N. Garnier

X-ray excited charge transfer luminescence of ytterbium-containing aluminium garnets

Abstract We report on measurements of X-ray excited charge transfer luminescence occurring in Yb 3+ -doped yttrium aluminium garnet (YAG:Yb) and ytterbium aluminium garnet (YbAG). The temperature dependence of luminescence was investigated in variously Yb 3+ -doped crystals. The fluorescence intensity and decay time drop drastically at low temperatures ( T

Entropy production and time asymmetry in nonequilibrium fluctuations.

The time-reversal symmetry of nonequilibrium fluctuations is experimentally investigated in two out-of-equilibrium systems: namely, a Brownian particle in a trap moving at constant speed and an electric circuit with an imposed mean current. The dynamical randomness of their nonequilibrium fluctuations is characterized in terms of the standard and time-reversed entropies per unit time of dynamical systems theory. We present experimental results showing that their difference equals the thermodynamic entropy production in units of Boltzmanns constant.

Work fluctuation theorems for harmonic oscillators.

We study experimentally the thermal fluctuations of energy input and dissipation in a harmonic oscillator driven out of equilibrium, and search for fluctuation relations. Both the transient evolution from the equilibrium state, and non-equilibrium steady states are analyzed. Fluctuation relations are obtained experimentally for both the work and the heat, for the stationary and transient evolutions. A stationary state fluctuation theorem is verified for various time dependences of the imposed external torque. The transient fluctuation theorem is satisfied for the work given to the system but not for the heat dissipated by the system in the case of linear forcing. Experimental observations on the statistical and dynamical properties of the position fluctuations of the torsion pendulum allow us to derive analytical expressions for the probability density functions of the work and the heat. We obtain for the first time an analytic expression for the probability density function of the heat. The agreement between experiments and our predictions is excellent.

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.

Temperature dependence of the charge transfer and f?f luminescence of Yb3+ in garnets and YAP

For single crystals of YAG–15 mol% Yb, LuAG–15 mol% Yb and YAP–8 mol% Yb the temperature dependence of the charge-transfer (CT) and f–f luminescence of Yb3+ excited in the CT absorption band in the temperature range 7–300 K has been studied. Simulation of the experimental data taking into consideration three processes, photoionization of the CT state and radiative and non-radiative energy transfer to the 4f levels, has demonstrated that for adequate description of the relaxation of the CT state it is essential to get information on the temperature dependence of the direct excitation efficiency of the f–f luminescence as well as to study the role of intrinsic luminescence in the energy transfer to ytterbium ions. Excitation spectra of the CT and IR luminescence in the energy region 4–20 eV at RT and 10 K are presented and discussed.

Ytterbium-based scintillators, a new class of inorganic scintillators for solar neutrino spectroscopy

Abstract The observed deficit of the solar neutrino flux is now well established. This puzzling problem of todays particle physics could be resolved soon. The most likely explanation would be the vacuum neutrino oscillation phenomenon, indirectly proving the non-zero mass of these fleeting particles. Following the proposition of Raghavan of using 176 Yb as a target for low-energy solar neutrino spectroscopy, an intense R&D work has started a few years ago to define a suitable scintillator incorporating a large amount of ytterbium. Recently, the observation of UV scintillation in mixed yttrium/ytterbium aluminium garnets opened the field of investigation to a new class of scintillating crystals with interesting luminescence properties, very attractive not only for neutrino physics but also for radiation detection, in general. Their luminescence properties present some peculiarities that make them interesting by themselves.

Fluctuations of the total entropy production in stochastic systems

Fluctuations of the total entropy are experimentally investigated in two stochastic systems in a non-equilibrium steady state: an electric circuit with an imposed mean current and a harmonic oscillator driven out of equilibrium by a periodic torque. In these two linear systems, we study the total entropy production which is the entropy created to maintain the system in the non-equilibrium steady state. The fluctuation theorem holds for the total entropy production in the two experimental systems, both for all observation times and for all fluctuation magnitudes.

Potential of existing growth methods of LuAP and related scintillators

Abstract LuAP and LuYAP are promising single-crystal scintillators for use in the next generation of positron emission tomographs. The practical crystal composition in terms of major components and Ce-doping level is however under discussion yet. Bridgman and Czochralski melt techniques are used to develop various crystal compositions for scintillation studies and final composition choice. In this paper we compare performance of 30 LuAP, LuYAP and YAP variously Ce-doped single-crystals grown by Bridgman technique, which reflect potential of existing materials and technologies. In addition to available optical properties, the chemical composition of solid solutions is described in terms of variation of Ce concentration and of Lu/Y ratio.

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.

Stationary modulated-amplitude waves in the 1D complex Ginzburg–Landau equation

We reformulate the one-dimensional complex Ginzburg-Landau equation as a fourth order ordinary dierential equation in order to nd stationary spatiallyperiodic solutions. Using this formalism, we prove the existence and stability of stationary modulated-amplitude wave solutions. Approximate analytic expressions and a comparison with numerics are given.