Potentialities and practical limitations of absolute neutron dosimetry using thin films of uranium and thorium applied to the fission track dating

Neutron dosimetry using natural uranium and thorium thin films makes possible that mineral dating by the fission-track method can be accomplished, even when poor thermalized neutron facilities are employed. In this case, the contributions of the fissions of (235)U, (238)U and (232)Th induced by thermal, epithermal and fast neutrons to the population of tracks produced during irradiation are quantified through the combined use of natural uranium and thorium films.If the Th/U ratio of the sample is known, only one irradiation (where the sample and the films of uranium and thorium are present) is necessary to perform the dating. However, if that ratio is unknown, it can be determined through another irradiation where the mineral to be dated and both films are placed inside a cadmium box.Problems related with film manufacturing and calibration are discussed. Special attention is given to the utilization of thin films having very low uranium content. The problems faced suggest that it may be better to substitute these films by uranium doped standard glasses calibrated with thicker uranium films (thickness greater than 1.5 x 10(13) mu m).

Er3+/Yb3+ co-activated silica-alumina monolithic xerogels

Monolithic silica xerogels doped with different concentrations of Er3+, Yb3+ and Al3+ were prepared by sol-gel route. Densification was achieved by thermal treatment in air at 950degreesC for 120 h with a heating rate of 0.1degreesC/min. We studied the luminescence properties of the I-4(13/2)-->I-4(15/2) emission band of Er3+ as a function of the Al/Er/Yb concentration and we paid particular attention to the alumina effects. Raman spectroscopy and Vis-NIR absorption were used to monitor the degree of densification of the glasses and the residual OH content.

Crystal nucleation kinetics of a metastable phase on cordierite glass surfaces

The crystal nucleation rates of a metastable phase (chi) on the surface of a near stoichiometric cordierite glass were determined for temperatures between 839 and 910 degrees C (T-g similar to 800 degrees C). The surface nucleation kinetics of that phase on our glass, as well as on a stoichiometric glass (2 MgO-2Al(2)O(3)-5SiO(2)) studied by other authors, were analysed in terms of the classical nucleation theory; for the first time. It was shown that the effective interfacial energy for surface nucleation is substantially lower than that for homogeneous volume nucleation in silicate glasses, vindicating the assumption of heterogeneous nucleation on free glass surfaces. The average wetting angle between the nucleating crystals and the active solid particles was estimated to be around 46 degrees C. The pre-exponential constant was several orders of magnitude higher than the theoretical values as found for volume homogeneous nucleation in oxide glasses.

Nonlinear refractive index measurements in antimony-sulfide glass films using a single beam nonlinear image technique

The nonlinear refractive index, n(2), of films based on the new glass system Sb(2)O(3)-Sb(2)S(3) was measured at 1064 nm with laser pulses of 15 ps, using a single-beam nonlinear image technique in presence of a phase object. The films were prepared from bulk glasses by RF-sputtering. A large value of n(2) = 3 x 10-(15) m(2)/W, which is three orders of magnitude larger than for CS(2), was determined. The result shows the strong potential of antimony-sulfide glass films for integrated nonlinear optics. (c) 2005 Elsevier B.V. All rights reserved.

Changes of physical properties of glass surfaces exposed to KNO3 vapors

Modifications of glass surfaces were studied after exposure of samples to an atmosphere resulting from the decomposition of molten KNO3. The diffusion coefficient of K+ ions migrating into the surfaces of float glass and synthesized glasses doped with up to 5 wt% SnO2 was calculated by the Boltzmann-Matano technique. The Vickers hardness and the refractive index increase with exposure time. Infrared spectra show that the migration of K+ is responsible for an increase in the number of non-bridging oxygens in the exposed samples. The spectra of the synthesized glasses present evidences that their surfaces undergo crystallization during the exposure. All results lead to the conclusion that the presence of tin in the glasses hinders the diffusion of K+ ions, thus affecting the Vickers hardness, the refractive index and the infrared spectra. It is shown that the exposure method can be used as an alternative process to promote the K+ migration into glass surfaces. (c) 2006 Elsevier B.V. All rights reserved.

