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Abstract
Ce3+- doped lithium borophosphate (LBPO) glasses were prepared with a melt quenching method. The structure and luminescence properties of Ce3+-doped LBPO glass after proton irradiation (API) are investigated and compared with those before proton irradiation (BPI). BO4 units and non-linear P-O-P bonds in the LBPO:Ce glass were broken after the proton irradiation, leading to the BO3 units, as well as more PO3 and PO4 units. There was a large red-shift (30 - 70 nm) of the absorption cut-off edge for the glasses after proton irradiation. A blue shift (about 10 nm) in the emission and excitation spectra was observed for the LBPO:Ce glass after proton irradiation compared with that before proton irradiation. This is due to the structure of LBPO:Ce glass and [CeOn] polyhedra becoming more flexible and larger after the proton irradiation. In addition, the integrated emission intensity decreased to about 20% for both under UV and X-ray excitation. The depths of the traps generated in the glasses during the proton irradiation process were estimated to be about 0.12 - 0.28 eV for different Ce3+ concentrations.
© 2017 Optical Society of America
1. Introduction
Neutron diagnostics is an indispensable tool for both inertial confinement fusion (ICF) and magnetic confinement fusion (MCF) research to effectively probe and control high-energy fusion plasma [1, 2]. The available technology for neutron detection is generally based on nuclear reaction. This relies on scintillating materials containing neutron- capture elements including 6Li, 10B, 155Gd and 157Gd [3]. The rare- earth (RE) doped scintillating glasses, with the considerable advantages of low-cost, large-volume, and easy shaping of elements, are always a desirable material for neutron detection [4]. Up till now, some Ce3+-activated silicate, phosphate and borate- based glasses containing concentrated 6Li or 10B elements have been explored for scattered neutron diagnostic purposes [5]. However, intrinsic structure defects in scintillating glasses, such as lots of anion vacancy and non-bridging oxygen, lower the energy transfer efficiency from the host glass to the incorporated emission centers. This leads to the relatively low light output [6]. On the other hand, the scintillating glasses containing a high content of 6Li elements usually demonstrate a tendency to diversify. This results in complex glass synthesis, particularly in the large- volume glasses [7]. Therefore, the trade-offs between lithium content and glass stability should be taken into account in the design of scintillating glasses for neutron detection. More importantly, the effect of high energy particle radiation on optical and fluorescence properties in scintillating glasses is important not only for understanding the irradiation mechanism in glasses, but also for exploration of novel scintillating glass with improved radiation hardness. However, there is few reference about the influence of high energy particle radiation, especially the proton irradiation on optical and fluorescence properties in Ce3+-activated glasses. For example, in 2009, K. Kadono et al investigated the effect of additive ions on the optical density and stability of the color centers induced by X-ray irradiation in soda-silicate glass [8]. Unfortunately, there is no data about the fluorescence properties of the soda-silicate glasses. In 2015, A. Borisevich et. al reported Di-silicate of barium (DSB) glass doped with Ce3+, which can be applied as scintillator [9]. It was found that the induced absorption in the scintillation range depends on the doping concentration, and the light yield of DSB: Ce glass after γ-ray irradiation can reach up to 80% to that before γ-ray irradiation, indicating an extremely high radiation resistance of the DSB glasses. However, there is no detailed discussion about the structure and fluorescence properties of the Ce3+- doped DSB glasses after γ-ray radiation, as well as the irradiation mechanism.
The luminescence properties of Ce3+- doped borophosphate- based glass have been reported with some interesting results [10-11]. It is well known that the properties and mechanisms of a scintillator’s radiation hardness are strongly related to their service performance. However, to our knowledge this has not been thoroughly investigated. Herein, the Ce3+ concentration- dependent luminescence properties of Ce3+- doped lithium borophosphate glass after proton irradiation (API) are discussed and compared with those before proton irradiation (BPI), using absorption, excitation and emission spectra, X-ray excited luminescence (XEL) spectrum, as well as thermal simulated luminescence spectrum (TSL). The mechanism of proton irradiation of Ce3+-doped lithium borophosphate glass is also investigated. This work paves the way not only for understanding the proton irradiation mechanism in the Ce3+- doped lithium-borophosphate glasses, but also for exploration of novel scintillation glass with improved radiation hardness.
