Oudomsack Viraphong

Purification of molybdenum, growth and characterization of medium volume ZnMoO4 crystals for the LUMINEU program

The LUMINEU program aims at performing a pilot experiment on neutrinoless double beta decay of 100Mo using radiopure ZnMoO4 crystals operated as cryogenic scintillating bolometers. Growth of high quality radiopure crystals is a complex task, since there are no commercial molybdenum compounds available with the required level of purity and radioactive contamination. This paper discusses approaches to purify molybdenum and synthesize compounds for high quality radiopure ZnMoO4 crystal growth. A combination of a double sublimation (with addition of zinc molybdate) with subsequent recrystallization in aqueous solutions (using zinc molybdate as a collector) was used. Zinc molybdate crystals up to 1.5 kg were grown by the low-thermal-gradient Czochralski technique; their optical, luminescent, diamagnetic, thermal and bolometric properties were tested.

One possible mechanism of spiral/footing growth of Cz-grown Li6Gd(BO3)3

Abstract Experimental observations on spiral growth phenomena in the Cz-growth of Li 6 Gd(BO 3 ) 3 single crystals are described. With careful observations at the meniscus part during the pulling, we found that the faceting formation is closely correlated with a “dog-leg” footing in combination with unsteady melt convection modes. It is assumed that one of predominant origin of the so-called spiral growth should be the faceting inclined against the pulling axis at the meniscus part of the solid–liquid interface. Several origins reported so far are also discussed.

Development and underground test of radiopure ZnMoO4 scintillating bolometers for the LUMINEU 0 nu 2 beta project

The LUMINEU (Luminescent Underground Molybdenum Investigation for NEUtrino mass and nature) project envisages a high-sensitivity search for neutrinoless double beta (0 nu 2 beta) decay of Mo-100 with the help of scintillating bolometers based on zinc molybdate (ZnMoO4) crystals. One of the crucial points for the successful performance of this experiment is the development of a protocol for producing high quality large mass ZnMoO4 crystal scintillators with extremely high internal radiopurity. Here we report a significant progress in the development of large volume ZnMoO4 crystalline boules (with mass up to 1 kg) from deeply purified materials. We present and discuss the results achieved with two ZnMoO4 samples (with mass of about 0.3 kg each): one is a precursor of the LUMINEU project, while the other one was produced in the framework of LUMINEU with an improved purification / crystallization procedure. The two crystals were measured deep underground as scintillating bolometers in the EDELWEISS dilution refrigerator at the Laboratoire Souterrain de Modane (France) protected by a rock overburden corresponding to 4800 m w.e. The results indicate that both tested crystals are highly radiopure. However, the advanced LUMINEU sample shows a clear improvement with respect to the precursor, exhibiting only a trace internal contamination related with Po-210 at the level of 1 mBq/kg, while the activity of Ra-226 and Th-228 is below 0.005 mBq/kg. This demonstrates that the LUMINEU purification and crystal-growth procedures are very efficient and leads to radiopurity levels which exceedingly satisfy not only the LUMINEU goals but also the requirements of a next-generation 0 nu 2 beta experiment.

A combined EPR and modulated photocurrent study of native defects in Bridgman grown vanadium doped cadmium telluride: the case of the tellurium antisite

The electrical and photoconductive properties of Bridgman grown vanadium–zinc co-doped CdTe bulk crystals are strongly influenced by one native intrinsic defect previously attributed to the Te vacancy. In order to identify this defect and control its formation mechanism, a correlated electron paramagnetic resonance (EPR) and modulated photocurrent (MPC) study has been made. The results obtained allow us to attribute this defect to the TeCd antisite, a double donor. By EPR and MPC its +/2+ level position has been determined to Ec - 0.20 eV. Four other centres of minor concentrations were characterized by MPC in as-grown crystals. From the MPC results the density of states for CdTe:V materials has been determined.

Laser demonstration with highly doped Yb:Gd 2 O 3 and Yb:Y 2 O 3 crystals grown by an original flux method

We present, to the best of our knowledge, the first laser demonstration of an Yb-doped Gd(2)O(3) cubic crystal. This crystal was obtained by the flux method using an original borate-based solvent, which was particularly well suited to the growth of rare earth sesquioxide crystals at half the working temperature of classical growth techniques. This flux method is a very interesting alternative for the production of laser sesquioxide crystals, not only because it provides access to new matrices of the cubic polymorph, but also because it permits high Yb(3+)-doping levels for these crystals. The first laser results of two highly Yb(3+)-doped sesquioxides, namely Gd(2)O(3) and Y(2)O(3), grown by this flux method are presented here, including the Ti:sapphire and diode pumping configurations. Laser efficiencies and emission spectra for these two crystals were studied and compared.

Growth by the heat exchanger method and characterization of neighborite, NaMgF3

The home-made heat exchanger method (HEM) apparatus has been adapted to the growth of NaMgF 3 ingots. By unseeded crystal growth using presynthesized NaMgF 3 the structure was maintained during growth. Twinning introduced by the high-temperature phase transition is observed in the crystal. Studies on the crystals, at high temperature, by optical microscopy, calorimetry and Raman scattering, show a unique phase transition, from orthorhombic (Pbnm) to cubic (Pm3m) structure, at about 770°C.

