The mobile confocal XRF scanner (CXRF) consists of a spectrometric head equipped with a low power microfocus X-ray tube (30W) with a Mo anode coupled to a highly focusing polycapillary optic. The spot of the beam coming out from the primary X-ray source is 10 microns at 10 kV at a focus distance of 3.5 mm. The detection system is positioned in a 90-degree geometry with respect to the beam direction. This consists of a SDD detector (with 50mm2 active area and <130 eV energy resolution at 5.9 keV) coupled to a secondary polycapillary with a focus equal to 10 microns at 10 keV at a distance of 5 mm. The intersection of the two foci allows defining the analytical volume with which to perform the XRF scanning of the sample along its thickness (1D elemental stratigraphy) or on the three XYZ directions (3D elemental stratigraphy). A long-range optical microscope (OM) with a 3 microns resolution is installed on the spectrometric head; it is used to guide the confocal XRF scanning and to carry out investigations of the materials in OM. A single scan with the CXRF instrument can cover a volume on the sample equal to 50x50x20 cm3. The system scans the sample with a maximum speed up to 50 mm/sec. In order to ensure sufficient counting statistics, measurements are usually made with a dwell-time per pixel in the range 0.1s and 3s, depending on the matrix material being studied. A point analysis and/or a 3D elemental mapping (depending on the application) can be performed on the samples in order to better elucidate its stratigraphy and its elemental composition along the thickness. Point analyses last typically 1000s while 3D CXRF mapping several hours. The spatial resolution is in the range of 10-15 microns depending on the detected chemical element. The system is equipped with a central unit (CU) for controlling the operational parameters of the scanner and to operate a dynamic analysis (full deconvolution of the pixel spectra on the fly) providing real-time elemental distribution images to users already during the scanning.
The confocal XRF measurements is suited to investigate samples with a complex stratigraphy providing information on the elemental compositon along its thickness (1D in-depth profiling) or providing 3D elemental distribution images of the layers. It is non-invasive and highly effective for the identification of inorganic elements with high chemical sensitivity. Precise information can be collected from the surface of a sample down to its ground evidencing the presence of surface contaminations, restoration products and allowing to better elucidate the elemental compostion of the original materials along the layer sequence.