2. Experimental section
Ce3+- doped lithium-borophosphate (LBPO) glasses with the nominal composition of 57.5Li2O-5B2O3-37.5P2O5-xCeO2 (x = 0.0, 0.5, 0.75, 1.0 and 1.25) were prepared from Li2CO3 (AR, ≥ 98.0%), H3BO3 (AR, ≥ 99.5%), NH4H2PO4 (AR, ≥ 99.5%), and CeO2. Batches of about 10 g raw materials were finely ground in an agate mortar and melted in a platinum crucible at 785-865 °C for 30 minutes in an ambient atmosphere. The homogeneous melts were quickly poured into a preheated stainless steel mold. The quenched glasses were annealed at 300 °C for 3 hours, then followed to cool naturally to room temperature. All glass samples were prepared homogeneously and polished to 2 mm. The theoretical lithium density of the glass was up 10.8 wt %. The compositions and theoretical optical basicity Λth of the LBPO:Ce glasses are shown in Table.1. The proton irradiation of the LBPO: Ce3+ was conducted on the proton beam line in a MC-50 cyclotron at the Korea Institute of Radiological and Medical Sciences (KIRAMS). The energy and current of the proton beam were fixed at 45 MeV and 1 × 1014 proton/cm2, and the irradiation time was 1,018 seconds.
Table 1. Compositions, theoretical optical basicity Λth and optical band gaps of the lithium borophosphate glasses before proton irradiation (BPI) and after proton irradiation (API).
Ce LIII- edge X-ray Absorption Fine Structure Spectroscopy (XAFS) measurements were carried out in fluorescence mode at room temperature at BL14W beamline of the Shanghai Synchrotron Radiation Facility (SSRF, China) to study the valence and local environment of the Ce ions. The electron energy in the storage ring was 3.5 GeV, with a current of 200 mA and the light emission angle was 1.5 × 0.1 mrad2. The beam size can be focused to 0.3 mm × 0.3 mm. The scanning range was from 5650 to 6100 eV. The energy steps were 0.2 eV (from 5650 to 6100 eV), which was appropriate for obtaining a clear Ce LIII- edge XAFS spectrum. Transmittance spectra of LBPO: Ce3+ glasses were measured with the Cary 5000, UV-VIS-NIR Spectrophotometer in the range of 200-800 nm. UV excitation and emission spectra were measured with a Hitachi F-4600 spectrometer. The scan speed was fixed at 240 nm/minute, the voltage was 400 V and the slits were fixed at 2.5 nm. The luminescence decay profiles were recorded on an Edinburgh Instruments (FLS980) spectrometer equipped with a continuous Xeon lamp, a nF920 lamp and a μF2 lamp as excitation sources. The X-ray excited luminescence (XEL) spectra were measured on a FluoMain X-ray excited luminescence spectrometer, equipped with a F30III-2 excitation source and Hamamatsu R928-28 photo receiver. All the spectra measurements were carried out at room temperature. Two-dimensional thermal simulated luminescence (TSL) spectra in the 323 - 673 K temperature range of LBPO: Ce3+ samples with proton irradiation were recorded on a ROSB-TL/OSL3DS measuring system (Department of Physics, Sun Yat-sen University, Guangzhou, China). The heating rate of 0.4 K/second was used in a 2D TSL glow curve. The LBPO: Ce3+ samples without proton irradiation were irradiated by X-ray at room temperature before TSL measurement for comparison.
