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Nuclear Instruments and Methods in Physics Research B 266 (2008) 2273–2278

NIMBBeam Interactions

with Materials & Atoms

Present and future role of ion beam analysis in the study of culturalheritage materials: The example of the AGLAE facility

J. Salomon a,*, J.-C. Dran a, T. Guillou a, B. Moignard a, L. Pichon a, P. Walter a, F. Mathis b

a Centre de Recherche et de, Restauration des Musees de France, CNRS UMR171, Palais du Louvre, 14 Quai Franc�ois Mitterrand, 75001 Paris, Franceb Centre Europeen d’Archeometrie, Universite de Liege, Belgium

Available online 18 March 2008

Abstract

The application of IBA to cultural heritage mostly relies on the use of PIXE because of its high sensitivity and its ease of implemen-tation at atmospheric pressure. The need for depth information not easily available with this technique has conducted to associate RBSalso in external beam mode. We have progressively developed a set-up that permits such a combination of techniques either simulta-neously or sequentially. The set-up is currently further improved to permit NRA measurement (depth profiles of light elements) in addi-tion to PIXE and RBS. The coupling of all these techniques provides a wealth of information on cultural heritage objects, not easilyattainable with any other single method.� 2008 Published by Elsevier B.V.

1. Introduction

Ion beam analysis has gained great popularity among thescientific community involved in the study of works of artand archaeological artifacts. The impetus has been givenby the development of external beam set-ups which permitnon-invasive analysis of large-sized items without samplingand greatly facilitate the handling and positioning of theobjects [1]. However for many years, the only techniquesreadily implemented on such set-ups were PIXE and PIGEin association. Even though these techniques are quite pow-erful in providing the panoramic elemental composition ofthe material, from major (generally light) elements to minorand trace elements (intermediate and heavy), they lack ofdepth information of particular significance in cultural her-itage materials. Indeed multi-layer structures are very fre-quent, as for examples glazed ceramics with luster orbronzes with patina and make global PIXE/PIGE analysissometime meaningless. Fortunately RBS can be used to cir-cumvent this difficulty and much effort has been made toapply this technique in conjunction with PIXE with an

0168-583X/$ - see front matter � 2008 Published by Elsevier B.V.

doi:10.1016/j.nimb.2008.03.076

* Corresponding author. Tel.: +33 140205405; fax: +33 147033246.E-mail address: joseph.salomon@culture.fr (J. Salomon).

external beam. In the present paper, we describe the pro-gressive evolution of the AGLAE facility to achieve thisgoal and highlight its present capability with a recent exam-ple. We also discuss the expected improvements.

2. Overview of the evolution of the AGLAE facility

The AGLAE facility, the first of its type to be fully ded-icated to art works, has been described in details in severalprevious papers [2–4]. Let us recall its main features. It isbased on a 2 MV electrostatic tandem accelerator fromNEC. Two ion sources (RF and duoplasmatron) providethe ions used for analysis (H, D, 3He, 4He). At the begin-ning of its operation, almost 20 years ago, measurementswhich mainly relied on PIXE, were performed in a conven-tional vacuum chamber. It became quickly obvious thatoperating under vacuum could not meet the constraint ofnon-destructive (non-invasive) analysis required for pre-cious items. Consequently we developed an external beamline to perform the analysis in air (actually in a heliumatmosphere to avoid attenuation of soft X-rays mostlyemitted by light elements). It is this line which has beenprogressively improved and is now used for the implemen-tation of practically all the ion beam analysis techniques on

Fig. 1. View of the first version of the external beam set-up used in PIXEmode. 1-Beam extraction nozzle with a Si3N4 window. 2-Laser co-linearwith the beam. 3-High energy Si(Li) X-ray detector equipped with a 6 lmBe window; solid angle 100 msr; mainly used for trace element analysis(Ca to U). 4-Low energy Si(Li) X-ray detector equipped with a deflectionmagnet; 0.25 lm BN window; He flux; solid angle 10 msr; mainly used formatrix identification (O to Fe). 5-Dose detector: PIN diode Peltier cooleddetector 6-Camera for positioning. 7-Magnetic triplet lenses. 8-Sample.

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all kinds of materials relevant to cultural heritage, includ-ing those potentially sensitive to radiation damage likepaintings, drawings, etc. We summarize below the mile-stones of the evolution of this beam line:

(i) In the early set-up, the mm-sized beam was colli-mated by a carbon diaphragm and extracted througha 8-lm Kapton foil. The dose was monitored via theparticles backscattered on the exit foil and collectedby a surface barrier detector. Due to the importantenergy straggling, only PIXE (associated with PIGE)could be performed. The detection equipment wasoriginally restricted to a single Si(Li) detector andthe analysis was performed with a beam of 3-MeVprotons and a funny filter. We shortly turned to adual detector system for low and high-energy X-rays[5].

