XEOM 1 - A novel microscopy system for the chemical imaging of heritage metal surfaces

Abstract

Background: 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.Results: 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. Conclusions: 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.

Synchrotron X-ray diffraction investigation of the surface condition of artefacts from King Henry VIII's warship the Mary Rose

“…Once more there is some doubt from these data about the presence or otherwise of nantokite because of its indistinguishability from sphalerite, but again there is no paratacamite hence no evidence of hydrolysis of nantokite. In addition, we show elsewhere (Dowsett et al, 2015) that Cu K-edge XEOL-XANES from the surface of this link does not contain a nantokite signature but is characteristic of cuprite. We therefore conclude that the main residual corrosion products are sphalerite and covellite with some cuprite.…”

“…XEOL-XANES data were also acquired on XMaS using the broadband photomultiplier (H8259-01, Hamamatsu Photonics KK) fitted to our XEOL full-field microscope system XEOM 1 (Dowsett et al, 2015;Sabbe et al, 2014). X-rays were incident at 60 to the surface, and take-off was at 45 .…”

“…However, the combination only gives information on the crystalline materials present. Therefore, in a parallel study we used a microscope (XEOM 1) (Dowsett et al, 2015) based on X-ray excited optical luminescence (XEOL) to examine the copper K-edge X-ray near-edge absorption structure (XANES). Some of these data have been published elsewhere (Dowsett et al, 2015;Sabbe et al, 2014).…”

Evaluation of an X-ray-Excited Optical Microscope for Chemical Imaging of Metal and Other Surfaces

“…The design and construction of XEOM 1 is described fully elsewhere. , It was manufactured using black acetal copolymer, shown to have a low intrinsic visible fluorescence under X-ray excitation, and comprises three main sections (Figure ). The first consists of the sample stage surrounded by a number of ports at 10°, 30°, 45°, and 60°.…”

Spatially and Time-Resolved Carrier Dynamics in Core–Shell InGaN/GaN Multiple-Quantum Wells on GaN Wire

“…In this context, a recent TRXEOL system has been further developed in the hard X-ray nanoprobe 23A of the Taiwan Photon Source . At macroscopic length scales, conventional XEOL layouts can be found in the Canadian Light Source, the Advanced Photon Source, the Cornell High Energy Synchrotron Source and the Diamond Light Source. − …”