Andrei Belsky

Luminescence and Scintillation Properties at the Nanoscale

This contribution is a review of the luminescence and scintillation properties of nanoparticles (NP), particularly doped insulators. Luminescence spectroscopy is an appropriate tool to probe matter at the nanoscale. Luminescence is also the last stage of the scintillation process. Specific surface and structural effects occurring in NP are reported. Their consequences on the NP luminescence properties are discussed. Parts of the effects are related to the preparation method. On the other hand some intrinsic properties of the nanostructures which can modify the optical properties are described: quantum confinement and dielectric confinement. The response under high-energy excitation is also discussed. It appears that their size can be used as a tool to describe the spatial distribution of electronic excitations induced by the relaxation process after the high-energy excitation. Finally, potentiality to grow transparent bulk materials based on small Nps agglomeration via soft chemistry route is presented. It is a promising approach toward the development of scintillating materials.

Scintillation Efficiency Improvement by Mixed Crystal Use

Analysis of the last years theoretical studies and track simulations to conclusion that primary stages (electron scattering and e-h thermalization) play the key role in the following scintillator efficiency. The long thermalization length comparing to Onsager radius is the main reason for geminate pair concentration decrease and later luminescence losses. The easiest way for thermalization length decrease is the scintillation crystal doping or even transfer to the mixed crystals (solid solution). The simple model of modification of electrons scattering and e-h pairs thermalization for the mixed crystals is proposed. It is shown that solid solutions have higher light output independently on the crystal type. Analysis of experimental data confirmed this conclusion. This phenomenon is found for halide, oxide and sulfates scintillators. The similar behavior is typical for mixed anion and/or cation systems. The key role of initial track formation stages is illustrated by the same trend for activated scintillators and pure crystal with intrinsic luminescence. These estimations and experimental data lead to the conclusion that the scintillation efficiency improvement by mixed crystal use can play an important role in the search and development of new scintillators.

Spectroscopic properties of CeF3 and LuF3:Ce3+ thin films grown by molecular beam epitaxy

Abstract First results on optical properties of CeF3 and LuF3:Ce3+ thin films deposited by molecular beam epitaxy on both calcium fluoride and silicon substrates are presented. For the first time, absorption spectra have been measured and analyzed, and oscillator strengths of the 4f→5d absorption transitions have been deduced. Extra absorption and excitation bands at higher energy have been assigned to Ce 4f→6s and F 2p→Ce 5d transitions and one sharp band near the fundamental absorption to the exciton formation.

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 6–8 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 8–11 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.

Growth and light yield performance of dense Ce3+-doped (Lu,Y)AlO3 solid solution crystals

Many Ce-doped (Lu,Y)AlO 3 solid solution single crystals of various Lu/Y ratio and dopant concentration have been grown using the vertical Bridgman and Czochralski processes. Light yields of around 200% of BGO have been measured under gamma-ray excitation. The as-grown and gamma-ray-induced color centers have been compared in crystals of various composition. The absorption properties were characterized in as-grown crystals and after exposure to 1 Mrad of 60 Co. The extent of radiation-induced optical absorption was found to be dependent on the initial light transmission properties of the crystals.

Fluorescence Properties of CeF 3 and of Some Other Cerium Doped Crystals and Glasses Under VUV and X-RAY Synchrotron Excitation

Reflectivity and fluorescence excitation spectra and fluorescence decays of CeF{sub 3} and LaF{sub 3}:Ce have been recorded and analyzed in the VUV range up to 120 eV and in the X-ray region using the synchrotron radiation. Dominant mechanisms are identified in various regions of excitation, leading to different shapes of the excitation spectra and typical profiles of the decay curves. Strong difference in the light yield of fluorides and oxides are shown and discussed in terms of relative energies of levels of cerium and of the valence and conduction bands of the solid compounds.

Progress in the development of LuAlO/sub 3/ based scintillators

LuAlO/sub 3/:Ce/sup 3+/ (LuAP) and Lu/sub x/Y/sub 1/-xAlO/sub 3/:Ce/sup 3+/ (LuYAP) crystals are used as scintillation materials for positron emission tomography. The actual study of these scintillators develops in three directions: (i) growth of large size LuAP crystals with stable properties, (ii) the relationship between the composition of LuYAP crystals and scintillation properties, and (iii) scintillation mechanisms in lutetium compounds. After improving of growth conditions a large size samples (length >40 mm) have been prepared. Crystals show a good correlation between growth parameters, light yield and transmission spectra. We studied a series of samples with calibrated size (2/spl times/2/spl times/10 mm3) and compare the light yield with standard BGO and LSO samples. Mixed crystals with composition of 0.6<x<0.8 show a significant increase of light yield. We suggest that the short order clusterisation in mixed crystals may play an important role in governing the scintillation efficiency. In order to clarify the scintillation mechanism in LuAP and LuYAP crystals measurements of absorption, emission and excitation spectra, thermoluminescence and scintillation decay curves have been performed.

X-ray excitation of luminescence of scintillator materials in the 7-22 keV region

According to usual models describing the scintillation process, the light yield 77 is expected to be constant under high energy excitation much larger than the gap (Es) of the materials. 1) can be defined as the product of three constant parameters [l] for each band of fluorescence emission:

Time-Resolved VUV Excited Luminescence of

Luminescence emission and excitation spectra as well as decay kinetics of Y2O3-Yb nanoparticles elaborated by the polyol mediated synthesis method have been measured at liquid helium temperature under VUV excitation. In nanoparticles the ratio of intensities of Charge Transfer Luminescence (CTL) and exciton excitation peaks differs from that for Y2O3-Yb single crystal, and changes with the particle size variation from 13 to 52 nm. This effect can be connected with the variation of excitation light scattering with particle dimensions. Two types of luminescence decay acceleration are observed. Acceleration of exponential decay time with decrease of nanoparticle size and the appearance of very fast decay component in small nanoparticles that we connected to energy transfer to the surface defects.

\hbox {Y}_{2}\hbox {O}_{3}\hbox {--} \hbox {Yb}

High energy excitation, after a very fast electron-electron diffusion cascade, leads to the creation of excited region with many electrons and holes of typical energies from few to several tens of electron-volts. At this stage, the scintillation process involves the same electronic relaxation and energy transfer mechanisms as those occurring under VUV excitation. Time-resolved luminescence spectroscopy with VUV and X-synchrotron radiation became today a principal investigation method of energy relaxation mechanisms in scintillation materials. High-order harmonic (HH) generation is a very promising new XUV source for these studies. The emission of HH occurring when an intense linearly polarized femtosecond laser pulse interacts with an atomic gas. We present the results of characterization of HH source mounted at C.E.L.I.A. laser (20 mJ, 20 fs, 1 kHz). The experimental setup giving the possibilities of many applications in physics and chemical-physics as well as a first time-resolved luminescence measurements of scintillation materials are described. The main advantages of harmonic radiation for spectroscopic study are: quasi continuous spectrum in the VUV region (10-100 eV), ultra-short pulse (ps and sub-ps), high pulse intensity (up to 106 photons/pulse/eV), relatively high frequency of pulses (1 kHz) and table-top size.