(ii) At its early phase, the facility was equipped with acommercial microprobe system supplied by OxfordMicrobeams (OM) and installed on a separate lineoperating under vacuum. For the above-mentionedreason, this system was not well adapted to artobjects and was thus dismantled and combined tothe external beam line to produce an externalmicro-beam. The choice of the exit window becamethen crucial for preserving the beam qualities (spotsize and energy straggling). Several metallic thin foils(Ti, Zn, Zr, etc.) of thickness down to 2 lm weretested and Ti was finally chosen for routine PIXE,thanks to its good mechanical resistance and thelow background due to nuclear reactions. Such a win-dow is more resistant to radiation effects than Kap-ton and does not evolve under the beam so thatbeam monitoring using backscattered particles ismore reliable. Under these conditions, the minimumachievable spot size was 150 lm [6]. Beside regularPIXE/PIGE measurements, a few attempts to applyRBS at atmospheric pressure were made. A surfacebarrier detector was fixed at 150� with respect tothe beam direction (maximum attainable diffusionangle because of the overall configuration of thebeam line end) and the space between the targetand the detector was filled with helium to minimizethe energy loss of the backscattered particles. Itturned out that only 3–4 MeV proton beams couldyield usable spectra.

(iii) In order to reduce the size of the external beam, welooked for much thinner exit windows and foundamong laboratory supplies ultra thin Si3N4 mem-branes of current use in transmission electron micros-copy as specimen holders. After testing membranes ofdiverse lateral dimensions and thickness, we selecteda membrane of 0.1 lm thick and 1 � 1 mm fixed ona silicon frame. This was the cornerstone in the devel-opment of our external beam since this kind of mem-brane allowed us to reduce the beam size by almostan order of magnitude. In spite of its low thickness

the membrane is very resistant to pressure and toradiation damage. The only drawback was that beammonitoring with the SB detector was no longer feasi-ble (too low count rate). We thus decided to use formonitoring the Si X-rays emitted by the exit windowand designed accordingly the exit set-up to fit a Sidetector cooled by Peltier effect (see Figs. 1 and 2).The choice of such windows opened the door to theuse of new beams like, 3He, 4He and even 16O, andthus extended the panoply of IBA techniques imple-mented in air (alpha RBS, alpha ERDA, ion–ionnuclear reaction analysis). In order to operate alphaRBS under the best conditions, the surface barrier(SB) detector was mounted in a housing kept undervacuum thanks to a second Si3N4 window of the sametype as that of the beam exit window [7].

(iv) Despite the breakthrough achieved, we did not con-sider the set-up as fully satisfactory for the followingreasons:

– The dose monitoring even via the emitted X-rays wasnot always reliable in particular under very low beamintensity (a few 100 pA) used for the analysis of fragilematerials.

– The study of layered samples needed the combination ofPIXE and RBS for both an easier spectrum processingand a more complete characterization of the materials.This could be done either simultaneously with a singleshot of protons (or alpha particles), but the correspond-ing RBS (or PIXE) spectrum has a poor quality, orsequentially using two successive beams of protonsand alpha particles [8]. In the latter case, both spectra

Fig. 2. Layout of the extraction nozzle (first version).

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are satisfactory but the probed zones are not strictlyidentical, not to speak of the duration of the analysisand the possible damage of the target.

– The 150� diffusion angle of RBS measurements imposedby the overall geometry of the system was not the mostfavorable for mass and depth resolution.

The above considerations led us to design a novel beamextractor which addresses most of the mentioned difficul-ties. The following section will describe in detail the inno-vation we made.

Fig. 3. The new version of the extraction nozzle which permits RBSmeasurement and dose monitoring, with the associated cold baffle.

3. Recent technical improvements of the external micro-beam

set-up

It is clear that the successive improvements of the set-upin the past have overloaded the micro-beam end with com-ponents (two X-ray detectors for PIXE, a Si Peltier cooleddose monitor, a SB detector for RBS and a HPGe detectorfor PIGE). This has led to a very cumbersome system,sometime poorly fitted to art objects of complex shape.The new idea is to reduce the oversize of our old set-upby integrating the SB detector within the exit nozzle andusing it for both RBS measurement and dose monitoring.For two reasons, we have chosen an annular SB detectorfor RBS measurements: diffusion angle of 170� more favor-able and larger solid angle. The resulting set-up is obvi-ously more compact than the previous ones, as shown byFig. 3. A detailed description is given by Fig. 4. Anotheradvantage provided by this new geometry of the exit nozzleis that the continuous helium flow can be directed along thepath of the incident beam. This means a much better con-trol of the atmosphere surrounding the sample and thusthat of the energy and angular straggling of the beam.

