Band artificial

For the correct interpretation of the luminescent bands, artificial apatite standards have been investigated, as nominally pure, as activated by different potential luminogen impurities. Natural carbonate-fluor-apatites not containing REE were heated with 1 - 5 wt. % of oxides of Eu, Pr, Sm and Dy at 900 °C in air and in vacuum. By changing the activation conditions the differentiation between isomorphous substitutions in different Ca-sites has been achieved. Under vacuum the compensation of the excessive positive charge by substitution of E by 0 is impossible and the luminescence centers in Ca(ll) sites may be less preferential. After heating at this temperature carbonate-fluor-apatite loses its carbonate content and becomes very similar to natural fluor-apatite. 

The natural barite in our study consisted of twenty-five samples of different origin. Concentrations of potential luminescence impurities in several samples are presented in Table 4.11. For the correct interpretation of the luminescent bands, artificial barite standards have been investigated, as nominally pure, and activated. The laser-induced time-resolved technique enables us to detect Ag+, Bi +, Bi, Eu, Ce +, Nd +, (U02) and several still not identified emission centers (Figs. 4.29-4.31). 

Intended Use The intended use of the model sets the sophistication required. Relational models are adequate for control within narrow bands of setpoints. Physical models are reqiiired for fault detection and design. Even when relational models are used, they are frequently developed bv repeated simulations using physical models. Further, artificial neural-network models used in analysis of plant performance including gross error detection are in their infancy. Readers are referred to the work of Himmelblau for these developments. [For example, see Terry and Himmelblau (1993) cited in the reference list.] Process simulators are in wide use and readily available to engineers. Consequently, the emphasis of this section is to develop a pre-liminaiy physical model representing the unit. 

Figures were obtained for the 295-385 nm band of 280 and 333 MJ/m2/year for Florida and Arizona respectively. Combining these with figures given by Davis and Sims [7] for total irradiance in Fondon and Phoenix (75 and 175 kcalories/cm2/year, respectively) and making several assumptions, rough acceleration factors were calculated for artificial light sources ...

The presence of Pr in apatite samples, up to 424.4 ppm in the blue apatite sample, was confirmed by induced-coupled plasma analysis (Table 1.3). The luminescence spectrum of apatite with a broad gate width of 9 ms is shown in Fig. 4.2a where the delay time of500 ns is used in order to quench the short-lived luminescence of Ce + and Eu +. The broad yellow band is connected with Mn " " luminescence, while the narrow lines at 485 and 579 nm are usually ascribed to Dy and the fines at 604 and 652 nm, to Sm +. Only those luminescence centers are detected by steady-state spectroscopy. Nevertheless, with a shorter gate width of 100 ps, when the relative contribution of the short lived centers is larger, the characteristic fines of Sm " at 652 nm and Dy + at 579 nm disappear while the fines at 485 and 607 nm remain (Fig. 4.2b). It is known that such luminescence is characteristic of Pr in apatite, which was proved by the study of synthetic apatite artificially activated by Pr (Gaft et al. 1997a Gaft... 

The spectral-kinetic parameters of the narrow band at 375 nm enable its confident identification as Eu + luminescence, which is confirmed by emission of synthetic BaS04 artificially activated by Eu (Fig. 5.15a). Such emission was also detected and interpreted by steady-state spectroscopy (Tarashchan 1978). It is interesting to note that very often such a band is absent in natural barite and appears only after heating in air at 600-700 °C (Fig. 4.31b). Such a transformation is reversible, at least partly. Under X-ray excitation the intensity of the UV band diminishes, and a new blue-green emission appears (Fig. 5.16). This shows some kind of transformation, which takes place in the barite lattice under these conditions. Several possibihties exist. It is possible that in barite the luminescence is quenched by the components with high-energy phonons. The water and organic matter may represent the latter. They are removed after... 

Garnet activated by trivalent Cr is a promising system for tunable laser appUcations and those systems have been well studied. Cr + replaces Ap" in octahedral sites with a weak crystal field. The transition involved in laser action is T2- A2, a vibrationally broadened band. At room temperature it has a maximum in the 715-825 nm range with a decay time in the 100-250 ps range depending on AE between the E and T2 levels. When the AE is maximal, narrow fines also appear from the E level. At low temperatures, when thermal activation of the T2 level is difficult, J -lines luminescence becomes dominant with the main fine at 687 nm (Monteil at al. 1988). We studied pyrope artificially activated by Cr and also found the two emission types described above (Fig. 5.26). 

In artificial phosphors the luminescence bands due to Mn exist from 620 to 715 nm. The spectrum has a structure consisting of several broad lines... 

The situation with Mn + center distribution between Ca(I) and Ca(II) positions in the apatite lattice is the opposite to this for REE + in artificially activated apatite the Mn(I) center clearly dominates the fluorescence spectra (Ryan et al. 1970), while in the natural one only Mn(ll) centers have been detected (Tarashchan 1978). In order to clarify the distribution in different Ca positions, luminescence spectra have been measured with different polarizations from one section or from prismatic and basal sections with the same analytical conditions. As was foimd earher (Barbarand and Pagel 2001) the shapes of the spectra are usually the same for both crystallographic orientations, while the major difference in the spectra is their intensity, with the mean intensity for the basal section lower than for the prismatic face. Nevertheless, in certain cases polarization changes lead to different spectra (Fig. 5.46). In this case spectra are composed mainly of the Nd and Mn with relatively weak Eu lines. The polarization change results in an inversion of the relative intensities of the liuninescence bands Mn +(I) emission dominates in one orientation and REE emission is practically not seen, but Mn " (II) in other orientations is much weaker compared to Mn +(I), while the... 

Artificially activated spinels of composition Mgi xAl204 x Mn were studied (Mohler et al. 1994). It was found that MgAl204 Mn " emits at 650 nm, while when X = 0.05 emission bands appear at 517 and 744 nm. They increase intensity and the 650 nm band disappears as x increases. The green emission is connected with Mn " in tetrahedral sites, while the red one evidently with Mn " in octahedral ones. 

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