B. Moine

U-Pb, Single Zircon Pb-Evaporation, and Sm-Nd Isotopic Study of a Granulite Domain in SE Madagascar

Madagascar is a poorly known Precambrian domain at the eastern border of the Mozambique belt, strongly affected by the Pan-African orogeny. This large island represents a key part of Gondwanaland and the accurate dating of the major metamorphic event is critical for the reconstruction of that supercontinent. In the southeastern zone of the island, metamorphism has reached high-temperature and low-pressure conditions (850°C and 5 kbar) with moderate retrogression. A U-Pb conventional multigrain, single zircon stepwise Pb-evaporation and Sm-Nd geochronological study on meta-felsic rocks has shown that basement of at least lower Proterozoic age was strongly metamorphosed around 570-580 Ma. This tectometamorphic event nearly obliterated any pre-Pan-African zircon isotopic memory. The granulite-facies metamorphism is associated with the syn-metamorphic emplacement of the Anosyan charnock-ites and granites. A strong channelized fluid contribution at 545 Ma has produced the (re)crystallization of biotite-apatite lenses containing giant monazites. Finally, the emplacement of zircon-bearing calcite veins at 520 Ma represents the latest Pan-African event in this SE Madagascar domain. This precise determination of the Pan-African overprint in this region facilitates comparative discussion of the geodynamic evolution of Madagascar and surrounding areas at the end of the Proterozoic. There is good agreement between the single zircon Pb-evaporation results and conventional isotopie dating; nevertheless slightly younger apparent ages are obtained by the former method. This discrepancy, which may have some important implications, could be related to the discordancy of the analyzed zircon populations.

One- and two-photon spectroscopy of Ce3+ ions in LaF3-CeF3 mixed crystals

Absorption, excitation and emission spectra and lifetime measurements in cerium-doped lanthanum fluoride single crystals were obtained using both one- and two-photon spectroscopy techniques. It is shown that two kinds of fluorescent centres are present. In weakly doped systems, a rapid fluorescence around 300 nm dominates, due to Ce3+ ions at normal sites. In heavily doped systems, an additional longer fluorescence appears at higher wavelengths due to Ce3+ ions occupying perturbed sites and acting as traps. In CeF3, the energy migrates among Ce3+ ions and is finally trapped so that only the trap emission is detected at room temperature. Significant energy losses in the fluorescence of Ce3+ at normal sites are explained by up-conversion and ionization processes and by non-radiative surface recombination phenomena for the case of strong absorption. Two-photon excitation spectra in the region of trap absorption confirm the role of traps in emission mechanisms and show narrow lines which could be vibronic structures of various kinds of traps existing in these materials.

Photoionization and luminescences in BaF2:Eu2+

Abstract The photoconductivity spectra of BaF 2 :Eu 2+ shows that the “anomalous” emission usually observed in BaF 2 :Eu 2+ is due to the fact that all the Eu 2+ excited states are in the conduction band of the crystal. The red shifted emission is due to an impurity trapped exciton which is the lowest excited state of this system. A model for the photoconductive and luminescence behavior of BaF 2 :Eu 2+ and SrF 2 :Eu 2+ is given in terms of configuration coordinate diagrams. A broad blue band is also observed under high energy excitation (> 4 eV) and is interpreted as the emission of a perturbed self-trapped exciton.

EXTENSIVE STUDIES ON CEF3 CRYSTALS, A GOOD CANDIDATE FOR ELECTROMAGNETIC CALORIMETRY AT FUTURE ACCELERATORS

Abstract In the framework of its search for new heavy, fast and radiation hard scintillators for calorimetry at future colliders, the Crystal Clear Collaboration performed a systematic investigation of the properties and of the scintillation and radiation damage mechanisms of CeF 3 monocrystals. Many samples of various dimensions up to 3 × 3 × 28 cm 3 were produced by industry and characterised in the laboratories by different methods such as: optical transmission, light yield and decay time measurements, excitation and emission spectra, gamma and neutron irradiations. The results of these measurements are discussed. The measured light yield is compared to the theoretical expectations. Tests in high energy electron beams on a crystal matrix were also performed. The suitability of CeF 3 for calorimetry at high rate machines is confirmed. Production and economical considerations are discussed.