3.1. Beam monitoring

An important factor for quantitative analysis is the mea-surement of the irradiation dose. Under conventional vac-uum conditions, this is simply done by current integrationon an insulated chamber. This technique can no longer beused in the case of external beam due to the loss of second-ary electrons. Different techniques have been tested in thepast, such as the use of the light emitted by the ion inducedplasma, the X-ray K-line of AR, the X-ray line of the con-stituent element of a chopper [9–11]. As described above,we have successively used the RBS signal of the exit foiland more recently the Si X-ray line of the Si3N4 membrane.The last version of our set-up relies on a system which ismore or less like a chopper. The difference is that the beamis not intercepted by a rotating foil but is intermittently

Fig. 4. Layout of the nozzle with integrated SB detector. 1-Annular SBdetector. 2-BNC connector. 3-Sample position. 4-Low energy Si(Li)detector with magnetic deflector. 5-Diaphragm for dose monitoring andangle of detection definition. 6-Path of backscattered particles fromsample and gold monitor. 7-Extraction steel cone. Note the newcomponents in the latest version: 8-Collimator to suppress the signalfrom the exit window. 9-A cold trap in the detector housing to enhance thevacuum and reduce the content in hydrocarbides.

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deflected on a steady 25 lm thick gold foil. The scanningcoils located in front of the magnetic triplet lenses are usedfor doing this operation. The same annular SB detector col-lects the RBS signal sequentially from both the sample andthe gold foil via two different electronic chains. To be cen-tered inside the extractor the beam has to travel on the axisof symmetry of the system (as shown by Fig. 4, the devicehas a cylindrical symmetry). The different parts constitut-ing the extractor (diaphragms, SB detector, shields, Si3N4

membrane, etc.) are mechanically aligned on this axis.When starting a new experiment, thanks to a homemadesoftware (Labview), we scan the beam vertically and hori-zontally (by adjustable steps) through the membrane on astandard in direct contact with the extremity of the extrac-tor. The middle of the X and Y flat-tops obtained helps usto position the beam on the symmetry axis of the system.Before describing the timing scheme of the electronic set-up it is worth mentioning that PIXE, PIGE and RBS canbe implemented simultaneously on this externalmicroprobe.

Using the following abbreviated labels: beam on sam-ple = BOS; beam out = BO; beam on foil = BOF; beamin = BI; beam on faraday cup = BOFC, the timing scheme is:

– When BOS then all ADC’s are freed, the dose ADC isblocked.

– When BO or BI then all ADC’s are blocked and no sig-nal can reach any detector during beam displacement.

– When BOF then all ADC’s are blocked, the dose ADCis freed.

The reliability of this beam monitoring approach hasbeen tested on a standard sample constituted of a 2 lm

gold layer on a silica slide. A large number of RBS spectrahave been acquired, for a dose of ions corresponding to afixed amount of particles backscattered from the monitorfoil. The reproducibility of the data for protons and alphaparticles as well. Some spectra have been obtained whileslightly modifying the voltage of the focusing lenses withno visible effect. It is worth mentioning the good reproduc-ibility of the He peak in the RBS spectra with protonswhich illustrates the stability of the helium atmosphere inthis new configuration. This He peak could even be usedfor monitoring proton beams.

– Finally, for evaluating the dose of ions received by thesample, it is even possible, after a run, to integrate thebeam current by attaching a tiny faraday cup at theextremity of the extractor. This device guaranties a goodcharge collection without secondary electron loss. Thecollected charge can then be directly related to theamount of particles backscattered from the gold foil.

This dedicated software permits not only to drive thedeflection coils and provide the right gate signals, but alsoto adjust the time ratio between dose measurement andacquisition.

3.2. New set-up for simultaneous PIXE/RBS measurements

Close examination of Fig. 5 indicates the presence ofvery small peaks in the RBS spectra of the gold standard.Some of them are easily identified and ascribed to N andSi due to particles backscattered from the Si3N4 membrane.The Cr peak is due to a very thin (a few 10 nm) layer ofchromium between the SiO2 substrate and the Au layerto insure gold adhesion. One also notes more marked peaksof C and O, the first one clearly evolving with cumulativeirradiation dose. All stable minor peaks do not hinderRBS measurements. However, the evolving (in height andwidth) carbon peak indicates the building up of a thin layerof carbon on the exit window likely due to the presence ofhydrocarbides in the residual atmosphere. This can lead tosome energy shift (up to 20 keV for 3 MeV alpha beam) ofthe RBS spectrum. In an attempt to solve this problem, wehave added a cold baffle and a small turbo-pump on theback of the beam extractor. The observed carbon peak ismuch reduced but still appears after longer irradiationtime. Further improvement of the baffle will certainlyreduce even more the amount of deposited carbon. How-ever, in the mean time we have relied to a totally differentapproach to circumvent the contribution of the exit win-dow to the RBS spectrum. Since the window is about 3mm ahead from the sample, we decided to build a kindof confocal system which prevents the signal from the win-dow to reach the detector. The layout of the last version ofthe external beam set-up is also shown on Fig. 4. The com-ponent of interest here is a small diaphragm (tube of 2 mmlong and 0.6 mm diameter). These dimensions and the dis-tance of the extremity of the tube to the exit window have

