV), lower energy photonscould leave the individual capillary channels of the lens in a conewith a larger opening angle than higher energy photons.

The description of the X-ray emission spectrum can beconsidered a prerequisite for performing an accurate quantita-tive analysis. In some cases, the description of the tube spectrumis not required to be performed in absolute terms, but at least tofit qualitatively the various spectrum components. It is difficultto directly measure the intensity of the excitation spectrum, sothe primary spectrum from X-ray source was recorded at theexit end of polycapillary optics by means of the scattering of X-ray using organic glass as the scatterer [8] (Fig. 2). There weresome impurities, such as Cu and W, of unknown origin (Wprobably resulted from contamination of the anode), and itshould therefore be regarded as inherent parts of the primaryspectrum. Polycapillary optics distorted the excitation spectrum,biasing it towards lower energies (5–10 keV), which werecaused by radiation of lower energy more efficiently

Fig. 4. The analytical position of Chinese ancient blue and white porcelain (AD1368–1344) by micro-XRF.

Fig. 2. The excitation spectra recorded with polycapillary optic using 1 mincounting time, 35 kV tube voltage and 10 mA tube current.

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transporting along the curved channels of the optics thanphotons of higher energy and it was helpful to excite the lightelement from K to Zn of archaeological objects; this effect wasvisible (Fig. 3) in the spectra of NIST (National Institute ofStandards and Technology) SRM 610 (Multi-element Glassstandard). Because the polycapillary optics has modified theenergy of the excitation spectrum, their components could bedetermined by Monte Carlo simulation [9]. The quantitativeanalysis with fundamental parameter methods is still in progressin our research group.

3.2. Analysis of Chinese ancient blue and white porcelain(AD 1368–1344)

In order to illustrate the potential of this instrument used inanalysis of archaeological objects, the blue glaze of a piece ofChinese ancient blue and white porcelain (AD 1368–1344) wasanalyzed by our micro-XRF system. The excitation spectra wereused to excite X-ray fluorescence in the analyzed sample with35 kV tube voltage and 10mA tube current. The micro-XRFsystem allowed the collection of X-ray mappings with a spatial

Fig. 3. Micro-XRF spectra recorded for NIST 610 with polycapillary opticsusing 1000 s counting time, 35 kV tube voltage and 10 mA tube current.

resolution of 50 μm. In each pixel a spectrum was collected for1 min. The step size during scanning was 50 μm and the analyzedarea of the blue glaze was 1 mm×3.5 mm. The analytical positionwas shown in (Fig. 4). All spectra, including the above-mentioned, were deconvoluted by the AXIL-QXAS softwarepackage, distributed by IAEA [10]. Typical spectra of the blueglaze and white glaze of ancient porcelain were given in Fig. 5; itis easy to discern that the blue glaze contains higher concentra-tions of the elements Ca, Fe, Mn, Co and Ni. The elementalmappings of K, Ca, Fe,Mn, Co and Niwere constructed using thenet peak areas, which were further plotted in intensity readoutsgraphs using the MatLab7.0 software package (Fig. 6).Comparing Fig. 4 with Fig. 6, it could be seen that only theintensities of Mn–Kα, Co–Kα and Ni–Kα were variable inagreement with the thickness of the blue glaze. For Chinesearchaeologists, the ratio of MnO/CoO is crucial to determine theprovenance and to identify a fake Chinese ancient blue and whiteporcelain [11]. In order to discuss the relationship of elements inthe blue glaze, correlation analysis of those elements were carried

Fig. 5. The typical spectrums of blue glaze and white glaze of Chinese ancientblue and white porcelain.

820 L. Cheng et al. / Spectrochimica Acta Part B 62 (2007) 817–823

out by the STATISTICA 6.0 software package (StatSoft, Inc.,Tulsa, USA) with 95% confidence, where the net peak areas wereregarded as variables. The result of statistic analysis showed thatcorrelation coefficients of Mn and Co were 0.99 (Fig. 7) while Nionly reached 0.46 and 0.54, respectively, which proved the Mnand Co come from the same cobalt pigments and Ni probably wasthe impurity of them.

Fig. 6. Distributions of elemental maps of K (a), Ca (b), Mn (c), Fe (d

3.3. Analysis of the Chinese ancient Ru porcelain and Junporcelain (AD 960–1276)

Chinese ancient Ru porcelain and Jun porcelain are very rareand famous in the world. Although they were analyzed by PIXE[12,13] and other associated methods, the formation ofseparated layers between the glaze and body are not understood

), Co(e), and Ni (f) in blue glaze of the blue and white porcelain.

Fig. 6 (continued ).

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by most archaeologists. To illustrate this question in view of thechemical compositions by micro-XRF, pieces of Ru porcelainand Jun porcelain were linearly scanned and analyzed fromglaze to body by our Micro-XRF. The experimental conditionshave been described above. Distributions of some elementswere variable from glaze to body and are shown in Fig. 8. Fromthe glaze to body, the concentrations of K and Ca had a variabledecreasing trend, while Ti and Fe had the reverse trend. Thereasons for the formation of a separate layer between the glaze

and body are very complex, many uncertain factors, such astemperature, material of glaze and body, can influence theforming of separate layer during the ancient porcelain firing. Inspite of those, it is useful to reveal the secret of the separatedlayer by examining the distribution of chemical compositionfrom glaze to body.

In order to prove the possibility of distinguishing ancientporcelain from fakes by micro-XRF, a piece of fake Junporcelain was measured in the same experimental conditions. A

Fig. 7. Correlations of intensity ofMn–Kα andCo–Kα, the linear relation of thoseis I (Intensity)Mn–Kα=−3234+6.9267ICo–Kα and r=0.99 with 95% confidence.

Fig. 8. The distribution of elements K, Ca, Ti, and Fe from glaze to body in Ru porcelain and Jun porcelain.

Fig. 9. Typical spectrum of Jun porcelain and the fake one.

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spectrum of it (Fig. 9) showed that the fake porcelains containedhigher minor elements Ti, Ba, Cu, and Zn and lower Mn and Fethan that of the ancient porcelain. Some minor or trace elementsBa and Zn are especially crucial to distinguishing ancientporcelain from fakes.

4. Conclusion

In this paper, the analytical characteristics and potential ofthe micro-XRF system in our lab have been described. It can beconcluded that the micro-XRF system based on rotating anode

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X-ray generator and polycapillary X-ray optics is a useful toolfor non-destructive analysis of archaeological objects, espe-cially when studying the decorative color glaze of ancientceramics.

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