XEOM 1 - A novel microscopy system for the chemical imaging of heritage metal surfaces
© Dowsett et al.; licensee Springer. 2015
Received: 13 October 2014
Accepted: 26 February 2015
Published: 1 May 2015
We describe a novel microscopy system which can obtain chemical maps from the surfaces of heritage metals in air or a controlled environment. The microscope, x-ray excited optical microscope Mk 1 (XEOM 1), forms images from x-ray excited optical luminescence (XEOL) induced by illuminating a few square millimetres of the sample with monochromated x-rays (broad beam or macroprobe illumination). XEOL is a spectroscopy tool in its own right and can, under the right circumstances, also be a vehicle for x-ray absorption spectroscopy. This (usually) synchrotron based technique provides information on the chemical state and short-range atomic order of the top few microns of a surface. It is thus well suited to heritage metal corrosion studies and is complementary to synchrotron x-ray diffraction.
Imaging can be performed by scanning the sample under an x-ray microprobe. We show elsewhere that the power density needed for image acquisition on a reasonable time-scale is high enough to damage a patina and modify its chemistry. Although the damaged region may be invisible to the human eye, the data are characteristic of the damage and not the native chemistry of the surface. A macrobeam power density can be 4 orders of magnitude smaller than that for a microbeam and no surface modification was observed on test samples. Features of the instrument are demonstrated using copper test surfaces with a spatially varying patination to establish the ground work for the imaging of copper, cuprite, nantokite and atacamite/paratacamite and a first application from a bronze chain mail link. In parallel we have developed a suite of imaging software which can process XEOM image stacks to produce reduced data sets characteristic of various aspects of the surface chemical map. These include edge-shift (oxidation state) images and edge height (high contrast) images and spectra from user defined regions of interest.
The technique can map the oxidation state of a surface from shifts in the absorption edge energy across columns of pixels in an image set, and map particular compounds from their characteristic XANES spectra. Optically filtered images give improved chemical selectivity and the data sets contain as yet untapped information sources.
One effect of the bombardment of material surfaces with x-rays is the emission of much lower energy electromagnetic radiation in the visible and near-visible bands. (herein referred to as trans-visible) [1-4]. This phenomenon, x-ray excited optical luminescence (XEOL), is the basis of a spectroscopy tool in its own right but, since the intensity of the emission is dependent on the probability of absorption of the x-rays onto core levels, it can also carry the information for x-ray absorption near edge spectroscopy (XANES) and extended x-ray absorption fine structure (EXAFS) [5-9]: This is because the trans-visible light will contain a contribution from particular radiative transitions excited downstream from the core-level photoexcitation, and therefore modulated in the same way. (However, this contribution must be sufficiently intense to compete with the emission arising from the non resonant excitation of states below the core level in question or it will not be discernible. This places limitations both on the minimum detectable concentration of the species whose core level is being probed, and the maximum concentration of emitters responsible for non-resonant background.) Thus, in favourable cases, by scanning the energy of the incident x-rays across the absorption energy of a particular core level (e.g. copper K-α, lead LIII) and beyond and measuring the intensity of the trans-visible emission, information on short range order, oxidation state, chemical species, etc. can be obtained. The resulting spectroscopies are known collectively as optically detected x-ray absorption spectroscopy (ODXAS) and individually as XEOL-XANES and XEOL-EXAFS. Elsewhere [8,10] we summarize some differences in the appearance of signals from XANES and XEOL-XANES. We also show that they easily detect far thinner layers of surface corrosion on heritage metals compared to x-ray fluorescence, the conventional way to measure XAS data from surfaces which has an information depth of several micrometres. In general, ODXAS is likely to be surface specific to around the skin depth at the optical frequency observed: i.e. 100–200 nm in good conductors, but potentially more in insulators. It therefore lies between conventional XAS and total electron yield XAS (TEYXAS)  in its surface specificity. The emission from metals and their corrosion products is broadband, typically covering the wavelength range from <250 nm to >1 μm. However, many different end-states contribute across this band so that emission from particular transitions may be studied by using colour filters, giving further selectivity.
