Decay Kinetics of Gd3Al2Ga3O12:Ce3+ Luminescence under Dense Laser Irradiation.

The decay kinetics of Gd3Al2Ga3O12:Ce3+ single crystal luminescence were studied under dense laser excitation. It was shown that the decay times as well as the intensity of Ce3+ luminescence depend on the excitation density. The observed effects were ascribed to the interaction between excitons as well as to the features of energy transfer from the excitons to Ce3+. The numerical simulation of the experimental results was performed for justification of the proposed model.

Deep traps can reduce memory effects of shallower ones in scintillators.

X-ray induced luminescence sensitization results have been obtained on three commercially relevant scintillators, namely CsI:Tl, YAG:Ce and LSO:Ce. The obtained curves have been used to validate a model based on the competition among trapping and recombination of free charge carriers. The model was able to accurately describe the complex phenomenology of the detected sensitization curves. We also used the model to predict the role of a high temperature and concentration trap in shaping the sensitization curves. Based on these modelling results we also proposed a novel, and rather counterintuitive, strategy to deal with the sensitization phenomenon based on the deliberate introduction of deep traps which can significantly reduce the bright burn effect.

Modelling energy deposition in nanoscintillators to predict the efficiency of the X-ray-induced photodynamic effect.

Scintillating nanoparticles (NPs) in combination with X-ray or γ-radiation have a great potential for deep-tissue cancer therapy because they can be used to locally activate photosensitizers and generate singlet oxygen in tumours by means of the photodynamic effect. To understand the complex spatial distribution of energy deposition in a macroscopic volume of water loaded with nanoscintillators, we have developed a GEANT4-based Monte Carlo program. We thus obtain estimates of the maximum expected efficiency of singlet oxygen production for various materials coupled to PS, X-ray energies, NP concentrations and NP sizes. A new parameter, ηnano, is introduced to quantify the fraction of energy that is deposited in the NPs themselves, which is crucial for the efficiency of singlet oxygen production but has not been taken into account adequately so far. We furthermore emphasise the substantial contribution of primary interactions taking place in water, particularly under irradiation with high energy photons. The interplay of all these contributions to the photodynamic effect has to be taken into account in order to optimize nanoscintillators for therapeutic applications.

Light yield sensitization by X-ray irradiation of the BaAl4O7:Eu(2+)ceramic scintillator obtained by full crystallization of glass.

We report the scintillation properties of BaAl4O7:Eu(2+), a transparent polycrystalline ceramic prepared by full and congruent crystallization of glass. We show that a small deviation from the stoichiometric composition as well as thermal treatment duration play a crucial role in the formation of charge carrier traps, leading to a strong influence on the scintillation yield. We demonstrate that when the traps are not entirely removed, X-ray irradiation allows them to be permanently filled in order to significantly enhance the scintillation output. Finally, the best sample obtained demonstrates performances able to compete with a commercially available scintillating material, CsI:Tl.

Interaction of intense femtosecond laser pulses with KDP and DKDP crystals in the short wavelength regime.

We investigate the electronic photo-excitation and relaxation mechanisms involved in the optical breakdown of potassium dihydrogen phosphate crystal (KH2PO4) and its deuterated form. The dynamics and spectroscopic properties of electron-hole pair formation are investigated using time-resolved measurement of the dielectric function, and luminescence spectroscopy. The non-common mechanical and electronic characteristics of these dielectric materials are revealed by the particular structure of ablation craters and also by the complex dynamics observed in the relaxation of excited carriers. This relaxation occurs in two steps, and varies with the initial carrier density and thus with the laser intensity. We show that the defect states play a key role in the excitation pathways, and also determine the relaxation stage. The latter also depends upon the initial amount of energy of the electron-hole pair after photo-excitation. A model based on kinetic equations describing the evolution of the different level populations allows us to successfully interpret and reproduce the experimental data.

A molecular precursor approach to monodisperse scintillating CeF3 nanocrystals.

A series of anhydrous cerium(III) trifluoroacetate complexes with neutral O-donor ligands, namely Ce2(OAc)(TFA)5(DMF)3 (1), Ce(TFA)3(L)x [x = 2, L = THF (2), DMF (3), DMSO (4); x = 1, L = diglyme (5)] and Ce2(TFA)6(DMSO)x(DMF)y [x = 6, y = 0 (6); x = 4, y = 2 (7)] (where OAc = acetate, TFA = trifluoroacetate, THF = tetrahydrofuran, DMF = dimethylformamide, DMSO = dimethylsulphoxide, and diglyme = MeO(C2H4O)2Me] were synthesized and completely characterized by elemental analysis, FT-IR spectroscopy and TG-DTA-MS studies. A partially hydrated complex [Ce(TFA)3(diglyme)(H2O)] (8) was obtained by slow evaporation of the THF solution of anhydrous 5 in the air. The single crystal X-ray diffraction analysis of 1, 3, 4, and 6­8 showed the versatile bonding mode of the TFA ligand (terminal, chelating and bridging). These complexes, on decomposition in 1-octadecene in inert atmosphere, gave CeF3 nanoparticles of 8­11 nm size. The complex 5 proved to be the best precursor in the series because of the ability of the diglyme ligand to act as capping reagent during decomposition to render the CeF3 particles monodisperse in organic solvents. The obtained CeF3 nanoparticles were characterized by FT-IR, EDX analysis and TEM studies and their luminescence and scintillation responses under UV and X-ray excitation were studied and compared with that of CeF3 single crystal.