Various Er/Ce co-doped SiO2-Al2O3-CaO glasses are prepared by the melt-quenching method. The radiation resistance of Er/Ce co-doped glasses is investigated under 5 kGy gamma-ray irradiation. The absorption spectra, up-conversion spectra, fluorescence intensity and lifetime of Er/Ce co-doped glasses before and after irradiation are measured and analyzed in details. The radiation induced absorption (RIA) of the Er/Ce co-doped silicate glasses can be suppressed due to Ce ions co-doping. The fluorescence intensity and lifetime of Er/Ce co-doped glasses have no apparent change after irradiation. Furthermore, the possible mechanism of Ce effect on the radiation resistance improvement is discussed. The result indicates that Er doped glasses with an optimal Ce concentration introduction can be used as active medias for radiation-resistant materials in harsh radiation environments.
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With the development of space exploring and communications, Er-doped fibers can be used in space for optical intersatellite links and other applications, such as amplifiers for highly-distributed data networks and broadband super luminescent sources for fiber gyroscopes [1–3]. The fibers would be withstand a total dose of ~500 – 2000 Gy of protons and electrons in the spacecraft lifetime which is a relatively large dose over 5-10 years in space . However, when exposed to gamma-ray irradiation, rare-earth doped fibers are more sensitive to irradiation than rare-earth free optical fibers [5–8]. As the carriers of the rare earth doped fibers can be excited into trap states and subsequently form defects or color centers in the glass matrix , which lead to the radiation-induced absorption (RIA) and radiation-induced luminescence (RIL) . There are many factors affect the radiation induced loss on doped fibers, such as the type of ionizing radiation, total absorbed dose, dose rate, rare-earth dopant concentrations and the fabrication parameters [7,10–13]. There is no doubt that the low radiation resistance of Er-doped fibers calls into questions to their space applications.
Research on the development of RIA of the glasses has been actively continued in the past few years [14–17]. The results show that the RIA arise from color centers generation due to the trapping of holes and electrons at pre-existing or radiation induced defect sites. The decrease in optical transmittance of the glasses with respect to ionizing-radiation dose and dose rate has been described by a power law [9,17]. It is well known that the radiation resistance of the passive and scintillating glasses is improved with Ce co-doping [18,19]. By the reason that Ce3+ and Ce4+ coexist in the glass, Ce3+ ions can capture the radiation-induced holes and then inhibit the formation of all other kinds of centers due to trapped holes while Ce4+ inhibit the formation of all other kinds of centers result from trapped electrons and inhibit recombination of electrons with trapped holes through capture the electrons [18,20]. Some investigations have been performed in the Er/Ce co-doped fibers or glasses and the results show that the telluride fibers have a low noise figure around 1.55 μm band when pumped at 980 nm , and the fluorescence characteristics at 1.55 μm of the fluoride host glasses can effectively be improved when pumped at 980 nm . However, few studies have been carried out on the radiation resistance of the Er/Ce co-doped silicate glasses. As we know, Ce3+ has a 4f1 ground electronic configuration with two free ion states separated by 2000 cm−1 and as to Ce4+ is a 4f0 system . The property of the Ce ions has very slight effect on the radiative transition of other rear earth ions that could not increase the non-radiative quenching and energy transfer. Furthermore, Ce ions could easily introduced in silicate glass with aluminum and other rare earth ions.
The aim of this work was to investigate the radiation resistance of the Er/Ce co-doped silicate glasses under 5 kGy gamma-ray irradiation. We demonstrate that the addition of Ce into glass matrix can improve the radiation resistance of Er-doped silica-based glass for the first time. The influence of Ce co-doping on optical properties of Er-doped glasses has also been investigated. The results imply that Er/Ce co-doped silicate glasses can be used for active materials in harsh radiation environments.
In this experiment, the glasses with composition in mol% was 65% SiO2, 10% Al2O3 and 25% CaO (SAC) were prepared by a conventional melting method under atmosphere. The doped samples were prepared with 0.5 mol% Er2O3 and different CeO2 concentrations (0, 0.2, 0.5, 0.7, 1.5, and 2 mol%). The glasses were prepared by mixing appropriate quantities of analytical grade in a sintered alumina crucible and melting the mixture for 2h at 1580°C in a normal atmosphere. The melts were poured on the pre-heated steel mold and then annealed for 2h at 550°C to obtain thermal and structural stability.