Cerium doped heavy metal fluoride glasses, a possible alternative for electromagnetic calorimetry

The article is an overview of the research activity performed in the framework of the Crystal Clear Collaboration to produce scintillating glasses. The manufacturing of heavy metal fluoride glasses doped with Ce3+ is discussed. The luminescence and scintillation characteristics as well as the radiation hardness properties are extensively studied in the case of Ce doped fluorohafnate , found to be the most convenient glass scintillator for high energy physics applications.

Further results on cerium fluoride crystals

Abstract A systematic investigation of the properties of cerium fluoride monocrystals has been performed by the “Crystal Clear” collaboration in view of a p

Time-resolved luminescence of CeF3 crystals excited by X-ray synchrotron radiation

Abstract Under X-ray synchrotron radiation (SR) of storage ring VEPP-3 excitation time-resolved luminescence spectra and decay curves for two different CeF 3 crystals have been measured. The temperature dependence of the decay kinetics of emission at 300 nm of new pure CeF 3 crystal has been studied. The maximum of the intensity of this emission was observed at 390 K under SR X-ray excitation. The results are compared with data obtained under UV SR excitation and are discussed in terms of two models of luminescence quenching.

Eu 2+ and Mn 2+ codoped Ba 2 Mg(BO 3 ) 2 —new red phosphor for white LEDs

A new red phosphor, Ba2Mg(BO3)2:Eu,Mn, was synthesized by the solid-state reaction method and its photoluminescence properties were investigated by excitation and emission spectra and decay curves. Its excitation band is extending from 250-450 nm, which is adaptable to the emission band of near-ultraviolet LED chips (350-420 nm). Upon the excitation of 365 nm light, the phosphor exhibits strong red emission centered at 615 nm. The relationship between Eu2+ and Mn2+ dopants was studied from the viewpoint of a crystal structure and by photoluminescence spectra and decay curves. The results show that the characteristic Eu2+ emission predominate in the emission band and Mn2+ promote the redistribution of Eu2+ at the cation sites of the host crystal.

One- and two-photon spectroscopy of f → d and f → f transitions of Eu2+ ions in M1-xNxF2 mixed fluoride crystals (M, N = Ba, Sr, Ca; 0 ⩽ x ⩽ 1)

Abstract Divalent europium ions exhibit either normal blue or anomalous yellow emission in fluoride crystals, the latter being due to an impurity-trapped exciton resulting from a photoionization process. In (Ca-Ba) and (Sr-Ba) mixed crystals both luminescences are observed. Absorption and emission spectra have been analyzed for several crystals, all doped with 0.02 at% Eu2+. As the Ba concentration increases, the broadening of the zero-phonon line is due to inhomogeneity and the smoothing of the first absorption band is interpreted as a Fano resonance due to the coupling of the excited “d” electron with the conduction band (i.e. the autoionization of Eu2+ ion). For the first time, the two-photon transition 8S7/2 → 6P7/2 in BaF2 as well as in M1-xNxF2, (M, N = Ba, Sr, Ca; 0 ⩽ x ⩽ 1) mixed crystals is presented. We analyze the evolution of the splitting of the 6P7/2 energy level in the mixed crystals and show that the lowering of the site symmetry from Oh to C2v induces a fourth component to grow up. Besides, this level is essentially broadened by the inhomogeneity of the crystal but no Fano resonance process with the conduction band occurs. This is in good agreement with the expected fact that the coupling of well shielded “f” electrons with the conduction band is much weaker than this of the outer “d” electrons.

Spectroscopic investigation of the copper (I)-rich phosphate CuZr2(PO4)3

The Nasicon-type phosphate CuZr2(PO4)3 exhibits two fluorescences occurring in the violet and green regions. They are assigned to two different centres: the violet one to isolated Cu+ and the green one to Cu+-Cu+ pairs occupying the same site. These assignments are in agreement with previous structural investigations using X-ray, neutron and EXAFS measurements and provide information about the location of copper ions in the crystal. A third luminescence is detected in the blue range at low temperature. The origin of this fast strong emission is not clearly established. It could be due to Cu+-Cu0 pairs in the M(1) site.