Radiation-Induced Centers

The reabsorption lines of have been found in zircon (Gaft et al. 1986) luminescence spectra. The strongest reabsorption lines at 592 and 656 nm are (Fig. 4.39b) clearly seen, which are totally identical to absorption lines of in zircon (Platonov et al. 1989). 

A distinctive feature of the O2 and S2 luminescence spectra in minerals is a quasi-linear vibrational structure of the broad luminescence band (Tarash-chan 1978). The O2 and S2 molecular ions are isoelectronic. From the molecular orbital diagram describing their electron structure the emission transition Eg- n l2 is determined. When observing luminescence spectra at 77 K, a fine structure associated with the frequency of intra-molecular vibrations of O2 and S2 is detected. This frequency depends on the type of the molecular ion, on inter-nuclear distance and upon the particular position of the molecular ion in the structures. For S2 the maximum of the emission band lies within the range of 600-700 nm with a mean vibration frequency of 500-600 cm , while for O2 the respective maximum is 450-550 nm with a frequency in the 800-1,200 cm range. 

Time-resolved luminescence spectroscopy of sodalite evidences that the vibration structure has a very short decay time and disappears after a delay of 250 ns. Such structure is superimposed on the very broad IR band (Fig. 4.65). 

As was already mentioned, the origin of this band was ambiguous, hi order to clarify this we studied the irradiation influence on laser-induced time-resolved luminescence of the following samples (Gaft et al. 2003b)  

The violet emission of the radiation-induced center (COs) is well known in steady-state luminescence spectra of calcite (Tarashchan 1978 Kasyanenko, Matveeva 1987). The problem is that Ce also has emission in the UV part of the spectrum. In time-resolved luminescence spectroscopy it is possible to differentiate between these two centers because of the longer decay time of the radiation-induced center. Its luminescence peaking at 405 nm becomes dominant after a delay time of 100-200 ns while emission of Ce is already quenched (Fig. 4.14f). 

648 and 645.9 nm, respectively. The excitation spectra obtained by monitoring the orange-yellow fluorescence at 300 and 10 K consist of a main band with a peak at 392 nm. The luminescence efficiency of the heat-treated sodalite from Xinjiang is about seven times as high as that of untreated natural sodalite. The emission spectrum of the S2 center in sodalite at 10 K consists of a band with a clearly resolved structure with a series of maxima spaced about 560 cm (20-25 nm) apart. Each narrow band at 10 K shows a fine structure consisting of a small peak due to the stretching vibration of the isotopic species of a main peak due to 

The emission and excitation spectra of yellow luminescence due to 82 in scapolite were observed at 300, 80 and 10 K (8idike et al. 2010b). Emission and excitation bands at 10 K showed vibronic structures with a series of maxima spaced 15-30 and 5-9 nm, respectively. The relative efficiency of yellow luminescence from one scapolite sample was increased up to 117 times by heat treatment at 1000 °C for 2 h in air. The enhancement of yellow luminescence by heat treatment was ascribed to the alteration of 803 and 804 to 82 in scapolite. 

Relatively broad luminescence bands with vibratiraial structure even at 300 K peaking at blue-green spectral region in several minerals have been ascribed to 

M-center consists of two electrons captured by F anion vacancies in fluorite. It is characterized by red luminescence peaking at 725 nm and excited in 370 and 530 nm spectral ranges (Tarshchan 1978). This center is characterized by long decay time and is detected with relatively broad gate width. Vk-center is a hole center, which is formed by F ions which loses its electron. It is characterized by UV luminescence band peaking at approximately at 290 nm with very short decay time (Fig. 5.106). 

Radiation induced luminescence centers and radiation influence on the luminescence properties of the other emission centers become more and more important both theoretically and for different applications (Nasdala et al. 2013). 

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Fig. 4.14. a-f Laser-induced time-resolved luminescence spectra of calcite demonstrating Mn ", Pb " and 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). 

Fig, 5.66. Laser-induced time-resolved luminescence spectrum (a) and excitation spectrum (b) of radiation induced center in calcite... 

Certain similarity may be seen between this luminescence and short-Hved orange emission in calcite, which has been ascribed to radiation-induced center (Fig. 5.67). It is possible that natural irradiation may be a reason of orange luminescence in apatite also. 

EPR Parameters of Radiation-Induced Centers in NHtY and Their Oxygen Adducts... 

Characterization of radiation-induced centers in the p -n-structures was carried out by means of deep level transient spectroscopy (DLTS) [3]. Concentrations, activation energies of charge carrier emission, and apparent capture cross sections of carriers were determined for all the traps observed. 

Fig. 3 displays the changes in the introduction rate of the vacancy-oxygen complex with the irradiation temperature. The introduction rates of the VO and C -Oi centers increase about twice upon raising the irradiation temperature from 350 to 685 K. An analysis of all the available results on the effect of irradiation temperature on introduction rates of radiation induced centers allows us to suggest that the observed increase of t] in the range of 350-685 K can be associated with suppression of the annihilation rate of Frenkel pairs with the temperature increase. 

Fig. 4.20 (a-b) Time-resolved lumineseenee spectra of blue (a) and orange (b) radiation induced centers in ealcite... 

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. 5.108 (a-d) Luminescence (a), decays (b, d) and excitation spectra (c) of blue radiation-induced centers in calcite... 

We have no geological evidence for the possible irradiation events leading to the formation of such radiation-induced centers in our calcite sample, but several indirect indications are present. The nature of pink coloration in calcite has been intensively studied and two possible models have been proposed. In Kazakhstan samples an elevated concentration of Pb was detected, from 0.1 % to 2 %. This color is thermally unstable and disappears after heating at 300 °C together with all absorption bands except one at 240 nm, which has been assigned to Pb absorption. The thermally bleached calcite has its pink color restored after X-ray, gamma ray or electron irradiation. The pink color has been ascribed to absorption bands at 370 and 500 nm connected with Ca vacancies and units, respectively,... 

Nevertheless, such interpretation cmitradicts the fact, that after heating at 800 °C the short-lived yeUow band disappeared and a usual long-lived Mn luminescence becomes visible (Fig. 5.111c, d). Time-resolved excitation spectrum of short lived yellow band consists of one main broad band with extremely low Stocks shift and is absolutely different from those for Mn + (Fig. 5.11 le-f). Certain similarity may be seen between this luminescence and short-lived orange emission in calcite, which has been ascribed to radiation-induced center. It is possible that natural irradiation may be a reason of orange luminescence in apatite also. 

Figure 6.13 presents PIE spectra of other luminescence centers different from trivalent RRE which present in well known luminescent minerals green luminescence centers of uranyl adsorbed by quartz (Fig. 6.13a), Mn in willemite (Fig. 6.13b) and calcite (Fig. 6.13c) with green and orange luminescence, correspondingly, blue emission of radiation-induced center in calcite (Fig. 6.13d) and red...

Fig. 6.26 (a-d) Short-lived orange luminescence spectrum (X x = 532 nm) of radiation-induced center in calcite and its decay as a function of delay time (a). Gated Raman spectra with excitation at 532 nm and gate widths of 10 ns (b) and 0.5 ns (d). CW Raman spectrum with excitation at 785 nm (c)... 

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