Luminescent properties of Tb3+ doped high density borogermanate scintillating glasses

JOURNAL OF RARE EARTHS, Vol. 35, No. 8, Aug. 2017, P. 787

Luminescent properties of Tb3+ doped high density borogermanate scintillating glasses QIAN Shan (钱 珊), HUANG Lihui (黄立辉)*, ZHAO Shilong (赵士龙), XU Shiqing (徐时清) (College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China) Received 22 October 2016; revised 3 May 2017

Abstract: Tb3+-doped high density borogermanate glasses were prepared by melt-quenching method. Their physical and luminescent properties including differential thermal analysis (DTA), density, transmittance spectra, photoluminescence, and X-ray excited luminescence spectra were investigated. The densities of the glasses doped with Tb3+ were in the range from 5.690 to 6.086 g/cm3. Under UV and X-ray excitations, the glasses showed intense green emissions. The lifetimes of Tb3+ doped borogermanate glasses were in the range from 1.597 to 1.869 ms. The results indicated that Tb3+ doped borogermanate glasses could be scintillator candidate used in X-ray detection application. Keywords: luminescence; Tb3+; scintillating glass; high density; rare earths

In recent years, as one of the most attractive hosts for active optical devices and optical amplifiers, rare earth ion doped transparent germanate glasses have attracted considerable attention due to the advantages in high rare earth ion solubility, low phonon energy, excellent chemical and thermal stability. Yb3+ ion[1,2], Dy3+ ion[3], Eu3+ ion[4,5], and Tm3+ ion[6–8] have been doped in germanate glasses to reveal their luminescence properties. As we know, heavy germanium oxide is an important component for glass with high density. Thus, rare-earth ions doped germanate glasses show great promise as candidates for X-ray scintillating materials. Meanwhile, among the scintillating performance required in practice, such as X-ray computer tomography (X-CT), glass density close to or exceeding 6.0 g/cm3 is particularly important because a high glass density increases the X-ray absorption cross-section, which results in significant increment of the image’s signal-to-noise ratio[9,10]. Recently, many researchers reported high density glasses by the addition of rare-earth oxides RE2O3 (RE=Y, La, Gd and Lu) to glass scintillators with suitable light yield[11–14] and many silicate, borosilicate, germanate, borogermanate and tellurite glasses containing high Gd2O3 contents have been used for glass scintillators[10–15]. Among these RE2O3 oxides, Lu3+ is optically inert, which is suitabe for matrix material and borogermanate glass with a high Lu2O3 content was rarely studied. Besides, dense Lu2O3 is a favorite raw material to elevate the density of the glass. Regarding to the borogermanate system, Eu3+-, Tb3+-activated boroger-

manate scintillating glasses with a density of 5.6–5.8 g/cm3 have been proposed to detect high energy X-rays[10,15,16]. Under X-ray excitation, Tb3+-doped borogermanate glasses show strong greenish fluorescence which could be applied in X-ray detection for slow event, especially. To our knowledge, there are much work on rare earth ions doped borogermanate or germanate glass. However, Tb3+-doped high density borogermanate scintillating glasses with a high Lu2O3 content are rarely studied and the strongest emission of Tb3+ is around 540 nm, which is convenient for direct coupling with silicon detector. Therefore, it is useful for the X-ray scintillation detection. In this paper, the physical and luminescent properties of Tb3+-doped high density borogermanate scintillating glasses containing dense Lu2O3 were investigated in detail.

1 Experimental Tb3+ doped borogermanate glasses samples with the composition of 20B2O3-40GeO2-20Lu2O3-5La2O3-(15–x) BaF2-xTbF3 (x=1, 2, 4, 6, 8, 10, 12 mol.%) were prepared by melt-quenching method. All raw materials were derived from H3BO3 (A.R.), GeO2 (99.999%), Lu2O3 (99.99%), La2O3 (99.99%), BaF2 (99.9%) and TbF3 (99.99%). About 20 g weighed raw materials were mixed in an agate mortar and melted in a covered high purity alumina crucible at 1500 ºC for 1 h in an electric furnace in the air atmosphere so that a homogeneously mixed melt was obtained. Then, the melt was poured into a preheated stainless steel mould, subsequently annealed for 2 h below the glass temperature

Foundation item: Project supported by the National Natural Science Foundation of China (11375166, 51372236) * Corresponding author: HUANG Lihui (E-mail: [email protected]; Tel.: +86-571-86835781) DOI: 10.1016/S1002-0721(17)60977-3

788

and then slowly cooled down to room temperature. The glass samples were polished for optical measurements with a thickness of 1.80±0.02 mm. DTA measurements were carried out by a Netzsch DTA 404PC at a heating rate of 10 K/min. The glass densities were measured by Archimedes’ method using distilled water as an immersion liquid. Transmittance spectra were obtained with a Shimadzu UV-3600 spectrophotometer in the range of 200–710 nm. Photoluminescence (PL) spectra and luminescence decay curves were recorded on a Jobin-Yvon Fluorolog3 fluorescence spectrophotometer using Xe lamp as an excitation source. The X-ray excited luminescence (XEL) spectra were measured using an X-ray tube (Copper target, 70 kV, 1.5 mA) and an Ocean Optical QE65000 spectrometer based on charge coupled device (CCD) detector. The spectral sensitivity of the detection system as a function of wavelength was corrected with a standard radon lamp. All the measurements were performed at room temperature.