3. Results and discussions
3.1 Transmittance spectra of the LBPO:Ce glasses BPI and API
The transmittance spectra of the LBPO:Ce glasses doped with different Ce3+ concentrations before and after proton irradiation are presented in Fig. 1(a) and 1(b), respectively. The absorption cut-off edge of the un-doped LBPO glass (G1) is lower than 200 nm. This can be attributed to the charge transfer from the non-bridging oxygen to metal ions [12]. However, for Ce3+-doped LBPO glasses, the absorption peaks around 212 nm and the absorption edges originate from the 4f-5d transitions of Ce3+. Note that there is an obvious red shift of the absorption edges in the LBPO:Ce glasses with increasing CeO2 when compared with the un-doped LBPO glass. This is shown in the inset of Fig. 1(a) and Table 1. The red-shift of the absorption edge of Ce3+- doped LBPO glasses (i.e. the decrease of the optical band gap) is due to the availability of more oxygen in the glass network by incorporating Ce3+. There is a large red- shift of the absorption edge of Ce3+ (1.25 wt%)- doped LBPO, that can be attributed to the absorption of Ce4+. This is demonstrated by the light yellow color of LBPO:Ce3+ (1.25 wt%) glass. The intensity of the absorption peak at 212 nm decreases with increasing Ce3+ concentration, and disappears completely when Ce3+ rises to 1.25 wt%.
where X1, X2, X3, …, Xn are equivalent fractions based on the amount of oxygen that each oxide contributes to the overall glass, and Λ1, Λ2, Λ3, …, Λn are the basicities assigned to the individual oxides.
Fig. 1 Transmittance spectra of the LBPO glasses doped with different Ce3+ concentrations before (a) and after (b) proton irradiation. The insets show the enlargement of the transmittance spectra and the LBPO:Ce glasses before and after proton irradiation.
It is interesting that the absorption cut- off edge of the Ce3+-doped LBPO glasses after proton irradiation is about 400 nm (Fig. 1(b)). This is demonstrated by the yellow color of samples as shown in the inset of Fig. 1(b). The large red-shift (30-70 nm) of the absorption cut-off edge can be attributed to the F-centers or defects introduced by proton irradiation. This will be discussed in detail below. The transmittance intensity of the Ce3+- doped samples didn’t change much after proton irradiation. However, for LBPO glasses without doping, the transmittance intensity decreases greatly from 90% to 10 ~60% in the wavelength range of 300 - 800 nm, indicating that the radiation hardness of LBPO glasses without doping is extremely poor. Fortunately, the radiation hardness of LBPO glasses has been greatly improved through Ce3+ doping.
3.2 Vibrational spectra of the LBPO:Ce glasses BPI and API
Spectroscopy is an important technique for the characterization of inorganic anion groups and may be used in conjunction with X-ray diffraction (XRD) to determine crystal structure and the local environment of the activators. However, it is impossible to use XRD to check the glass structure due to the long-range disorder of atoms. Therefore, the vibration spectra investigation of LBPO:Ce glass was carried out. The infrared spectra of LBPO:Ce glass BPI and API are shown in Fig. 2. All the experimental results are collected and tabulated in Table 2. For the infrared spectra of LBPO:Ce glass BPI, the relatively strong band at 575 cm−1 and weak band at 1043 cm−1 are ascribed to the stretching mode v4 and v3 of BO4 units, respectively [13, 14]. The weak absorption band located around 751 cm−1 and strong absorption band at 904 cm−1 were correlated to the symmetric and asymmetric stretching vibration of P-O-P chains, respectively [13, 15]. The symmetric and asymmetric stretching vibration of PO3 units can be observed at 998 and 1106 cm−1 [13, 16]. The appearance of a band at 1184 cm−1 is attributed to the asymmetric stretching vibration of the PO4 units [13, 17]. It was reported that the structure of B2O3-based glass can be modified by converting BO3 groups into BO4 groups by incorporating alkali oxide [14]. As a result, the vibration mode of the BO3 unit cannot be observed in the LBPO:Ce glass BPI. In addition, the position and relative intensity of absorption peaks in the FT-IR spectra of LBPO:Ce glass BPI with different Ce3+ concentrations are quite similar, indicating that the incorporation of Ce3+ didn’t introduce the glass structure distortion. After proton irradiation, the FT-IR spectra of the LBPO:Ce glass are similar with those before proton irradiation. However, it is of note that the new weak absorption band at 1439 cm−1 can be found in the LBPO:Ce glass API, which is assigned to the B-O stretching vibrations of the BO3 units [15, 17]. In addition, the absorption bands at about 1134 and 1227 cm−1, attributed to the asymmetric stretching vibration of PO3 and PO4 units are stronger than the asymmetric stretching vibration of the P-O-P chains. This observation indicates that BO4 units and non-linear P-O-P bonds in the LBPO:Ce glass were broken after the proton irradiation, leading to the BO3 units, as well as more PO3 and PO4 units. As a consequence, the structure of the LBPO glass is more flexible after proton irradiation compared with that before proton irradiation.
