The mobile XRD scanner consists of a measurement head equipped with an high brilliant monochromatic source (two sources with with Ag or Cu anodes respectively are available) focused with a Montel multilayer optic down to a spot-size in the range of 100 microns and with a divergence of about 7.5 mrad. The primary X-ray source is coupled to a 1D hybrid photon counting (HPC) detector covering an angular range of some tens of degree. The HPC detector is chracterized by 1000 silicon strips presenting 50 micron width, 8mm length and 450 micron thickness each. The use of the large area HPC detector allow us to avoid the angular scanning usually performed in tabletop XRD device operated in the laboratory. In fact, the diffraction pattern is taken with a one-shot measurement per pixel (lasting about 1s-10s depending on the investigated material). The detection efficiency of the HPC detector is high enough in the energy range between 5keV-40keV to work with different anode materials in the monochromatic source (i.e., typically the Cu-source at 8.041 keV is used in a reflection geometry while the Ag-source at 22.10 keV is used in a transmission geometry). An updated version of the XRD scanner, still under development, will be based on a 2D HPC detector allowing to cover a larger solid angle and to reduce drastically the measurement time. The irradiation geometry per pixel is based on a 90 degree incidence angle for the primary beam with resepct to the sample surface, while the angular range for detecting the scattered X-rays covered by the HPC detcetor is about 10-70 degrees (2theta) at a distance of 10 cm. In the XRD imaging procedure the sample is fixed while the measurement head is moved by a XYZ travel system to perform the scanning of the sample. A single scan, performed pixel-by-pixel, covers an area of 50x50cm2. The Z axis is coupled with a laser triangulation device able to operate a dynamic correction in order to maintain the source-sample-detector geometry uniform during the scanning. The point analyses or the 2D mineral-phase mapping are performed in order to better elucidate the chemical composition the investigated material. Point analyses last typically 10s-60s while the XRD mapping can last several hours.The system is equipped with a central unit (CU) for controlling the operational parameters of the XRD scanner and to operate the analysis (based on a ROI method) of the diffraction patterns providing in real-time the mineral-phase distribution images to users.
Potential Results
The XRD technique is highly effective for determing the phase composition of policrystalline materials. In Heritage Science, the cases of interest for its application are mostly paintings and metals. The registered diffraction pattern provides information on the chemical composition of the original components of the investigated materials (e.g., the nature of pigments in a painting) and/or the texture of the material (e.g., trace of the manufacturing technology in metallic alloys). Finally it is particularly suited to detect degradation products of corrosion patinas providing information necessary to addres a proper conservation procedure.