Implementation of ionoluminescence in the AGLAE scanning external microprobe

Nuclear Instruments and Methods in Physics Research B 348 (2015) 68–72

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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb

Implementation of ionoluminescence in the AGLAE scanning external microprobe L. Pichon a,b,⇑, T. Calligaro a,c, V. Gonzalez a,c, Q. Lemasson a,b, B. Moignard a,b, C. Pacheco a,b a

Centre de recherche et de restauration des musées de France, C2RMF, Palais du Louvre – Porte des Lions, 14 Quai François Mitterrand, 75001 Paris, France Fédération de recherche NewAGLAE, FR3506 CNRS/Ministère de la Culture/UPMC, Palais du Louvre, 75001 Paris, France c PSL Research University, Chimie ParisTech-CNRS, Institut de Recherche Chimie Paris, UMR8247, 75005 Paris, France b

a r t i c l e

i n f o

Article history: Received 19 December 2014 Received in revised form 30 December 2014 Accepted 12 January 2015 Available online 4 February 2015 Keywords: External beam IBIL PIXE Ionoluminescence Imaging

a b s t r a c t The scope of this work is to present the implementation of an IBIL imaging system in the scanning external microprobe of the AGLAE facility so as to correlate luminescence and composition maps provided by PIXE, RBS and PIGE. The challenging integration of the optical spectrometer, due to incompatibility of acquisition timings and data formats with the other IBA channels has motivated the development of a specific acquisition system. This article details the IBIL setup and explains the technical solutions retained for the coupling of IBIL with IBA techniques in order to produce fast and large IBIL-IBA maps. The IBIL maps stored in the same format as the PIXE, RBS and PIGE ones can be visualised and compared using the dedicated AGLAEmap program or the PyMCA processing package. An example of such a coupled mapping on Mesoamerican jade is presented to emphasise the interest of performing simultaneously IBA and IBIL large mappings. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Ion beam induced luminescence (IBIL) is an auxiliary IBA technique increasingly employed in combination with PIXE, RBS and PIGE for the characterisation of materials [1–3]. It provides complementary information that can be used e.g., to detect trace levels of rare earth elements, differentiate polymorphs minerals and visualise crystalline defects. Its application for the study of complex and heterogeneous Cultural Heritage targets is particularly promising [4–7], especially through the coupling of IBIL with IBA imaging techniques at various scales. Although IBIL measurements are generally carried out in single spot analyses where a single spectrum is recorded, there is a growing interest in the acquisition of IBIL maps [7–11]. The present work details the implementation of an IBIL system in the AGLAE external scanning NMP that permits to acquire full spectrum IBIL maps simultaneously with PIXE, PIGE and RBS maps. In fact, the AGLAE facility is undergoing a major upgrade program labelled NEWAGLAE. Within the framework of this program, the PIXE setup of the external end-station has been upgraded by installing five large SDD detectors coupled with a multiparameter acquisition ⇑ Corresponding author at: Centre de recherche et de restauration des musées de France, C2RMF, Palais du Louvre – Porte des Lions, 14 Quai François Mitterrand, 75001 Paris, France. E-mail address: [email protected] (L. Pichon). http://dx.doi.org/10.1016/j.nimb.2015.01.010 0168-583X/Ó 2015 Elsevier B.V. All rights reserved.

system. Today this system routinely works in imaging mode for PIXE, PIGE and RBS analyses. The recently installed acquisition system where the IBIL setup has been implemented exhibits peculiar features. Elemental maps of large areas (mm to cm) with a resolution down to 30 lm are drawn at high speed. This is achieved by combining a rapid vertical magnetic deflection of the beam with a mechanical translation of the target and acquiring data in list mode, i.e., event-by-event. These two specifications, which complicate IBIL implementation in the setup, have motivated the development presented here.

2. Experimental For PIXE analysis, the AGLAE external end-station has been upgraded with four 50 mm2 SDD detectors dedicated to high energy X-rays and one SDD for low energy X-rays [12]. Their preamplifier signals are digitised using two XIA MERCURY-4 DSP’s. For RBS analyses, the pulses from the PIPS detector (Passivated Implanted Planar Silicon) are fed to a CANBERRA 9660-DSP. For c-ray measurements, signals from a HPGe detector are sent to a CANBERRA 426 NIM pulse processor and digitised using a CANBERRA 6117 NIM ADC (Analog-to-Digital Converter). Each channel provides its converted data to an input port of the 12-channel multiparameter acquisition system (FASTCOMTEC MPA-3) with an ADC communication protocol.

