Unlike paintings, most Cultural Heritage objects do not always possess a plane surface, and even paintings themselves are often not flat. Highly topographic objects range from metal or wooden sculptures to ceramics, glass, stone objects, and various forms of contemporary art. So far, topography is usually considered the main limiting factor of imaging analysis with a high spatial resolution. Recent approaches to overcoming this challenge include constructing equipment that can follow the surface topology [1, 2] and, for open beam systems measuring directly in the air, the post-processing of data to correct working distance variations using the Ar-signal [3]. Following the surface topography is impractical for intricate or highly irregular objects, where the spectrometer cannot always be positioned in an ideal perpendicular geometry to the surface. Even correction based on the Ar signal has its limitations.
We are adopting a different approach. In this case, a double 60 mm² SDD M6 JETSTREAM system is used. This instrument, equipped with a polycapillary lens, maintains good spatial resolution due to its low divergence design. The polycapillary lens irradiates the sample with a perpendicular beam (Fig. 1a), typically with a smallest spot size of approximately 30-50 µm (Fig. 1b). Due to the beam's low divergence, the minimum spot size is achieved at ~3 mm closer to the sample surface than the first calibrated position at a 100 µm spot size.
Generally, the M6 JETSTREAM has five calibrated spot size positions based on increased working distances and the corresponding spot divergence of the polycapillary lens. It is important to note that a longer distance results in a loss of intensity (Fig. 1b).
The M6 JETSTREAM's patented Aperture Management System (AMS) further reduces the divergence of the X-ray beam. The AMS is a specially designed aperture that can be positioned automatically between the X-ray source and the polycapillary lens entrance. The selected aperture narrows the diameter of the primary radiation entering the lens, excluding the outer capillaries from the transmission process (Fig. 2a). This approach significantly reduces beam divergence. As shown in Figure 2, the standard divergence of the M6 JETSTREAM lens is ~30 µm/mm; when using the smallest aperture, this is reduced to 10 µm/mm.
The AMS comes in two variants: the AMS 1000 and the AMS 500, which differ in the diameter of the opening shaping the primary beam before it enters the lens. Effectively, at a distance of 10 cm, the spot size would be around 4 mm wide, but the AMS 1000 and AMS 500 can reduce this to 2.5 mm and 1.2 mm, respectively (Fig. 2b). However, this reduction in divergence comes with a trade-off: the AMS reduces intensity by approximately 1/3 for the AMS 1000 and 1/7 for the AMS 500. Therefore, the AMS is only available for double 60 mm² SDD systems to help compensate for this loss in intensity.
How does this now apply in practice? Measuring an object like the polychrome painted wooden sculpture of Jesus depicted below, the working distance of the M6 JETSTREAM is limited by the highest position of the sculpture, in this case the sculpture’s knee . This means, that the hands of the small sculpture are 5 cm out of focal plane.
Micro-XRF elemental distribution images of a M6 JETSTREAM scan without the AMS reveals the presence of lead white (Pb) and vermilion (Hg) in areas below the focal plane, but longer measurement times and the AMS 500 enables the visualization of cracks in the paint on the hand as well as the fine brushwork of the vermilion-red paint above the lead white flesh tone.
The same principles apply to all types of materials which either have a strongly diverging surface topography or cannot be aligned parallel to the measuring head. Even very fine structures and trace amounts can be visualized to a better degree.
The following cases reveal the power of the AMS feature:
As discussed above, using the AMS reduces the intensity up to 1/7 for the AMS 500. Nevertheless, as in the case of the tea cup, even without enhancing the measurement time per pixel, a gain in resolution is notable. This means, that even if the overall time available for conducting an analytical campaign cannot be prolonged, the AMS offers an opportunity to enhance results without having to consider the lower intensity.
The first measurement of the tea cup shown below was conducted with excitation conditions of 50 kV and 600 µA. The pixel size is fixed to 250 µm, while the dwell time per pixel is at 20 ms. In the following example, the cup was remeasured without changing any of the measurement parameters but using the AMS 500. While only 1/7 of the intensity is expected, the rose decoration still becomes visible to a high resolved extent. With a 7x times longer measurement time per pixel (140 ms), application traces of the various colourants are clearly notable.
As shown in the examples above, the AMS is a powerful feature that allows users to retain a high spatial resolution of the micro-XRF elemental distribution images. Though its use results in a decrease of the relative intensity, this can be easily circumnavigated by increasing measurement time or incoming flux. Even if no additional time is available, a higher resolution in areas far out of focus can be gained.
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