Local field effects on the radiative lifetimes of Ce3+^{3+} in different hosts

For emitters embedded in media of various refractive indices, different theoretical models predicted substantially different dependencies of the spontaneous emission lifetime on refractive index. It has been claimed that various measurements on $4f\to 4f$ radiative transition of Eu$^{3+}$ in hosts with variable refractive index appear to favor the real-cavity model [J. Fluoresc. 13, 201 (2003) and references therein, Phys. Rev. Lett. 91, 203903 (2003)]. We notice that $5d\to 4f$ radiative transition of rare-earth ions, dominated by allowed electric-dipole transitions with line strengths less perturbed by the ligands, serves as a better test of different models. We analyze the lifetimes of $5d\to 4f$ transition of Ce$^{3+}$ in hosts of refractive indices varying from 1.4 to 2.2. The results favor the macroscopic virtual-cavity model based on Lorentz local field [J. Fluoresc. 13, 201 (2003)].Comment: 9pages, 1 figures, presented on AMN-2 and to appear on Current Applied Physic

Scintillateurs denses et rapides pour la détection de rayonnement gamma. Monocristaux à base de silicates de lutécium dopés Ce3+.

Ce travail concerne l'étude de cristaux scintllateurs et en particulier, le développement de silicates de lutécium dopés cérium: Lu2Si2O7:Ce3+ (LPS) et Lu2(1-x)Y2xSiO5:Ce3+ (LYSO). Une densité et un numéro atomique effectif élevés ,une reponse rapide, un rendement lumineux élevé sont quelques-unes des propriétés qui rendent les cristaux de LPS et de LYSO intéressants pour la detection de rayonnement gamma. En raison de leur application potentielle en imagerie médicales (TEP° et en géophysique, le développement de ces cristaux est envisagé. Un travail important d'optimisation des conditions d'élaboration par la méthode Czochralski a été effectué afin d'obtenir des cristaux performats de grande taille, et ce, de façon reproductible. Ce développement ne peut se faire sans une bonne compréhension des mécanismes. Dans ce sens, plusieurs approches (XPS, spectroscopie optique résolue en temps...) sont proposées pour localiser les niveaux d'energie des lanthanides par rapport à ceux de la matrice

Scintillateurs denses et rapides pour la détection de rayonnement gamma (monocristaux à base de silicates de lutécium dopés Ce3)

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Production of Hydroprocessed Esters and Fatty Acids (HEFA) – Optimisation of Process Yield

Both Fischer-Tropsch (FT) and Hydroprocessed Esters and Fatty Acids (HEFA) Synthetic Paraffinic Kerosine (SPK) fuels are considered as leading alternative replacements for conventional jet fuel. To satisfy the requirements of Civil Aviation Authorities (CAA), their drop-in incorporations have been subjected to a rigorous certification process. To reach the ambitious incorporation targets, new routes for biofuels incorporation may need to emerge, involving optimizing the production processes and the blending strategies. This paper focuses on a new strategy for incorporating HEFA, allowing the process yield to be optimised. One of the major steps limiting the process yield for HEFA remains the isomerisation that allows production of a biofuel with very good cold flow properties. But this step introduces a substantial decrease of the overall yield (fuel component per kg of starting material) due to the production of light compounds, unsuitable for conventional jet fuel. In this work relaxing the freezing point requirement for the neat HEFA component (by decreasing the severity of the isomerisation step) is proposed in order to minimize the production of less valuable light compounds. This strategy could lead to a significant additional biofuel yield with respect to the oil compared to a process making a better freeze point component. This allows the land surface area necessary for HEFA feedstock cultivation to be reduced for a given amount of bio-jet fuel produced