Table 2. Infrared band wavenumbers (cm−1) and assignments a for LBPO:Ce glass BPI and API.
3.3 Valence state and luminescence properties of Ce in LBPO glasses
Figure 3 compares the normalized Ce LIII- edge XANES spectra of LBPO:Ce glass BPI and API, along with two standard spectra of CeF3 in hexagonal phase and CeO2 in cubic phase. The strongest peak in the Ce LIII edge spectrum of CeF3 locates at about 5725.5 eV (Fig. 3(a)), which is attributed to the dipole-allowed transition of 2p state to 4f15d final state [18]. The Ce LIII edge spectrum of CeO2 (Fig. 3(b)) is in agreement with the results of Zhang and Kaindl et al [19, 20]. Because of the cubic crystal-field splitting of Ce 5d states, the shoulders A and B are associated to the transitions of Ce 2p to the Ce 4f15d eg (L) and t2g (L) states, where L denotes an oxygen ligand 2p hole, 4f1 refers to an electron from an oxygen to 2p orbital to a Ce 4f orbital, which is similar to the charge transfer. The energy difference between A and B is about 3.6 eV, which is in good agreement with the previous works [19, 21]. The peak C is due to the different final state configuration 4f05d. It can be observed from Fig. 3(d) that the position of the LIII-edge of Ce in LBPO glasses BPI is similar to that of CeF3 (5725.5 eV), locating at lower energy than that of CeO2. This means that Ce in LBPO glasses BPI is found in the trivalent state. However, for the LBPO:Ce API (Fig. 3(c)), there are two peaks locating at 5725.5 and 5737.3 eV, attributing to the LIII edge of trivalent and tetravalent states of Ce, respectively. The coexistence of Ce3+ and Ce4+ in LBPO glass after proton irradiation indicates that the some Ce3+ has been oxidized to Ce4+ during the interaction between proton and the glass.
Fig. 3 The X-ray absorption near-edge structure (XANES) spectra of Ce3+ in LBPO glasses before (a) and after (b) proton irradiation, CeO2 (c) and CeF3 (d).
The typical excitation and emission spectra of LBPO: Ce3+ (0.5 wt%) glass before and after proton irradiation (BPI and API) are shown in Fig. 4. There is one principle excitation band in the excitation spectrum of the BPI glass when measured at room temperature (Fig. 4(a)). This excitation band is in the wavelength range of 200-325 nm, with the maximum and shoulders at about 272, 221 and 318 nm, respectively, attributing to the 4f - 5d transitions of Ce3+ ions. When the BPI glass is excited at 272 and 318 nm, a broadening symmetrical emission band, centering at 339 nm in the wavelength range of 300 - 450 nm, is observed due to the allowed transition from the lowest 5d excited state of the Ce3+ ion to the 4f ground state. Due to the absence of any specific symmetry or long range periodicity in glass, a large broadness of the emission band is expected. The BPI glass displays a broad symmetrical emission band in the wavelength range of 300 - 450 nm with a peak centered at about 339 nm. This can be attributed to the allowed transition from the lowest 5d excited state of the Ce3+ ion to the 4f ground state. Generally, the emission of Ce3+ ions shows doublet character bands due to the spin-orbit splitting of the ground state (2F5/2 and 2F7/2) with an energy difference of about 2000 cm−1 [22]. However, no doublet emission bands were observed since the doublet character of the emission band depends on the temperature and Ce3+ concentration (self-absorption), and is not always found [23]. There is no significant change in the position and shape of the excitation and emission spectra with increasing Ce3+ concentration (Fig. 4(c)). This indicates that there are no significant changes in the crystal field strength around Ce3+.
Fig. 4 The excitation and emission spectra of LBPO: Ce3+ (0.5 wt%) glass before (a) and after (b) proton irradiation, the integrated emission intensity and wavelength as a function of Ce3+ concentration (c).