Fig. 5. RBS spectra with 6-MeV 6 alpha particles on a 2 lm thick gold standard. Note several small peaks originating from particles backscattered fromthe exit window.

J. Salomon et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2273–2278 2277

been accurately adjusted so that the signal from the win-dow is totally suppressed whereas that from the sample isfully collected. The resulting RBS spectrum illustrates thebenefit resulting from this confocal set-up: the unwantedpeaks have disappeared. Of course the build up of the car-bon film on the window still occurs and after a while canproduce a slight energy shift.

4. Illustration with an example

For illustrating the reliability of this new set-up, we havechosen to analyze a Japanese silver drop dispenser from the19th century (Fig. 6). This pear-shaped object of about5 cm long is decorated with black inlays characteristic ofShakudo patina. We have used a beam of 6-MeV 4He

Fig. 6. PIXE spectrum obtained with 6-MeV 4He ions on a blackdecoration (patina) of a 19th century Japanese drop dispenser (shown ininsert).

beam to record simultaneously the PIXE and RBS signalsof several inlays. The resulting data are quite identical fromone inlay to the others. The PIXE spectrum (Fig. 6) is pro-vided by the high-energy X-ray detector (the low energydetector is useless for metallic matrices in general, and alsoin this particular case because of its low solid angle). Onenotes in decreasing order of energy, the K-lines of Ag,the L lines of Au and the K-lines of Cu. The presence ofthese three elements is typical of the Shakudo alloy (com-posed of Cu + a few% of Ag and Au). The sole PIXE spec-trum cannot explain the dark color of the inlay which islikely due to a chemical transformation of the alloy. Theassociated RBS spectrum, presented on Fig. 7, providesthe right answer in clearly showing evidence of a coppercompound at the surface revealed by a small depletion onthe copper signal, correlated with an oxygen bump. Onealso notes on this spectrum very small but well definedAu and Ag steps, as well as a small C peak likely due toa protective varnish layer. In order to identify unambigu-ously the copper compound responsible for the blackpatina, we performed an X-ray diffraction analysis at graz-ing angle on one of the inlays. The corresponding diffracto-gram indicated the presence of cuprite (CuO). Pure cupriteis known to be red, but in this case the black color couldoriginate from the presence of Ag and Au in the alloyunder a form still debated.

5. Future prospects

The development of a sophisticated external beam set-up has permitted to implement simultaneously or sequen-tially PIXE (PIGE) and RBS with beams of either protonsor 4He ions. The ability to produce external beams of 4He

Fig. 7. RBS spectrum associated with the PIXE spectrum of Fig. 6.

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ions with quite good energy definition constitutes a signif-icant progress. The resulting RBS measurements are almostas good as under vacuum, while the corresponding PIXEdata, although inferior in sensitivity than those obtainedwith protons, do provide useful information on the bulkcomposition. The set-up is currently under improvementto permit simultaneous NRA measurement (depth profilesof light elements) in addition to PIXE and RBS (by using amulti-sector annular detector). In particular, the use of 3Hebeams could be quite advantageous for implementingsimultaneously PIXE, RBS and NRA. For the sameenergy, the rate of X-ray production should be larger for3He than for 4He and 3He induces several nuclear reactionswith light nuclei that should be of interest for NRA.

6. Conclusion

After almost 20 years of operation, the AGLAE facilityof the Center for Research and Restoration of the Museumsof France, has reached the status of a very versatile toolsable to study all kinds of cultural heritage objects in air withthe full panoply of IBA techniques. It should be men-tioned that due to the increased availability of synchrotronradiation facilities, X-ray based analytical techniques andspecially SRXRF, appear more and more as a good alterna-tive to PIXE. However, the coupling of all the IBA tech-niques still provides a wealth of information on culturalheritage objects, not easily attainable with any other singlemethod. In particular, the analysis of light elements and the

depth profiling capacity with a single shot is the specificityof IBA techniques and will insure their use in the future.

Acknowledgements

We thank D. Robcis for the loan of the drop dispenserwhich illustrates the capability of the AGLAE facility. Weare indebted to M. Aucouturier for his help during dataprocessing.

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