Samuel Blahuta

Improved scintillation time response in (Lu0.5Gd0.5)2O3 : Eu3+ compared with Lu2O3 : Eu3+ transparent ceramics

The scintillation properties of two sesquioxides ceramics Lu2O3 : Eu3+ and (Lu0.5Gd0.5)2O3 : Eu3+ were studied. Both ceramics present comparable transparency and light yield whereas (Lu0.5Gd0.5)2O3 : Eu3+ showed an order of magnitude reduced afterglow in the 3–300 ms range. A thorough study of the location and behaviour of Eu3+ dopant ions at C2 and S6 sites of Lu2O3 and (Lu0.5Gd0.5)2O3 structures was carried out with low-temperature selective excitation of Eu3+. This revealed that (i) at both C2 and S6 sites, Eu3+ 4f–4f lifetime is shorter in (Lu0.5Gd0.5)2O3 : Eu3+ than in Lu2O3 : Eu3+, (ii) the host matrix (Lu0.5Gd0.5)2O3 as compared with Lu2O3 favours the location of Eu3+ at C2 site. As decay times of Eu3+ in C2 and S6 sites are 1.0 ms and 3.8 ms, respectively, the preferred occupation of C2 in (Lu0.5Gd0.5)2O3 : Eu3+ implies a much shorter decay time for (Lu0.5Gd0.5)2O3 : Eu3+ in the 3–20 ms range. Reduction of afterglow in the 20–300 ms range is illustrated by thermally stimulated luminescence peaks presenting a highly reduced intensity for (Lu0.5Gd0.5)2O3 : Eu3+ compared with Lu2O3 : Eu3+ implying reduced charge trapping defects in (Lu0.5Gd0.5)2O3 : Eu3+ ceramics.

Control of the point defects in oxide materials to enhance functionalities in imaging

Last generation medical imaging equipments require materials which possess outstanding performances. For scintillators in the high energy imaging field (PET), crystals with high light yields allow a decrease of the irradiation dose received by the patients during medical application and a more accurate diagnostic. Thermally stimulated luminescence (TSL) data provides the depth of hole or electron traps which can limit the efficiency and increase the kinetic. If these traps are due to lanthanide ions, the level schemes can predict the depth values. Thanks to comparison between TSL glow curves and energy diagrams, the traps inside oxide-based-hosts can be identified. Two examples are proposed here, first, the scintillation in the Ce:LYSO crystals which can be improved by thermal annealing and where divalent cations are used for charge compensation and traps removal and second, optical imaging using a new approach where persistent luminescent nanoparticles are used for in-vivo imaging. In both cases, traps depth should be carefully controlled.

Improving Ce3+ doped scintillating materials for medical imaging applications

Ce:LYSO (Lu1.90Y0.1SiO5:Ce) single crystals were prepared by the Czochralski technique. We report some effects of the X-ray irradiation on the shape of the emission spectra. For instance a large decrease of the longest wavelength emission (at about 500 nm) is observed with an increase of the light yield after one hour under X-ray irradiation. Electron paramagnetic resonance (EPR) evidences no change in the two cerium site occupancy along with the irradiation time. A mechanism taking into account the evolution of the oxygen vacancies as neighbors of Ce3+ ions is proposed.