L. Pichon et al. / Nuclear Instruments and Methods in Physics Research B 348 (2015) 68–72

The beam scanning of an area on the sample is achieved with an independent system built upon a NATIONAL INSTRUMENT USB6353 module programmed with LABVIEW which controls the vertical magnetic deflection of the beam and the horizontal/vertical mechanical translation of the sample holder (Fig. 1). The beam is vertically deflected through the external beam exit window [13–19] with an analog triangular signal (0–10 V) sequenced by a free-running hardware clock of a few kHz (pixel clock). This signal is send to the OM40e dual scan amplifier (OXFORD MICROBEAM) which drives the magnetic scanning coils. It is sequenced by a free-running hardware clock of a few kHz (pixel clock). The OM40e scanning size is adjusted so that for the maximum analog voltage range, the scanning beam extends up to 640 lm to fit within the exit window, forming what we call a ‘beam column’. This beam column contains a number of Y positions that depends on the requested resolution (e.g., 32 positions at 20 lm resolution). The scanning system simultaneously outputs the corresponding digital vertical position (Y) to the MPA-3 input port but the actual values are not readable. It also controls X/Y translation stages and sends the related digital positions (X) to the MPA-3 input port. The Y position is determined by the magnetic deflection of the beam and controlled by the independent scanning system. The Y linear stage is only activated at the end of the line to move the target vertically (e.g., 640 lm) to the next line. The rapid vertical scan permits a homogeneous distribution of the beam dose among the pixels while keeping a constant integrated charge in each column. Once the integrated charge per column is reached, the sample is moved to the next horizontal position. In this manner, the target horizontally scrolls in front of the beam column till the end of the line where it moves vertically to the next line, drawing a maze pattern. The drawback of this architecture is that the actual vertical position of the beam within a column is neither controlled nor monitored. The mean horizontal velocity in mm/s depends on many factors (preset beam charge, beam intensity, X resolution) but the time to move to the next column, once a the present beam dose per column is reached, is 50 ms. The vertical velocity is fixed to 2 mm/s. accordingly the maximum duration time to move to the next line is 320 ms with a beam column of 640 lm. During all mechanical movements, all DSP’s and ADC’s are gated off to disable acquisition. For all the events occurring in the detectors during the course of the scan, the acquisition system records the energy E and the X, Y coordinates in list format. With such a design, any additional detection channel must provide its data event by event with an ADC communication protocol, which is impossible with our CCD-based spectrometers. At the end of the run, the PIXE, RBS and PIGE maps are extracted from this list file in EDF (ESRF Data Format) files which is used in all mapping software used at AGLAE and detailed in PyMCA documentation and open source code [20]. The implementation of the IBIL system within the acquisition setup was achieved by means of an independent measurement channel distinct from the multiparameter system used for PIXE, RBS and PIGE. Because the bulk of detectors does not leave enough space to accommodate focusing optics, the luminescence emitted by the beam column is directly collected with a 1 mm diameter optical fibre placed at 45° angle (Fig. 2). To keep the beam column within the homogeneous collecting field of view of the fibre, the beam scan is either reduced to 300 lm and the fibre placed at 3 mm distance or the scan left at 640 lm and the fibre moved 20 mm away. The light is conducted to a research grade OCEAN OPTICS QE65000 spectrometer [21] recording from 200 to 1000 nm with a resolution of 3 nm FWHM (100 lm entrance slit). The spectra are read through a USB 2.0 port at a maximum rate of one spectrum every 8 ms. Thanks to its thermoelectrically cooled detector, the spectrometer exhibits a good sensitivity and the IBIL spectrum collected in 8 ms presents an acceptable signal to background ratio. The light must be switched off during experiment

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Fig. 1. IBIL data acquisition block diagram showing its integration within the existing scanning system.

Fig. 2. View of the implemented IBIL setup in the external microprobe. 1 = optical fibre, 2 = high energy SDD detectors, 3 = Low energy SDD detector, 4 = RBS annular detector, 5 = HPGe gamma-ray detector, 6 = SDD dose detectors, 7 = Si3N4 exit window, 8 = videomicroscope.

