nan Sangeeta

Study of growth imperfections, optical absorption, thermoluminescence and radiation hardness of CdWO4 crystals

The effect of starting charge composition used for growth on the crystalline perfection, optical, thermoluminescent and radiation hardness of CdWO 4 (CWO) single crystals has been investigated. One of the major problems faced with the growth of defect free crystals is the presence of a core-like defect. The loss of CdO by evaporation, which makes the melt progressively rich in WO 3 , is found to be the main cause of core formation. This problem has been overcome by employing a starting charge rich in CdO, the pull rates in the range of 2-3 mm/h and maintaining a convex solid-melt interface during the growth. The crystals grown from starting charges containing ∼53 mol% CdO exhibit excellent optical transmission, high radiation hardness towards gamma rays and extremely weak thermoluminescence (TL) emission, the characteristics which are the most desirable for any scintillator crystal. In the TL glow curves recorded for stoichiometric crystals, only one glow peak around 148°C is observed. Minute stoichiometric deviations are found to shift the glow peak temperature towards higher temperature side and enhance the TL output considerably. The stoichiometric crystals are found not to develop coloration for gamma exposures of 10 Mrad.

Effect of sodium doping on thermoluminescence and optical properties of barium borate (BaB2O4) single crystals

Abstract Thermoluminescence (TL) emission from undoped and sodium doped α- and β-BaB 2 O 4 (BBO) crystals has been investigated. A single TL glow peak is observed for the undoped crystals of both phases. The glow peak temperatures found for α- and β-BBO crystals over the temperature range 35–300°C are 104 and 80°C, respectively. The presence of minute amount of sodium ions in the lattice gives rise to a glow peak around 200°C. For impurity concentrations higher than 0.02% (by wt), additional glow peaks around 123 and 240°C are also observed. The ratio of the TL intensities corresponding to the first and 200°C peaks is observed to increase with impurity concentration. This fact provides an easy and sensitive method of detecting the presence of sodium ions in BBO lattice. The results also show that sodium ions go substitutionally into the lattice and act as charge traps. The optical absorption in the ultra violet region and the crystal hardness towards ionizing radiations are found to be adversely affected by the presence of this impurity.

Effect of highest temperature invoked on the crystallization of LiB3O5 from boron-rich solution

Abstract The crystallization behavior of LiB3O5 (LBO) has been investigated by performing differential thermal analysis (DTA). When the measurements are carried out with the highest temperature limited to a few degrees above the melting point, a clear indication of the crystallization is revealed by the presence of exotherms at 787°C and 772°C in the DTA plot. On the other hand, the crystallization of LBO phase gets inhibited if the melt is subjected to a much higher temperature, prior to cooling. Powder X-ray diffraction patterns recorded in the two cases are also in agreement with these observations. The results obtained have been explained in terms of the microstructure of the melt, which besides other factors, is shown to be dependent upon the highest temperature to which the melt is raised prior to crystallization. The inference drawn has been vetted by growing LBO single crystals by top seeded solution growth technique. The results obtained also help to understand the difficulties encountered in nucleating LBO from a boron-rich solution with a Pt wire, as reported in literature [1] .

Effect of non-stoichiometry on some properties of lead tungstate single crystals

The effect of stoichiometric deviations on the optical absorption, gamma rays induced absorption and thermoluminescence (TL) from PbWO4 (PWO) single crystals has been investigated. The stoichiometric deviations are found to develop yellowish coloration in this crystal. This problem has been overcome by purifying the starting powder for minute stoichiometric deviations through repeated crystallisations. In the case of non-stoichiometric crystals a broad absorption over the wavelength range 400–500 nm is observed. This absorption gets further enhanced when the crystal is subjected to γ-ray irradiation. Whereas, the stoichiometric crystals, which are colourless, are found not to develop coloration for gamma exposures as high as 10 Mrad. The presence of weak TL in both types of crystals is observed for the radiation exposures of 10 Mrad. For colourless crystals, only one TL glow peak at 170°C is recorded over the temperature range 30–400°C. A shift in the glow peak temperature towards the lower temperature side is observed in the case of non-stoichiometric crystals. This result can be used to detect minute stoichiometric deviations in PWO.

Thermally stimulated luminescence from single crystals of LiYF4 and its constituent fluorides

Abstract Thermally stimulated luminescence (TSL) from YF 3 , LiYF 4 and LiYF 4 (U 4+ ) crystals has been investigated for the first time. While an undoped LiYF 4 crystal exhibits very weak TSL, the emission from YF 3 is found to be intense. On doping a LiYF 4 crystal with uranium, its TSL output is observed to get considerably enhanced. The emission spectra recorded for all the crystals are found to be identical and characteristic of the constituent Y 3+ ions. This is an important result which shows that in the process of TSL, U 4+ ion behaves as a donor of excitation energy to Y 3+ ions.

Investigations on the solidification behavior of alkaline earth borates

The solidification behavior of materials with general formula XB2O4 (X=Ca, Sr, Ba) has been investigated by differential thermal analysis (DTA). The melts of these materials are found to undergo large supercooling, which is strongly dependent on the atmosphere provided during the thermal cycle. The DTA plot of BaB2O4 shows the presence of a single exotherm, implying normal freezing behavior of the material. In contrast, for CaB2O4 or SrB2O4 two exotherms are observed, suggesting their solidification behavior to be complex in nature.

Effect of atmosphere on the crystallization temperature of BaB2O4 melt

Abstract The differential thermal analysis (DTA) on BaB 2 O 4 (BBO) single crystals and polycrystalline material has been performed under both oxidising and inert atmospheres. The melt under oxygen is found to supercool down to 966°C, while under argon the crystallization temperature is further lowered to 887°C. This novel observation has been found to be extremely useful for the growth of β-BBO crystals by the Czochralski technique. The results obtained have been explained in terms of the melt oxygen stoichiometry.

Effect of doping concentration on spectroscopic and structural properties of Nd:NaY(WO_4)_2 crystal

The optical and structural properties of NaNd(x)Y(1-x)(WO(4))(2) (NYW) single crystals grown from melt containing 1-10 at. % of Nd have been investigated. Optical absorption, photoluminescence, x-ray diffraction, and x-ray florescence studies have been carried out on grown crystals. The crystal is found to exhibit a broad absorption peak of ~16 nm width at 806 nm for 6 at. % of Nd doping. A decrease in fluorescence lifetime from 150 to 80 mus with an increase in Nd(3+) concentration is observed for 1.06 mum emission. Spectroscopic quality parameters (Omega(4)/Omega(6)) and quantum efficiency calculated from the above measurements and application of Judd-Ofelt theory are found to be 1.1 and 68%, respectively. Structural analysis through Rietveld refinement of x-ray diffraction data showed that the oxygen sublattice of NYW is not affected by changes in Nd dopant concentration, though the lattice dimensions are found to increase proportionately. The study for the first time shows that the high doping levels do not introduce any strain into the lattice. As a consequence the optical quality of the crystal does not deteriorate at high doping levels. The selection of doping concentration is therefore solely governed by the decay time requirement for a particular application.

EXAFS investigations on PbMoO4 single crystals grown under different conditions

Extended X-ray absorption fine structure (EXAFS) measurements on PbMoO

Preparation and characterisation of radiation hard PbWO4 crystal scintillator

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