Confocal X-ray Fluorescence (XRF) Microscopy: A New Technique for the Nondestructive Compositional Depth Profiling of Paintings

Abstract

A confocal x-ray fluorescence microscope was built at the Cornell High Energy Synchrotron Source (CHESS) to determine the composition of buried paint layers that range from 10-80 µm thick in paintings. The microscope consists of a borosilicate monocapillary optic to focus the incident beam and a borosilicate polycapillary lens to collect the fluorescent x-rays. The overlap of the two focal regions is several tens of microns in extent, and defines the active, or confocal, volume of the microscope. The capabilities of the technique were tested using acrylic paint films with distinct layers brushed onto glass slides and a twentieth century oil painting on canvas. The position and thickness of individual layers were extracted from their fluorescence profiles by fitting to a simple, semi-empirical model.

2D-imaging of sampling-probe perturbations in laminar premixed flames using Kr X-ray fluorescence

“…The polycapillary optic only allowed X-rays from a limited region (300 µm FWHM) of the beam path (along the z-axis) to reach the detector. As such, the measurement probes a discrete volume 5 x 7 x 300 µm FWHM in size, with a technique known as X-ray confocal microscopy [31].…”

Multilayer X-ray optics at CHESS

“…Some of the recent CHESS projects based on microbeam fluorescent analysis, such as reading ancient inscriptions (Powers et al, 2005) or studies of art objects (Woll et al, 2005), generated a lot of interest in extending ML optics to 30 keV (Ag, Sn K-edges) and higher. The obvious practical limit is the geometrical design constraints of double-crystal monochromators, i.e.…”

Single-bounce monocapillaries for focusing synchrotron radiation: modeling, measurements and theoretical limits

“…With the CHESS D line bending-magnet source, a 1% multilayer monochromator, and with the accelerator storage ring running at 125 mA, we obtained 1.0 Â 10 12 X-rays s À1 at 10 keV in the full spot after blocking the direct through-beam. This arrangement was successfully used for trace-element microbeam fluorescence mapping (Bilderback et al, 2002), and more recently in a confocal XRF geometry [a monocapillary for focusing the incident beam and a polycapillary at 90 for receiving (Woll et al, 2005)] to obtain an elemental profile along the sample depth direction, similar to the method of Proost et al (2002).…”

“…This improvement was obtained mainly by making the capillary length equal to twice the focal distance, which is the optimized length for best focusing as discussed in this paper. With further refinements of the glass puller control system, capillary profile accuracy has also been significantly improved, enabling us to make dozens of capillaries with total divergence controlled very well at various values (from 2 to 8 mrad) for a variety of applications at CHESS, such as microcrystallography, microbeam X-ray standing wave (Kazimirov et al, 2004), scanning microspectroscopy (Bilderback et al, 2002), microbeam hightemperature high-pressure diffraction with diamond anvil cells and confocal X-ray fluorescence (Woll et al, 2005). Optical formulae are developed below to easily explain SBC focusing properties even without numerical coding, which helps both beamline scientists and users to choose capillary parameters to best fit their applications.…”