In general, for any analytical method, chemical and structural imaging can be done in one of two ways: A fine probe (a micro or nanoprobe) of the incident radiation may be scanned across the sample (or where the probe is difficult to scan, the sample may be moved across the probe) [7,12-14]. The emission is recorded as a function of probe position. An advantage of this method is that different information channels can be collected from the same point on the sample using parallel detection . Alternatively, a relatively large area of sample may be illuminated simultaneously with a broad beam (a macroprobe), and the emission imaged onto a detector with lateral resolution, e.g. a pixelated detector such as a charge coupled device (CCD) [6,14-18], sometimes with energy resolution on each pixel [19,20]. This is just applying the principle of the microscope. An advantage of this method is that movement of the beam, due, for example to monochromator scanning, does not shift the image. However, achieving uniform x-ray illumination over the imaged area may not be straightforward because the beam from a synchrotron source often contains internal structure.
Both approaches will result in dead-time (in the sense of time when data cannot be taken). For example, in both the scanning of the monochromator and the mechanical rastering of a sample no data can be taken until the position has stabilized. With full field imaging, the read out speed of a CCD must be moderated to reduce noise in the images. These effects must be considered when designing experiments to minimize the elapsed time, and the overall dose accumulated in the sample.
Both the microprobe and the macroprobe are ubiquitous in electron microscopy, ion imaging techniques, x-ray methods, optical spectroscopy and so on. (Near-field optical techniques may also be used  but these have similar beam density implications to microprobes.) The lateral resolution of a microprobe is ultimately limited either by the probe size, or by the dimensions of the interaction volume. For the macroprobe, the ultimate resolution is determined by aberrations and diffraction in the image-forming device or by limitations such as pixel size and cross talk between pixels in the detector. (Practical lateral resolution is often determined by statistical fluctuation in either case .) The microprobe method usually offers the highest lateral resolution but the microscope method may be orders of magnitude faster in image collection since each part of the image is simultaneously acquired. Moreover, to achieve practical collection times, the power density in the microprobe must be orders of magnitude higher than in the macroprobe (since the same total dose must be delivered to a pixel for the same statistical precision).
As we show elsewhere [8,10], the x-ray power density used in a microprobe for XAS and ODXAS imaging alters copper and bronze corrosion products on a timescale from milliseconds to tens of seconds through a variety of processes. Such images are therefore unlikely to be representative of the original surface. For example , we have shown that using a microprobe with a power density of 60 W mm-2, a thin nantokite coating (observed in the ODXAS channel, but too thin to be seen in conventional XAS) is progressively hydrolysed to paratacamite in the beam over a period of seconds to minutes. In contrast, in a macroprobe of 20 μW mm−2 (typical of a bending magnet) the ODXAS observed from the thin patina and a bulk reference of CuCl are stable over time and identical . Effects analogous to the bleaching seen in conventional fluorescence microscopy  are also seen in our microprobe data . Since optical components and pixelated light detectors are widely available and relatively inexpensive, imaging using XEOL provides an attractive alternative for collecting XAS images using a macroprobe. We therefore developed a broadband microscopy system (XEOM 1)  which will transfer XEOL from an area of 4 mm2 to a broadband CCD detector. XEOM 1 allows the sample to be in air, in a controlled atmosphere, or even immersed in an electrochemical cell.
XEOM data structure
XEOM 1 data from cuprite and nantokite test samples
A bronze chain mail link from the Mary Rose
Imaging of copper patinas using high intensity X-ray microprobes can change the composition of the surface thereby invalidating the results. A similar effect would be expected for other surfaces, especially when x-ray energies close to absorption edges of the material are used, and irrespective of the actual technique in use e.g. XAS, synchrotron x-ray diffraction SR-XRD, XEOL and other photoluminescence-based techniques. It should never be assumed that the input flux from a synchrotron is non-destructive; this has to be demonstrated for any particular material system. By using a large area beam, with a power density several orders of magnitude lower, XEOL microscopy has the potential to provide similar information in a less destructive or non-destructive way.
