Zircon radiation-induced centers

Luminescence of Pr + in zircon is very difficult to detect under UV excitation even by time-resolved spectroscopy. The reason is that it has a relatively short decay time similar to those of radiation-induced centers. Visible excitation, which is not effective for broadband luminescence, allows the revealing of Pr + luminescence lines, using high-resolution steady-state spectroscopy. Under such experimental conditions each element has individual lines, enabling confident identification of the spectrum to be possible (Gaft et al. 2000a). Only if radiation-induced luminescence in zircon is relatively weak, the lines of Pr may be detected by UV excitation (Fig. 4.38c). 

X-ray Fluorescence Spectrometry and Inductively Coupled Plasma analysis reveal the presence in the zircons of all existing REE. The steady-state luminescence in natural zircons is dominated by broad emission arising from radiation-induced centers and narrow emission lines of Dy " (Trofimov 1962 Tarashchan 1978). These emissions obscure the spectra of other REE. The thermal treatment enables to solve this problem in certain cases using the fact that the intensity of broad band luminescence quickly decreases after heating at 700 °C-800 °C, while the intensities of the REE lines remain nearly constant (Shinno 1986, 1987). Even after heating the samples not all the REE can be identified by steady-state spectroscopy since the weaker luminescence lines of certain REE are obscured by stronger luminescence of others. For example, luminescence of Pr " is difficult to... 

Fig. 4.39. a-d Laser-induced time-resolved luminescence spectra of zircon demonstrating intrinsic radiation induced, luanyl and Fe center... 

Approximately 50 natural zircons have been investigated together with synthesized analogs, as nominally pure and activated by potential liuninogens. Concentrations of potential impurities in several zircxon samples are presented in Tables 4.14-4.15 The laser-induced time-resolved technique enables us to detect the following emission centers radiation induced trivalent rare-earth elements such as Gd ", Ce ", Tb ", Tm ", Er +, Ho ", Dy ", Eu ", Sm ", Yb + and Nd + (U02) Fe + and Cr + (Figs. 4.38-4.40). 

Figure 6.14 presents PIL of several centers which are characterized by short and very short intrinsic decay times, such as 1300 ns for Eu in fluorite Cap2 (Fig. 6.14a), 20-30 ns for orange luminescence in calcite (Fig. 6.14b) and 25-30 ps for radiation induced yellow luminescence of zircon ZrSi04 (Fig. 6.14c). Such luminescence detection after delay time of 125-150 ps, where it has to be totally quenched, implies that the life time of luminescence is actually much longer than decay time determined by intrinsic transition probabilities. Actually, it is three orders of magnitude longer.

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