Optical spectroscopy and imaging of colour centres in lithium fluoride crystals and thin films irradiated by 3 MeV proton beams

Lithium fluoride is a well-known dosimeter material and it is currently under investigation also for high-resolution radiation imaging detectors based on colour centre photoluminescence. In order to extend their applications, proton beams of 3 MeV energy, produced by a linear accelerator, were used to irradiate LiF crystals and thin films in the fluence range of 1010–1015 protons/cm2. The irradiation induces the formation of colour centres, mainly the primary F centre and the aggregate F2 and F3+ defects, which are stable at room temperature. By optical pumping in the blue spectral region, the F2 and F3+ centres emit broad photoluminescence bands in the visible spectral range. By conventional fluorescence microscopy, the integrated photoluminescence intensity was carefully measured in LiF crystals and thin films as a function of the irradiation fluence: a linear optical response was obtained in a large range of fluence, which is dependent on the used LiF samples. Colour centres concentrations were estimated in LiF crystals by optical absorption spectroscopy. It was possible to record the transversal proton beam intensity profile by acquiring the photoluminescence image of the irradiated spots on LiF films.