as any stray light can be detected by the spectrometer. The sensitivity limit is set by the faint lines emitted by the atmosphere atoms (helium, nitrogen, oxygen) under the passage of the protons. For the acquisition of IBIL maps the operation of the variablerate (dose-controlled) scanning system must be synchronised with the fixed-rate IBIL spectrometer. First, the dwell time of the scanning system and the integration time of the spectrometer are set to the same value. For an integration time of 8 ms the internal frequency clock of the vertical scanning system is set to 125 Hz. Secondly, the IBIL acquisition system must accumulate short time IBIL spectra collected during the fast scan until the dose per column is reached. To do so, the system maintains an array of IBIL spectra related to all beam positions within a column. As the instant beam position within a column is not available with a shared software variable, the index of a particular IBIL spectrum in this array is obtained by digitising the above-mentioned triangular analog signal driving the coils. The last recorded IBIL spectrum is added to the spectra of the corresponding pixel position within the column. The reduction of scanning frequency can however degrade the dose homogeneity among the pixels within a column. To prevent this situation, the dose per column has to be increased so that the beam passes on each pixel several tens of times. All in all, the time spent on each column when using IBIL has to be extended to several

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Fig. 3. Global IBIL spectrum and decomposition of the global IBIL spectrum in distinct components (NNMA) for a geological jade sample.

seconds, while without IBIL this time can be less than a second, which slows down the scanning in consequence. At the end of each line, IBIL data are directly saved in EDF files. The resulting IBIL maps can then be directly visualised and compared to the PIXE, RBS and PIGE ones simultaneously acquired, using the PyMCA imaging toolkit or the AGLAEmap program [12]. The interpretation of the IBIL map can be performed by extracting the areas of the most intense bands. However, this simple approach can miss weak or overlapping bands and does not show band correlations. To circumvent these limitations, an automated peak search algorithm has been implemented in the AGLAEMap program. Alternatively, it is possible to use the Non-Negative Matrix Approximation algorithm (NNMA) [22] which is included in the PyMCA package. 3. Examples: IBIL/PIXE coupled imaging of Mesoamerican jade Jade (jadeite form) [23] is one of the most important materials in pre-Columbian cultures. The determination of the precise provenance of this archaeological material relies on the comparison from jade from archaeological artefacts with geological samples from known deposits. Jadeite is a rare mineral, and in Mesoamerica the only sources identified yet are located in the Motagua river valley, Guatemala. PIXE analyses of jade have not permitted so far to

establish a clear link between archaeological objects and geological evidence. The strong luminescence of jadeite, either induced by electrons or by ions, constitutes a promising complementary signature for the determination of its provenance [24]. To illustrate the capabilities of the described setup, the coupled PIXE/IBIL imaging was applied to a reference jade sample from the Motagua river. IBIL and PIXE maps of a 4.2  3.8 mm2 area (210  190 pixels, pixel size 20 lm) were collected from a 19  16 mm2 flat and polished sample. The scanned area exhibits visually two main colours: light green and dark green. The area was scanned for a total integrated charge of 93 lC using a 3 MeV proton beam of 8 nA and 30 lm diameter. The counting rates were ca. 10 kcounts/s in the low energy detector and 20 kcount/s in the four high energy (trace element) detectors that were screened by a 50 lm aluminium absorber. The IBIL signal was recorded all over the scanned area. Due to their small size compared to the beam diameter and the range of 3 MeV protons in jade (57 lm), several crystallites might have been hit simultaneously. Moreover, the emitted light was sometimes partly reflected or guided in the neighbouring crystals, modifying the recorded IBIL signal. In the global IBIL spectrum, three strong emission bands were observed at 350 nm (band I), 570 nm (band II) and 720 nm (band III), in agreement with previous IBIL studies on jade [25]. In a first approach, the application of our peak search algorithm to IBIL map evidences an additional band at 800 nm (band IV) overlapping band III. The application of the NNMA multivariate method to the entire IBIL map automatically reassigned IBIL emission into four basis vectors which are labelled NNMA 0–3 (Fig. 3), largely confirming the previously identified bands. NNMA-0 mainly corresponds to band I with a weak additional band at 700 nm. NNMA1 corresponds to band III. NNMA-0 and NNMA-1 appear in the same areas in the sample, but a close examination reveals that they do not exactly overlap. NNMA-2 corresponds to band II accompanied by a small contribution of band I. NNMA-3 corresponds mostly to band IV. The PIXE quantitative maps of Na, Mg, Al, Si, Ca, Cr, Mn and Fe (Fig. 4) were obtained using the TRAUPIXE software build upon the GUPIXWIN engine [25], as described earlier [26]. They enable to distinguish two principal areas corresponding to the jadeite (NaAlSi2O6) mineral phase and to the omphacite (Ca,Na)(Mg,Fe,Al)Si2O6 phase which incorporates trace amounts of Cr and Ni (Table 1). The two can be visually distinguished as light and dark green areas.