Then the sample glasses were cut and polished into with dimensions of 15 × 15 × 2 mm3 for subsequent measurements. The absorption spectra before and after irradiation in the ultra-violet and visible wavelength ranges were performed by using a Perkin Elmer Lambda 35 UV-Vis spectrophotometer. The fluorescence spectra in the near infrared region and the up-conversion spectra in the visible range were measured on a ZOLIX SBP300 spectrophotometer under an 976 nm LD excitation. Photoluminescence decay measurements were carried out by exciting the samples with a modulated 980 nm LD, and monitoring 1535 nm emission with an InGaAs photodetector in TRIAX550 spectrofluorometer. The fluorescence data was processed by a Tektronix oscilloscope to determine the lifetime. In order to investigate the radiation resistance of the silicate glasses, the samples were exposed to gamma rays from a 60Co γ-source under normal atmosphere up to total doses of 5 kGy at a dose rate of 250 Gy/h. To avoid noticeable radiation-induced absorption relaxation during the measurements, the spectra of the glasses were measured within one day after finishing the irradiation. All of measurements were carried out at room temperature.
3. Results and discussions
The initial and 5 kGy irradiated glass samples are shown in Fig. 1 . It is known that Ce ions exhibit both trivalent and tetravalent oxidation state (Ce3+ and Ce4+) in oxide glasses under normal conditions of melting. Ce3+ in oxide glasses has very little, if any, absorption in the visible region, therefore, the color of Ce-containing glasses origins from Ce4+ due to the tail of strong charge transfer absorption band in the near ultraviolet region . From Fig. 1 we know that the color of the single Er-doped glass changes from light pink to brown owning to irradiation while no obvious color change is observed in the Er/Ce co-doped glass. This can be confirmed from Fig. 2 , the change in the optical absorption coefficient spectra of corresponding sample glasses upon irradiation.
The distinctive feature of the Fig. 2 was a red shift of the UV cut-off positions, which corresponded with the increasing concentration of CeO2. From Figs. 2(a) and 2(b), we can see that the absorption coefficient of the host material and single Er-doped glass in the visible range have a more substantial increase after irradiation while the change of the Er/Ce co-doped glasses is small. The absorption peaks have little excursion after irradiation which indicates that the microscopic environment of the Er3+ ions is not much affected by the gamma radiation . In Er/Ce co-doped glasses, a strong broad absorption band is observed in the UV region and that extends to the visible region (~400 nm). This broad absorption band is assumed to be a superposition of both charge transfer transitions of Ce4+, i.e., Ce4+ + O2-→Ce3+ + O-, where O is an oxygen ligand, and 4f1→4f5d1 transitions of Ce3+. Since the f – f related transition of Ce3+ is located in the far-infrared range (~5 μm) where the silicate glass is opaque. The ratio of the two valence states being dependent on the composition of the glass, the temperature of melting  so that the amount of Ce ions in each valence state is very difficult to estimate. As shown in the inset of Fig. 2 the absorption peak around 980 nm has little shift after irradiation, and the absorption coefficient of the single Er-doped sample witnesses an increase about 0.75 after irradiation while the absorption coefficient does not change in Er/Ce co-doped glasses. The strong absorption in the UV region (<300 nm) is considered to be the absorption edge of the host material.
The RIA and up-conversion are important characteristic parameters for evaluating the performance of radiation resistance materials. The RIA can be obtained by digitally subtracting the absorption coefficient spectrum of the un-irradiated glass from that measured after each exposure. The RIA spectra of the host material and the co-doped glasses with the concentration of CeO2 was 0, 0.2, 0.5 mol% are shown in Fig. 3(a) ; Fig. 3(b) shows the up-conversion spectra with the CeO2 concentration was 0, 1.0 mol% in the visible range. As can be seen, the free CeO2 silicate glass form a strong broad absorption band around 400 nm and extends to the near infrared region while there is slight change in the Er/Ce co-doped glasses. From the spectra of the up-conversion in Fig. 3(b) we know that the intensity of Er doped glasses obviously minished after irradiation while the intensity of Er/Ce co-doped glasses with no change. It is suggested that co-doped CeO2 could improve the radiation resistance of the sample via inhibiting the formation of color centers in silicate glasses under gamma radiation. Therefore compared with the glass free of CeO2, the induced absorption bands of the Er/Ce co-doped glass extending from the UV into the visible insight range are strongly suppressed. The Ce3+ and Ce4+ could coexist in the silicate glass, and Ce ions could effectively trap ionization charge induced by radiation due to its ability to change the charge state between Ce4+ and Ce3+: Ce4+ + e-→Ce3+ or Ce3+ + h+→Ce4+, thus diminishing the potential for gamma-induced color-center formation. In all the glasses and solutions, Ce4+ produces a very strong absorption around 250 nm . The 4f-5d transitions of Ce3+ make a strong absorption between 320 ~360 nm. The absorption coefficient of the Er/Ce co-doped glasses have a tendency of increase around 400 nm, we speculate that is related to the valence change of the Ce ions during the radiation.