Energy transfer between Tm3+ and Er3+ ions in a TeO2-based glass pumped at diode laser wavelength

In this paper we investigate the energy transfer processes in TM3+/Er3+ doped telluride glass pumped at the commercial diode laser pump wavelength similar to 800 nm. Tailoring the rare-earths content in the glass matrix, seven main energy transfer channels within the doping range considered were identified, A 6-fold enhancement of the Er3+ visible frequency upconversion fluorescence at similar to 660 nm is observed due to the inclusion of Tm3+ ions. This is evidence of the relevant contribution of the route Er-1(I-4(11/2)) + Er-2(I-4(13/2)) -> Er-1(I-4(15/2)) + Er-2(F-4(9/2)) to the process. Energy migration among pumped I-4(9/2) level reducing the efficiency of the upconversion emission rate (H-3(11/2), S-4(3/2), and F-4(9/2)) is observed for Er3+ above 1.5 wt%. The rate equations regarding the observed energy transfer routes are determined and a qualitative analysis of the observed processes is reported. (c) 2006 Elsevier B.V. All rights reserved.

Local order around of germanium atoms in Ga10Ge25S65 glass by EXAFS

Irreversible photoexpansion effect has been observed in amorphous Ga10Ge2S65 glasses when its surface was exposed to light with energy greater than the band gap, 3.52 eV. A volume change of about 5% was reached in bulk samples by controlling illumination time and the laser power density. To understand the atomic scale processes of the photoexpansion effect, extended X-ray absorption fine structure (EXAFS) spectroscopy has been used as a local probe of the germanium environment in the glass samples before and after illumination. Modifications are observed in the average coordination shell around Ge atoms in the illuminated sample compared to the non-illuminated one. For the non-illuminated sample, the Ge coordination shell is described by a distorted tetrahedron of sulfur atoms at around 2.20 Angstrom. After illumination, the EXAFS signal can be explained by introducing an additional contribution to this average environment. Based on an analysis of the EXAFS data we proposed a two-shell model of 0.5 oxygen atoms at 2.01 Angstrom and 3.6 sulfur atoms at a 2.20 Angstrom. The existence of Ge-O bonds in the glass after illumination was confirmed by infrared measurements. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Thermally enhanced cooperative energy-transfer frequency upconversion in terbium and ytterbium doped tellurite glass

Infrared-to-visible frequency upconversion through cooperative energy-transfer and thermal effects in Tb3+/Yb3+-codoped tellurite glasses excited at 1.064 mum is investigated. Bright luminescence emission around 485, 550, 590, 625 and 65 nm, identified as due to the D-5(4) --> F-7(J) (J= 6, 5, 4, 3, and 2) transitions of the terbium ions, respectively, was recorded. The excitation of the D-5(4) emitting level of the Tb3+ ions is assigned to cooperative energy-transfer from pairs of ytterbium ions.. The effect of temperature on the upconversion process was examined and the results revealed a fourfold upconversion enhancement in the 300-500 K interval. The enhancement of the upconversion process is due to the temperature dependence of the Yb3+-sensitizer absorption cross-section under anti-Stokes excitation. A rate-equation. model using multiphonon-assisted absorption for the ytterbium excitation combined with the energy migration effect between Yb-Yb pair, and Tb3+ ground-state depopulation via multiphonon excitation of the F-7(J) excited states describes quite well the experimental results. (C) 2003 Elsevier B.V. B.V. All rights reserved.

Phosphate functionalization of spongiolite surface

Spongiolite from Mato Grosso do Sul (Brazil), natural inorganic composite constituted of silica needles, was treated with concentrated phosphoric acid at high temperatures. Superficial coating of the needles was proved to be constituted of silicon diphosphate, a compound offering six-coordinated silicon sites. Owing to the affinity of three charged ions to phosphate groups, this coating acts as specific adsorbent for the rare earth elements which prefer octahedral coordination (starting from samarium, yttrium included). The uptake of lanthanum and neodymium are significantly lower due to different coordination tendencies. Lanthanide fixation upon silica with PO4 groups anchored on its surface may be useful in the manufacturing of special phosphate-silicate glasses. (C) 2003 Elsevier B.V. All rights reserved.