Fig. 4. Corresponding quantitative Na, Mg, Al, Si, Ca, Cr, Mn and Fe elemental maps obtained using PIXE. Intensity scale in ppm.

L. Pichon et al. / Nuclear Instruments and Methods in Physics Research B 348 (2015) 68–72 Table 1 Mean elemental composition of the four NNMA components and of the two mineral phases identified. NNMA-0

NNMA-1

NNMA-2

NNMA-3

Jadeite

Omphacite

(wt%) Na2O MgO Al2O3 SiO2 CaO Fe2O3

12.4 0.3 25.0 61.1 0.5 0.5

12.5 0.4 24.9 60.9 0.6 0.6

11.8 0.6 24.2 61.2 1.0 1.0

10.4 2.4 20.6 59.6 4.4 2.3

12.2 0.5 24.6 60.9 0.9 0.7

8.9 3.7 17.5 58.6 7.4 3.6

(ppm) SO3 Cl K2O TiO2 TiO2 Cr2O3 MnO CoO NiO CuO ZnO SrO ZrO2

257 180 331 130 142 42 311 5 10 2 113 3 1

369 140 347 85 123 58 640 7 13 2 265 1 3

144 248 552 176 202 88 224 8 33 2 23 9 0

196 127 378 352 415 906 593 15 214 2 86 15 7

203 148 352 258 280 80 255 2 18 3 95 3 2

128 98 357 359 445 1221 1049 20 426 3 147 33 15

The NNMA maps show complementary information to the PIXE maps as two types of jadeite can be distinguished by IBIL. A first one characterised by the combined NNMA-0 and NNMA-1 components and a second one restricted to NNMA-2. While NNMA-0 and NNMA-2 do not appear to be correlated to a particular detected element, NNMA-1 component seems to be linked to traces of Mn in certain areas. Concerning the omphacite phase, its luminescence is considerably weaker but exhibits the NNMA-3 component in correlation with traces of Cr. The above observations depart from the interpretation of the luminescent bands commonly found in the literature [27]. The cathodoluminescence (CL) emission of the same area was captured using a cold cathode equipment CITL Mk5 mounted on a petrographic microscope with an electron gun operating at 20 kV and 0.6 mA. The CL image appears similar to the false colour

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image resulting from the combination of the NNMA pattern maps (Fig. 5) with however a higher spatial resolution provided by the CCD camera. On the other hand, the CL image is only a RGB snapshot while the IBIL maps contain a more detailed picture through the underlying NNMA patterns. The measured IBIL patterns might possibly be correlated with unseen elements (below LOD or out of range) or linked to the mineral phase crystallinity (growth zones, crystal defects, etc.) to which IBA methods are notably insensitive. 4. Conclusion The main achievement of this work is the development of an imaging system based on IBIL and its integration within the existing elemental imaging system of the AGLAE external NMP. This system allows the recording of full IBIL spectrum maps without impacting the operation in PIXE, RBS and PIGE modes and offers the potentially informative coupling of the maps. Indeed the IBIL emission of certain materials can be used to address cultural heritage issues, e.g., to identify polymorph constituents in pigments or to determine raw material provenance. However due to the complexity of the luminescence phenomenon, there is not always a clear correlation between IBIL and chemical composition as exemplified by the case of jade. Notwithstanding this limitation, IBIL could routinely be used in Arts and Archaeology if a rich luminescence signature database for major heritage materials (pigments, stones, ceramics, glasses, etc.) were constituted. In some cases, this approach can be a much simpler alternative to chemical or structural characterisation. Acknowledgements We are indebted to Dr. O. Jaime-Riveron for loaning the jade sample from Guatemala. Many thanks to Dr. Yvan Coquinot for providing the cathodoluminescence imaging. We wish to thank D. Jimenez-Rey for his precious advices on the choice of the IBIL spectrometer. We greatly acknowledge the fruitful discussion and the careful revision of the manuscript by Dr. J-C. Dran. This work is realized in the framework of the Equipex Project New AGLAE (‘‘Investissements d’avenir’’ program) and with the

Fig. 5. Combined view of the sample, the scanned area, distribution maps of the four IBIL components labelled NNMA-0 to NNMA-3. Comparison with cathodoluminescence with false colour combination of NNMA components (NNMA-0 = red, NNMA-1 = blue, NNMA2 = green, NNMA-3 = grey). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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financial support from the French Government through the grant from the Agence Nationale de la Recherche ANR-10-EQPX-22.

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