We also measured the emission properties of the sample glasses before and after irradiation, as shown in Fig. 4 . Figure 4(a) shows the fluorescence spectra of typical non- and 5 kGy irradiated samples under 980 nm excitation, and the peak wavelength near 1535 nm. All spectra have been divided by the peak wavelength intensity of un-irradiated Er-doped glass for comparison. Figure 4(b) shows the intensity of Er/Ce co-doped glasses as a function of CeO2 concentrations at a wavelength of 1535 nm. As can be seen, the FWHM (full width at half maximum) and the peak wavelength have slightly change while the luminescence intensity of the glasses decreased obviously with increasing CeO2 concentration and only half when the CeO2 concentration up to 1.5 mol%. The intensity of the Er/Ce co-doped glass has barely changed after irradiation, as compared with single Er-doped glass. This also means that the radiation resistance of the Er-doped glass can be greatly improved with CeO2 co-doping.
The decay time of the photoluminescence (PL) intensity at 1535 nm versus CeO2 concentration (Ce = 0,0.2,1.5 mol%) before irradiation and that of Ce co-doped sample (0.2 mol%) in non- and 5 kGy-irradiated glass are shown in Fig. 5 . All the curves of the samples were approximated by the exponential decay, and in Fig. 5(a) the lifetime of the samples were 6.5, 6.4, and 3.4 ms, respectively. That is to say low concentration of CeO2 has little effect on the fluorescence lifetime of the upper level in the Er3+, while the fluorescence lifetime of Er3+ decreased significantly in the higher CeO2 concentration samples. This is opposite with previous studies. Many investigations have shown that Ce3+ co-doping effectively improve the 1.55 µm fluorescence characteristics of Er3+-doped fluoride glasses or tellurite glasses with 980 nm excitation [22,24]. The resonant energy transfer between Er3+ in the 4I11/2 state and Ce3+ in 4F7/2 state results in the improvement in the 4I11/2→4I13/2 transition of Er3+, thus reduce excited state absorption (ESA) without shorting the lifetime of the 4I13/2 state, which could improve the fluorescence quantum yield and the characteristics of gain significantly. We think that the contrary may be result from the difference of the host material and may be more likely to the reason that most of Ce ions is in the form of Ce4+ in the samples. There is a overlap between the charge transfer absorption band of the Ce4+ and the absorption band of the Er3+ upper level, part of the pump power can be transferred to Ce4+ via the ESA of the Er3+, which immediately reduced the intensity of the glasses. The lifetime of the samples in Fig. 5(b) were 6.4 and 6.3 ms, respectively. Co-doped with Ce could improve the radiation resistance of the silicate glasses when compared with the lifetime of the single Er-doped is 4.1 ms (not given in the figure) after irradiation.
We have demonstrated that Ce ions co-doping by some amount could effectively improve the radiation resistance of the Er-doped silicate glasses. The radiation induced loss of Er/Ce co-doped glass diminished significantly as compared with single Er-doped glass. However, taking into account the influence of Ce ions on the luminescent characteristics of Er3+ around 1.55 µm, an optimal concentration of Ce should be exist. We can reduce the negative impact of doped Ce to acceptable levels by controlling the ratio of Ce3+ and Ce4+ via adjusting the redox atmosphere.
This work was financially supported by the National High-Technology Research and Development Program of China (2011AA030201). The authors would also like to thank Huazhong University of Science &Technology Analytical and Testing Center for the spectroscopic measurement.
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