X-ray phase contrast and X-ray scattering images of pearls Michael S. Krzemnicki 1 , Vincent Revol 2 , Carina Hanser 3 , Laurent Cartier 1 , Henry A. Hänni 1 1 Swiss Gemmological Institute SSEF; michael.krzemnicki@ssef.ch 2 CSEM Centre Suisse d’Electronique et Microtechnique, SA, Alpnach, Switzerland 3 Institute of Earth and Environmental Sciences, University of Freiburg, Germany Keywords X-ray radiography, X-ray phase contrast, X-ray scattering, tomography, pearl structures Introduction The separation of natural pearls from cultured pearls is mainly based on the analysis and observation of their inter- nal structures. Due to their value and their importance as a cultural and even archaeological heritage, it is absolutely mandatory that such testing is non-destructive. Traditionally, pearl testing is mainly based on X-ray radiography (Anderson 1931, Strack 2006, Sturman 2009), visualizing slight variations in X-ray absorption within a pearl. These variations are linked to the presence, concentration, and orientation of organic matter or voids within the calcium carbonate pearl matrix. In recent years, X-ray microtomography (Xray-μCT) has strongly contributed to a better understanding of the spatial distribution of such internal features (Wehrmeister et al. 2008, Krzemnicki et al. 2010). In the present study, we have investigated the potential of X-ray phase contrast and X-ray scattering as new and promising complementary methods for pearl analysis. Instrumentation and samples Conventional X-ray imaging (radiography, CT scans) relies on the absorption of X-rays (attenuation) when transmit- ted through a sample material but does not take into account the phase shift of the X-ray beam caused by the sam- ple. One of the main drawbacks in conventional X-ray imaging is the limited possibility to increase the absorption contrast, especially for materials of similar absorption properties (e.g. biological samples) (Zhu et al. 2010). Although widely used in gemmological laboratories, X-ray radiography has a number of limiting factors due to in- strument parameters (e.g. focal size of the X-ray tube), analytical geometry (X-ray exposure is cone-shaped, resulting in lateral geometrical distortions of any 3D-object on the detector) and dynamic range and resolution of detectors (or X-ray films). But the most limiting factors are intrinsic properties of the pearl, such as its spherical geometry and thus variable path length (and attenuation) of the transmitted X-rays, and the tiny dimension and geometric orien- tation of internal structures (e.g. organic matter in the centre of the pearl). By using a combination of gratings it is possible to build an X-ray interferometer and obtain X-ray phase contrast images, for which the beam phase shift is transformed into intensity variations, which are then recorded by a detec- tor. Phase contrast X-ray imaging substantially improves contrast information compared to conventional attenua- tion-based methods. Especially for biological samples (e.g. tissues) made up of low-Z elements, the phase contrast effect is much more pronounced than the X-ray attenuation (Zhu et al. 2010). Based on ground-breaking research on synchrotron X-ray interferometry by research groups at the Paul Scherrer Institute (PSI) in Switzerland and the University of Tokyo (David et al. 2002, Momose et al. 2003), Pfeiffer et al. (2006) were able to transfer the grating-based method to conventional laboratory X-ray tubes. The same research group developed an additional imaging method, using the X-ray signal scattered at microstructures of the sample (Pfeiffer et al. 2008). 34 th IGC 2015 – Vilnius, Lithuania Saturday 29 th August 2015 117