It is interesting to find that there is no significant change in the profile of the excitation and emission spectra of the LBPO:Ce glass after proton irradiation (API). However, the emission peak of the LBPO:Ce glass shifts from 339 to 329 nm after proton irradiation (API), as shown in Fig. 4(b). Similarly, four peaks located at about 211, 228, 255 and 292 nm were observed in the excitation spectrum, with a small blue shift compared with BPI glass. The increase in excitation peaks observed indicates that the 5d excited levels were split into four different energy levels after proton irradiation. The observed excitation and emission bands of Ce3+ in LBPO glass BPI and API are tabulated in Table 3. From these, the values of Stokes shift, crystal field splitting εcfs and spectroscopic redshift D(A) are calculated [24]. Apparently, Ce3+ shows analogous luminescence behavior in the LBPO glass BPI and API, even though their local Ce3+ coordination environment may be different. As it is well known that the crystal field splitting behavior of Ce3+ 5d levels in the oxides is mainly determined by the symmetry and size of the [REOn] polyhedral. This corresponds respectively to the angular part and radical part of the crystal field [24]. According to the results of Fig. 4 and Table 3, it is reasonable to deduce that the local environment of Ce3+ in LBPO glass BPI resembles that in API. This agrees with the results observed in FT-IR. However, several differences can be observed. The Stokes shift of LBPO:Ce3+ glass BPI (1910 cm−1) is smaller than that of API (2890 cm−1), indicating the more rigid nature of LBPO:Ce3+ glass BPI [25]. In addition, the slightly larger values of εcfs and the D(A) of Ce3+ in LBPO:Ce glass BPI show a stronger crystal field. This probably arises from the smaller size of the [CeOn] polyhedra. From the these results, it is reasonable to deduce that the structure of LBPO:Ce3+ glass and [CeOn] polyhedra became more flexible and larger after proton irradiation.
Table 3. Observed excitation and emission bands of LBPO:Ce glass BPI and API.
It is of note that under an excitation of 272 nm, the integrated emission intensity of BPI and API LBPO: Ce3+ (x wt%) (0 ≤ x ≤1.25) glass decreases as the Ce3+ concentration exceeds 0.75 wt% as a concentration quenching. However, the integrated emission intensity of API glass is only 20% compared with BPI glass, indicating that the proton irradiation introduces defects leading to the luminescence degradation. This will be discussed in detail below.
Radioluminescence is one of the most important properties for a scintillator used in practical application. Therefore, the X-ray excited luminescence (XEL) spectra are illustrated in Fig. 5. For LBPO:Ce glass BPI, the symmetrical emission band centering at 339 nm in the wavelength range of 275 - 425 nm is observed to originate from the allowed transition between the lowest 5d excited state of Ce3+ ion and the 4f ground state (Fig. 5(a)). However, the peak position of XEL band shows a blue shift (about 10 nm) compared with the samples before proton irradiation (Fig. 5(b)). In addition, the luminescence intensities reach the maximum at 0.75 wt% for both LBPO:Ce BPI and API, and the integrated emission intensity of API is about 20% of the BPI glass (Fig. 5c). However, it is of note that there is no emission from the un-doped LBPO glass before and after proton irradiation for both under UV and X-ray excitation, as shown in Fig. 4(c) and Fig. 5(c), because there is no Ce3+ ion as a luminescence center in the glasses. It is difficult to make the comparison of radiation influence on optical and fluorescence properties with other Ce3+-doped glasses (e.g. DSB: Ce glass) because of the different glass hosts (Di-silicate vs borophosphate) and irradiation sources (X-ray, γ-ray vs proton).
Fig. 5 The XEL spectra of LBPO: Ce3+ glass before (a) and after (b) proton irradiation with different Ce3+ concentrations, the integrated XEL intensity as a function of Ce3+ concentration (c).
