Confocal Micro X-Ray Fluorescence: A New Paradigm in Materials Characterization

Abstract

Micro x-ray fluorescence (MXRF) is a microscopic analysis and imaging technique that is used to characterize the elements in a material non-destructively. Micro XRF instruments use an x-ray source to shine x-rays on a sample, and a detector to detect the characteristic x-rays given off. These fluorescent x-rays have very specific energies corresponding to specific electron energy transitions. Therefore, it is possible to detect and identify all of the elements present in a sample (typically above sodium) as well as measure their concentrations. This technique is widely used for the characterization of materials including polymer and metallic foams, powder samples, forensics applications, geological samples, works of art and nuclear fuels. Commercial MXRF instruments use a fused silica optic (mono or polycapillary) to focus the x-rays on the sample with no optic on the detector (Figure 1a).

Three‐dimensional density measurements of ultra low density materials by X‐ray scatter using confocal micro X‐ray fluorescence spectroscopy

“…Benefits of this technique include high sensitivity, low background, data collection at each point, moderate to high spatial resolution and non‐destructive to the sample. Some of the drawbacks include low signal counts (for this particular instrument design) leading to long acquisition times, decreasing signal due to absorption of the signal with increasing depth and artifacts in the signal due to instrument geometry . As seen from the data presented here, this technique can generate 1D line scans, 2D contour plots, as well as full 3D maps of density variations.…”

“…All measurements were conducted with a custom‐built, bench‐top confocal X‐ray fluorescence instrument (Fig. ).…”

“…Dwell time for this 3D image was 10 s per point; imaging the entire piece took ~ 4 weeks. Other spectral regions of interest could also be collected, which were usually set to specific elements, and multiple element 3D maps can be overlaid . The scatter region could then be truncated to avoid any elemental signal; however, that was not required for any of these samples.…”

“…μXRF has been applied in the characterization of samples in both the materials and biological sciences. These include polymers, pharmaceuticals, forensics, art and archeological materials, paint chips, and bone . The instrumentation used is quite varied and may include either a synchrotron or a small laboratory‐based high voltage anode as the X‐ray source.…”

“…Elemental correlations have also been investigated using 2D μXRF . In our laboratory, confocal μXRF has shown to be very useful for imaging graded copper doped beryllium capsules, examining thin films, coatings, and layered materials . Finally, confocal μXRF has been used synergistically with 3D micro X‐ray computed tomography to overlay lower resolution elementally specific images with high resolution X‐ray attenuation information, which can greatly enhance sample characterization.…”