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Confocal or Normal Micro-XRF spectrometer for light element analysis

Since most available micro-XRF spectrometers operate in air, which does not allow the analysis of low-Z elements (Z ≤ 14), a special micro-XRF spectrometer has been designed to extend the analytical range down to light elements (Z ≥ 6). It offers improved excitation and detection conditions, necessary for light element analysis. To eliminate absorption of the exciting and fluorescent radiation the system operates under vacuum condition. Sample mapping is automated and controlled by specialized computer software developed for this spectrometer by the Atominstitut.

This system consists of an air cooled low power x-ray tube (50W) with molybdenum anode and a thin (125µm) exit window. An optional beam filter can be inserted to reduce spectral background. The beam is focused onto the sample using a polycapillary x-ray optics, offering a focal spot of about 31µm FWHM for Mo-Kα. Characteristic X-rays from the sample are detected by means of a Si(Li) detector with ultra thin window. An optical microscope attached to a high resolution CCD camera is used to control the measurement position. Sample positioning and scanning is performed using a motorised xyz sample stage.

Recently the spectrometer has been adapted to confocal geometry by inserting a second polycapillary X-ray optics in front of the detector. This allows for 3D elemental imaging of light elements in the laboratory. The new setup can be seen in Fig.1. The result of a 3D measurement of a test structure can be seen in Fig.2.

The new spectrometer offers improved excitation and detection conditions, necessary for light element analysis. The thin window of the x-ray tube allows both, the molybdenum L-lines and K-lines to sufficiently excite the sample over a wide energy range. Detection of the low energetic characteristic radiation is possible due to the ultra thin window of the detector. To eliminate absorption of the exciting and fluorescent radiation in air the system operates under vacuum condition. Sample scanning is automated and controlled by specialized computer software developed for this spectrometer (Fig.3).

Fig.1: Photo of the inside of the spectrometer in confocal geometry.
Fig.2: 3D scan of a Cu wire cross (red) on a Gd screen (yellow) fixed with sticky tape (blue). Scan size: 230 x 230 x 230 µm³.
Fig.3: Screenshot of the measuement software.