The fluorescence decay curves of Ce3+ in LBPO: Ce3+ (0.5 wt%) BPI and API, together with their fitted curves are shown in Fig. 6. The monitoring excitation and emission wavelength were fixed at the effective 5d - 4f emission of Ce3+ in the specific samples, i.e. λex = 272 nm, λem = 339 nm for LBPO: Ce3+ (0.5 wt%) BPI, and λex = 252 nm, λem = 329 nm for API, respectively. For LBPO: Ce3+ (0.5 wt%) BPI and API, the fluorescence decay curves fit well using a single exponential functional function
where I0 is the initial spectral intensity, τ1 is the decay time constant of the emission. The fitting decay time of Ce3+ is in the range of 30 - 40 ns, showing the typical decay time value of Ce3+ 5d - 4f emission in many other host lattices [26–29]. The similar decay time profile of Ce3+ in LBPO glass BPI and API shows that no more new emission centers were generated in the glass during the proton irradiation process. Considering the low-cost, large-volume, relatively high light yield, fast decay time and easy shaping of elements of Ce3+-doped LBPO glass, one has reason to assume that this glass might find an potential application as a potential scintillating material.Fig. 6 Fluorescence decay curves of Ce3+ 5d-4f emission for LBPO: Ce3+ (0.5 wt%) BPI (a) and API (b) at room temperature.
The determination of the type and depth of the traps generated through proton irradiation is important for understanding the correlation mechanism between protons and the glass. It is well known that thermal simulated luminescence (TSL) is a powerful tool to calculate the trap depth in a specific host lattice. Therefore, the thermal simulated luminescence spectra of LBPO: Ce3+ glass before and after proton irradiation with different Ce3+ concentrations were collected and shown in Fig. 7 (a). There is no emission peak for the LBPO: Ce3+ glass BPI when heated up to about 350 °C. This indicates that there is no trap in the glass. However, for the LBPO: Ce3+ glasses API, there are strong emission peaks located in the temperature range of 200 −250 °C. This means that some traps were generated after proton irradiation on the LBPO: Ce3+ glasses. This phenomenon can be demonstrated by the yellow color and the shrinkage of the optical band gap for the LBPO: Ce3+ glasses after proton irradiation. The TSL peaks indicate the existence of traps which are responsible for the trapping of shallow as well as deep traps in the LBPO glass API. They can trap electron and hole under X-ray irradiation and result in the reconstruction of local charge balance. Considering an isolated TSL peak, the trap parameter is such that thermoactivation energy E can be estimated by the total glow method [30, 31]. The dependence of the intensity of luminescence I on the temperature is expressed as
Fig. 7 The thermal simulated luminescence spectra of LBPO: Ce glass after proton irradiation with different Ce3+ concentrations (a) and the plot of Lnδ versus 1/T for the TSL measurement of LBPO:Ce glass API (b).
in which β is the heating rate and k is the Boltzman constant. Giving , Eq. (1) can be written as
Figure 7(b) shows the plot of ln δ vs 1/T for the TL curves of LBPO:Ce glasses with different Ce concentrations after proton irradiation. As shown in Fig. 7(b), the simulated curves at the beginning of the heating can be fitted with a line, indicating simple first-order kinetics. In addition, the thermoactivation energy E in LBPO glass without Ce3+ doping was estimated to be about 0.28 eV, and the thermoactivation energy E decreases to 0.26, 0.22, 0.14 and 0.12 eV for the 0.5, 0.75, 1.00, 1.25 wt% Ce3+ concentrations, respectively.
4. Conclusions
We have reported the luminescence properties of Ce3+-doped lithium borophosphate glass after proton irradiation. It was found that the BO4 units and non-linear P-O-P bonds in the LBPO:Ce glass were broken after the proton irradiation, leading to the BO3 units, as well as more PO3 and PO4 units. A large red-shift (30 - 70 nm) of the absorption cut-off edge for the glass after proton was observed. Moreover, under UV and X-ray excitation, the integrated emission intensity decreases to about 20% compared with that before proton irradiation attributing to the traps generated in the glasses during the proton irradiation. The depths of the traps were estimated to be about 0.12 - 0.28V for different Ce3+ concentrations according to the thermal simulated luminescence spectra.
Funding
Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2015M2B2A4033073, 2016R1D1A1B03933488 and 2015R1D1A1A01058991); National Natural Science Foundation of China (Grant No. 51772185).
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