Structural organization and thermal properties of the Sb2O3-SbPO4 glass system

The structural organization of Sb2O3-SbPO4 glasses has been studied by FTIR, Raman, P-31 MAS and spin echo NMR, Mossbauer and X-ray absorption spectroscopy (EXAFS and XANES at K and L-3,L-1-Sb edges). The combined results can be explained in terms of two potential mechanisms describing the change of the Sb(m) local environment upon incorporation of Q((4))-type phosphate. The formation of the latter species requires anionic compensation that may be adjusted by (a) formation of non bridging oxygen or (b) formation of SbO4E- groups (E = non-bonding electron pair). The second model is favored.

Optical recording in Se(Te)/As2S3 multilayers

The development of Te containing nanoscaled, compositionally modulated Te/As2S3 multilayer structures enables the realisation of optical recording in real time with a CW He-Ne laser illumination (lambda=633 nm) with improved recording parameters in comparison with known Se/As2S3 structures. It is established that the efficiency of the recording process depends on the light stimulated interdiffusion and thickness of sub-layers.

Energy upconversion luminescence in neodymium-doped tellurite glass

Infrared-to-visible upconversion luminescence emission in Nd3+-doped 60TeO(2)-10GeO(2)-10K(2)O-10Li(2)O-10Nb(2)O(5) tellurite glasses under cw excitation around 800 nm is investigated. Blue (430, and 475 nm), green (5 0 nm) and yellow-orange (590 nm) energy upconversion emission owing to the P-2(1/2) --> I-4(j) (j=9/2, 11/2, 13/2 and 15/2) transitions of the Nd3+ ions, respectively, was recorded. The dependence of the upconversion intensity upon the excitation wavelength and pump power is also studied. The upconversion excitation mechanism responsible for the observed emission signals is attributed to stepwise multiphoton absorption. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Er-doped silica-based waveguides prepared by different techniques: RF-sputtering, sol-gel and ion-exchange

Erbium-activated silica-based planar waveguides were prepared by three different technological routes: RF-sputtering, sol-gel and ion exchange. Various parameters of preparation were varied in order to optimize the waveguides for operation in the NIR region. Particular attention was devoted to the minimization of the losses and the increase of the luminescence efficiency of the metastable I-4(13/2) state of the Er3+ ion. Waveguide properties were determined by m-line spectroscopy and loss measurements. Waveguide Raman and luminescence spectroscopy were used to obtain information about the structure of the prepared films and about the dynamical processes related to the luminescence of the Er3+ ions.

Study of the most suitable new glass laser to incorporate ytterbium: alkali niobium tellurite, lead fluorborate or heavy metal oxide

Diode-pumped Yb-doped glass lasers have received considerable attention for applications such as high-power beam production or femtosecond pulses generation. In this paper, we evaluate the laser potential of three different glass families doped with Yb3+ : alkali lead fluorborate (PbO-PbF2-B2O3), heavy metal oxide (Bi2O3-PbO-Ga2O3) and niobium tellurite (TeO2-Nb2O5-K2O-Li2O). Spectroscopic properties were studied for the samples and calculations of the minimum laser pump intensity (I-min), saturation fluence (U-sat) and the theoretical limit of peak power (P-max) are also presented. A comparison of laser properties of these three different glasses and their importance is shown and analyzed. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Blue cooperative luminescence properties in Yb3+ doped GeO2-PbO-Bi2O3 vitreous system for the production of thin films

Germanate glasses are of interest for optoelectronic applications because they combine high mechanical strength, high chemical durability and temperature stability with a large transmission window (400 to 4500 nm) and high refractive index (2.0). GeO2-PbO-Bi2O3 glasses doped with Y-b(3+) were fabricated by melting powders in a crucible and then pouring them in a brass mold. Energy Dispersive Spectroscopy showed that the glass composition has a high spatial uniformity and that the Yb concentration in the solid sample is proportional to the Yb concentration in the melt, what was confirmed by absorption measurements. Intense blue emission at 507 nm was observed, corresponding to half of the wavelength of the near infrared region (NIR) emission; besides, a decay lifetime of 0.25 ms was measured and this corresponds to half of the decay lifetime in the infrared region; these are very strong indications of the presence of blue cooperative luminescence. Larger targets have been produced to be sputtered, resulting in thin films for three dimensional (3D) display and waveguide applications. (c) 2006 Elsevier B.V. All rights reserved.