X-ray fluorescence (XRF) is a commonly used technique
for elemental analysis. Compared to other technologies,
XRF is non-destructive, easy to use, and requires
minimal sample preparation. Micro-XRF applies the same
principles as XRF but on a much smaller (micro) scale.
To perform micro-XRF, a micron-sized beam of X-rays is required to hit the sample. This small beam can be achieved using a conventional pinhole aperture or X-ray optics. One challenge faced with using a conventional pinhole aperture is that it will block a large portion of the X-rays emitted from the source thereby reducing the number of X-rays hitting the sample, resulting in much lower fluorescence. Another challenge presented is that the output beam from the pinhole will be divergent, resulting in a larger excitation area on the sample. Both challenges impact the detection sensitivity and spatial resolution of micro-XRF analysis.
It has been proven that the use of polycapillary focusing optics greatly enhances micro-XRF analysis. Polycapillary optics offer many benefits; a large collection solid angle, high flux density, and a focused micron sized beam. Polycapillary optics transmit a polychromatic beam that can achieve a focused spot size as small as 5μm, FWHM at Rh Kα energy (20.2keV) with intensities greater than 107 photons/second. These performance attributes provide a great impact to micro-XRF across many different applications. Figure 1 below is an example of a mapping application.
1D XRF scan (bottom), of a copper PCB sample (top), using both a regular and a halo reduction polycapillary optic, each with a 15μm focal spot. The high-energy halo effect is clearly visible with regular optic while it is eliminated with the halo reduction optic.