Growth and spectroscopic properties of 6Li- and 10B-enriched crystals for heat-scintillation cryogenic bolometers used in the rare events searches

We present the crystal growth of centimeter-sized 6Li6Eu(10BO3)3 and Li6(Eu,Gd)(BO3)3 single crystals by combined Czochralski and Kyropoulos methods, initiated on specifically oriented seeds, with a heretofore unexplored concentration range for heat-scintillation cryogenic bolometers application. The crystal structure and lattice thermal expansion of the former, obtained for the first time by single crystal X-ray diffraction (XRD), and the spectroscopic characterizations together with a consistent set of related thermodynamical properties measurements (magnetic susceptibility, specific heat), also unknown to date, are discussed. Finally, preliminary scintillation measurements at low temperature between 300 and 1100 nm under X-ray excitation are shown.

Anisotropy of the spectroscopic, optical and thermo-mechanical properties of Li 6 Eu 1-x Gd x (BO 3 ) 3 crystals optimized for heat-scintillation cryogenic bolometers

Optical, spectroscopic and thermo-mechanical properties of monoclinic Li6Eu1-xGdx(BO3)3 (x = 0,0.25,0.35) bulk single crystals, grown to be used in the design of heat-scintillation cryogenic bolometers (HSCBs), were investigated. The linear thermal expansion was determined along the a, b, c and c* directions over the temperature range 303–873 K, and its tensor principal coefficients were calculated for both x = 0.25 and x = 0. In addition, the anisotropic thermal conductivity was measured over the temperature range 20-400 K in Li6Eu0.75Gd0.25(BO3)3 (LGEB7, x = 0.25), and the principal components of its tensor at 300 K were established. Spectroscopic properties such as polarized absorption, polarized emission, Raman spectroscopy and optical refractive indices are also reported for the first time. Based on the polarized emission spectra, the line and oscillator strengths, the radiative lifetimes and fluorescent branching ratios were obtained. The intensity parameters Ωλ (λ = 2,4) were obtained and then predicted via the Judd–Ofelt theory. The crystal field parameters and the 7F1 level splitting were discussed using the simple overlap model (SOM) and the method of equivalent nearest neighbours (MENN). The 613 nm emission originates from the 5D0→7F2 transition and the associated stimulated emission peak cross section reaches its maximum value in π-polarization, ≈1.07 × 10−20 cm2 in LGEB7. The thermo-mechanical characterizations and spectroscopic analysis in LGEB7 suggest that this crystal has better optical properties than the Eu3+-doped Li6Y(BO3)3 crystals. However, its potential for solid-state laser applications is still quite speculative, unless a clever thermo-mechanical management of the crystal under laser operation is designed.

Continuous cross-over from ferroelectric to relaxor state and piezoelectric properties of BaTiO3-BaZrO3-CaTiO3 single crystals

Optimal properties like piezoelectricity can be found in polarizable materials for which the structure changes sharply under small composition variations in the vicinity of their morphotropic phase boundary or the triple point in their isobaric temperature-composition phase diagram. In the latter, lead-free (Ba0.850Ca0.150)(Ti0.900Zr0.100)O3 ceramics exhibit outstanding piezoelectric coefficients. For the first time, we report the growth of piezoelectric lead-free single crystals in the BaTiO3-BaZrO3-CaTiO3 pseudo-ternary system. The stoichiometry control in the CaO-BaO-TiO2-ZrO2 solid solution led to single crystals with various compositions ranging from (Ba0.857Ca0.143)(Ti0.928Zr0.072)O3 to (Ba0.953Ca0.047)(Ti0.427Zr0.573)O3. We evidenced a continuous cross-over from a ferroelectric state at high titanium content to a relaxor one on increasing the zirconium content. Such a property tuning is rather seldom observed in lead-free ferroelectrics and confirms what was already reported for ceramics. Single crystal with (Ba0.838Ca0.162)(Ti0.854Zr0.146)O3 composition, which has been grown and oriented along [001] crystallographic direction, displayed electromechanical coefficients d31 and k31 of 93 pC.N−1 and 0.18, respectively, near the room temperature (T = 305 K).

Optimal sintering parameters for Al2O3 optoceramics with high transparency by spark plasma sintering

Transparent Polycrystalline Alumina (PCA) optical ceramics were fabricated at a high heating rate and low temperature by spark plasma sintering (SPS). Maximum pressure (100 MPa) at dwell time keeps the grain size small irrespective of the dwell time. A heating and cooling rate of 100°C min−1 at the sintering temperature of 1150°C for a dwell time of 1 h at 100 MPa yielded highly densified samples with the good transparency of 63 and 83% in visible and infra-red region, respectively. Optoceramics yielded a mechanical hardness of (3000 Hv)/ 29.42 GPa and a thermal conductivity of 21 Wm−1 K−1.