Improvement of radiopurity level of enriched 116^{116}CdWO4_4 and ZnWO4_4 crystal scintillators by recrystallization

As low as possible radioactive contamination of a detector plays a crucial role to improve sensitivity of a double beta decay experiment. The radioactive contamination of a sample of $^{116}$CdWO$_4$ crystal scintillator by thorium was reduced by a factor $\approx 10$, down to the level 0.01 mBq/kg ($^{228}$Th), by exploiting the recrystallization procedure. The total alpha activity of uranium and thorium daughters was reduced by a factor $\approx 3$, down to 1.6 mBq/kg. No change in the specific activity (the total $\alpha$ activity and $^{228}$Th) was observed in a sample of ZnWO$_4$ crystal produced by recrystallization after removing $\approx 0.4$ mm surface layer of the crystal.Comment: 14 pages, 5 figures and 2 table

Radioactive contamination of ZnWO4 crystal scintillators

The radioactive contamination of ZnWO4 crystal scintillators has been measured deep underground at the Gran Sasso National Laboratory (LNGS) of the INFN in Italy with a total exposure 3197 kg x h. Monte Carlo simulation, time-amplitude and pulse-shape analyses of the data have been applied to estimate the radioactive contamination of the ZnWO4 samples. One of the ZnWO4 crystals has also been tested by ultra-low background gamma spectrometry. The radioactive contaminations of the ZnWO4 samples do not exceed 0.002 -- 0.8 mBq/kg (depending on the radionuclide), the total alpha activity is in the range: 0.2 - 2 mBq/kg. Particular radioactivity, beta active 65Zn and alpha active 180W, has been detected. The effect of the re-crystallization on the radiopurity of the ZnWO4 crystal has been studied. The radioactive contamination of samples of the ceramic details of the set-ups used in the crystals growth has been checked by low background gamma spectrometry. A project scheme on further improvement of the radiopurity level of the ZnWO4 crystal scintillators is briefly addressed.Comment: 15 pages, 8 figures, 6 tables, submitted for publicatio

High-pressure Raman spectroscopy and lattice-dynamics calculations on scintillating MgWO4: A comparison with isomorphic compounds

Raman scattering measurements and lattice-dynamics calculations have been performed on magnesium tungstate under high pressure up to 41 GPa. Experiments have been carried out under a selection of different pressure-media. The influence of non-hydrostaticity on the structural properties of MgWO4 and isomorphic compounds is examined. Under quasi-hydrostatic conditions a phase transition has been found at 26 GPa in MgWO4. The high-pressure phase has been tentatively assigned to a triclinic structure similar to that of CuWO4. We also report and discuss the Raman symmetries, frequencies, and pressure coefficients in the low- and high-pressure phases. In addition, the Raman frequencies for different wolframites are compared and the variation of the mode frequency with the reduced mass across the family is investigated. Finally, the accuracy of theoretical calculations is systematically discussed for MgWO4, MnWO4, FeWO4, CoWO4, NiWO4, ZnWO4, and CdWO4.Comment: 36 pages, 9 figures, 4 table

Liquid crystal transitions induced by spherical particles in anisotropic polymer brushes: Molecular dynamics simulation

A change in the structure of a polymer brush formed by chains composed of anisotropic monomers brought about by filling the volume of the system with free spherical particles was studied by means of molecular dynamics simulation. Microphase separation takes place at a certain concentration of spherical particles inside the brush. A microphase that contains a small amount of isotropic particles and is characterized by a high density and high ordering of the monomers is formed near the wall. A disordered microphase with a low monomer density and a high concentration of spherical particles is located at the periphery of the brush. The microphases are separated from one another by a distinct boundary. As the concentration of isotropic particles further increases, the ordered microphase expands until the entire volume of the brush becomes occupied. It was also found that the brush in the ordered state forms different LC structures, namely, smectic or nematic entities at relatively low concentrations or at higher concentrations of spheres, respectively

ZnWO₄ luminescent films obtained by hydrothermal method

ZnWO₄ films of 10–20 μm thickness can be produced by hydrothermal synthesis method without additional subsequent treatment. The data obtained show that films possess wolframite structure and demonstrate luminescent properties which are similar to bulk ZnWO₄

Computer simulation of anisotropic polymer brushes

A polymer brush with chains consisting of anisotropic monomers, in a liquid consisting of free spherical particles, is studied by the molecular dynamics method. It is shown that, at some value of the concentration of spheres, a liquid-crystal or oriented- domain transition occurs in the brush. A densely-grafted brush and a sparsely-grafted brush are studied; for this system, the transition point seems not to depend strongly upon the grafting density. In the case of the densely-grafted brush, a liquid-crystal transition proceeds via an intermediate microphase segregated state. One microphase, located near the grafting surface, is characterized by high density and high degree of ordering of monomers. This part of the brush contains only a small concentration of spheres. On the periphery of the brush, a disordered microphase with low monomer density is located. This part of the brush is enriched with spherical particles. The two microphases are separated by a welldefined boundary. On increasing the sphere concentration, the boundary between microphases shifts towards the periphery, and eventually the ordered microphase extends through the whole brush volume. The monomers of the densely-grafted brush in their ordered state form different structures, namely, a smectic structure at relatively low values of sphere concentration, and a structure of orientationally ordered domains at the higher sphere densities

Luminescent and structural properties of ZnxMg1-xWO4 mixed crystals

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