Study of luminescence properties in lapis lazuli diopside crystals during micro-IBIL measurements

Micro-Ion Beam Induced Luminescence or micro-IBIL consists of the photon emission in the IR-VIS-UV range from a crystalline material that occurs when a significant amount of energy is transferred to it by an ion probe. The intensity and wavelength of the luminescent light provide information on the nature of luminescence centers, such as trace-level impurity interstitial and structural defects of the matrix. In the framework of the Ion Beam Analysis applied to cultural heritage studies, IBIL is usually acquired simultaneously with other complementary techniques, such as PIXE, generally guaranteeing a completely non-destructive approach, which is suitable in the case of archaeological or artistic samples. However, the interaction of the proton beam with the atoms of the target crystal, with the possible creation of local radiation-induced structural damages, can affect the luminescence response and so the detection of a signal useful for analysis. In the present work, a preliminary investigation of the radiation hardness of a reference diopside crystal (CaMgSi2O6) inside a lapis lazuli rock is presented. This is realized by following the evolution of its IBIL signal upon proton irradiation and by comparing the results obtained repeating the same measurement after 2 years with the same experimental setup. This work falls in the framework of the provenance study of lapis lazuli that the Solid State Group of the University of Torino is carrying on since 2008; in the provenance study, the position of the luminescence bands of diopside crystals represents a useful marker for distinguishing among different sources of the raw rock used for lapis lazuli artworks in antiquity.