The early results from XEOM 1 presented here show chemical speciation of copper compounds on test structures and a heritage metal surface. Spectra extracted from columns of pixels in image stacks give chemical speciation with lateral resolution in the range 10 micrometres. Speciation can be improved by taking images through colour filters.
Construction of XEOM 1
The camera may be replaced by a broad band photomultiplier tube, Hamamatsu H8259-01 (Hamamatsu Photonics K. K., Japan) for filtered total ODXAS. This device can also be mounted on the sample manifold in parallel with an X-ray fluorescence detector to give total ODXAS (no optical filter) and total XAS channels simultaneously with image acquisition. An optical spectroscopy system with parallel detection is under design for the microscope.
The optical system
Instrument control, acquisition and data processing
XEOM 1 is controlled remotely using custom software esaXAS  running on a laptop PC which drives a pair of Agilent U2300A USB multifunction cards which provide control functions for the optics and sample handling. The software also controls the cameras using the relevant software functions. The software allows the user to set up a large variety of acquisition scenarios, automatically acquires and stores the image stacks and synchronizes image acquisition with the stepping of the monochromator on the synchrotron beam line.
Like the control software, the data processing software is custom written and forms part of the esaProject code (©2006-2014 EVA Surface Analysis) .
The data were acquired on beam line BM 28 (XMaS) at the ESRF. X-rays were incident at 60° to the sample surface in a footprint around 1 × 2 mm2 containing 1011-1012 photons s-1. At 9 keV this corresponds to an input power density <7.2 × 10-3 W mm-2. Image stacks containing 51 to 112 images were acquired at uniform intervals across x-ray energy range 8.96 - 9.01 keV, thus spanning the Cu-K absorption edge. Image acquisition times vary between 20 and 300 seconds in this work, but are constant for a given stack acquisition. In a typical experiment on a new material system, image stacks or total XEOL-XANES data (substituting a photomultiplier tube for the camera) are taken through a range of narrow band filters across the bandwidth of the device. Along with similar measurements on reference materials, this allows us to decide which filters to use in future analyses of similar samples.
Synthetic nantokite was made on the surface of copper electron microscope grids 20 μm thick and ∅3.05 mm with a pitch of 127 μm (Athene Old 200, Agar Scientific Ltd. UK). The grid has bars 35 μm wide and holes 90 μm square. Cleaning these is impractical so they were used directly from the packaging. They were immersed for 1 h in a saturated aqueous solution of copper II chloride (Aldrich, >99%) and then rinsed very briefly with deionized water to remove residual CuCl2. (This must be done rapidly or much of the CuCl patina will hydrolyse to Cu2O ). Cuprite was produced on ∅12.5 mm × 2 mm thick coupons of 99.9% pure copper (Goodfellow, UK). The copper surface was cleaned as described elsewhere  and heated to a dull red in a reducing Bunsen flame. A patina of >90% cuprite with a residue of tenorite forms immediately on air exposure .
The XEOM 1 project is greatly indebted to numerous organizations and individuals viz: The Research Foundation-Flanders (FWO) and the Paul Instrument Fund of the Royal Society for the funding which supported the construction; EVA Surface Analysis (UK) which supplies the XEOM instrument engineering and the data processing software (esaProject); the beam-line team on BM28 (XMAS) at the ESRF, especially Simon Brown and Laurence Bouchenoire; Adrian Lovejoy, David Greenshields, and Bob Day of the Electronics Workshop at the University of Warwick who built the electronics; The Computing Group at the ESRF for advice on interfacing to ESRF systems; Nigel Poolton, Manolis Pantos and Rob Pettifer for their early help and guidance. MH and P-JS are grateful to the UK Engineering and Physical Sciences Research Council (EPSRC) and Ghent University respectively for their PhD studentships. XMaS is funded as a Mid-Range Facility by the EPSRC. Finally, the author’s thanks go to the Trustees of the Mary Rose and Professor Mark Jones MBE, Head of Collections, for allowing them access to material from this historic ship and to Rosie Grayburn, PhD student, who took care of the samples on the beam line.
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