2 Results and discussion Fig. 1 shows the DTA curve of 10 mol.% Tb3+ doped borogermanate glass. The glass transition temperature (Tg) is around 571 ºC. Two exothermic peaks are observed at 755 and 956 ºC. The former peak could be the precipitation of BaF2 and the latter is due to the crystallization of the glass. There are few reports on the thermal analysis of germanate scintillation glasses. Meanwhile, the Tx-Tg value of glass is higher than other glasses[17,18], which indicates that the present glass has better thermal stability. The densities of the borogermanate glasses doped with different Tb3+ concentrations (1, 2, 4, 6, 8, 10, 12 mol.%) are shown in Fig. 2. The density of the glass decreases slightly from 6.086 to 5.690 g/cm3 with increasing the concentration of Tb3+. This result may be attributed to the content of heavy Lu2O3 slightly decreased while that of light TbF3 increased. It is worth noting that all the densities of the Tb3+ doped borogermanate glasses are around 6.0 g/cm3. This indicates that the Tb3+ doped borogermanate glasses could be potential glass scintillator can-

JOURNAL OF RARE EARTHS, Vol. 35, No. 8, Aug. 2017

Fig. 2 Densities of borogermanate glasses doped with different Tb3+ concentrations

didates for practical applications. Fig. 3 shows the transmittance spectra of Tb3+-doped borogermanate glass with a thickness of 1.80±0.02 mm. It is obvious that the transmittance of the glasses decreases with increasing Tb3+ concentration, but the overall has good transmittance in the visible spectrum region and the UV cut-off wavelength is around 265 nm. It indicates that the obtained glasses may be used as scintillator material. The absorption bands of Tb3+ ions with peaks at 317, 340, 352, 369, 378, 487 nm could be observed in the spectra as well, which are due to the transitions from the ground state 7F6 to the 5H7 and higher 5 D states of Tb3+. Under UV light, Tb3+-doped borogermanate glasses show greenish fluorescence. Fig. 4 shows the emission spectra of the borogermanate glasses doped with 1, 2, 4, 6, 8, 10 and 12 mol.% Tb3+ under 376 nm light excitation. The emission spectra contain four emission bands peaked at 489, 543, 589, and 622 nm, which are attributed to the transitions 5D4→7F6, 5D4→7F5, 5D4→7F4, and 5D4→7F3 of Tb3+, respectively. Besides, there is a shoulder peak at about 549 nm in Fig. 4. It may be caused by asymmetry of the glass matrix results in splitting of the energy level[19]. The emission at 543 nm is the strongest

Fig. 3 Transmittance spectra of Tb3+ doped borogermanate Fig. 1 DTA curve of 10 mol.% Tb3+-doped borogermanate glass

glass with a thickness of 1.80±0.02 mm

QIAN Shan et al., Luminescent properties of Tb3+ doped high density borogermanate scintillating glasses

Fig. 4 Emission spectra of Tb3+ doped borogermanate glasses

among them. The intensity of 543 nm emission increases with the increment of Tb3+ concentration until it reaches 10 mol.%, then the intensity decreases due to concentration quenching. Other emissions also have the same trend. The results indicate that the optimum concentration of Tb3+ in the borogermanate glass is 10 mol.%. Fig. 5 shows the excitation spectra of the borogermanate glasses doped with different Tb3+ concentrations monitored at 543 nm emission. The spectra consist of six excitation bands with peaks at 284, 302, 317, 350, 376, and 483 nm can be attributed to the 7F6→5H4, 5H6, 5H7, 5 D2, 5D3, 5D4 of Tb3+, respectively. The excitation band with that peak at 376 nm is the strongest. With increasing Tb3+ concentration, the excitation intensity increases up to 10 mol.%, then decreases. These results are consistent with the emission spectra results. The luminescence decay curves of 543 nm emission of Tb3+ doped borogermanate glasses excited by 376 nm were recorded. All the curves are single exponential. Fig. 6 shows the decay curve of 10 mol.% Tb3+ doped borogermanate glass. The lifetime of the 543 nm emission determined by the least-squares fitting of the decay curve with single-exponential function is 1.690±0.035 ms. The lifetimes of 543 nm emissions of Tb3+ doped germanate glasses in the range from 1.597 to 1.869 ms are shown in inset in Fig. 5. The lifetime decreases monotonically with the increment of Tb3+ concentration. The

789

Fig. 6 Decay curve of 543 nm emission in the glass doped with 10 mol.%Tb3+ (Inset: the lifetime dependence of 543 nm emission on Tb3+ concentration)

self-generated quenching occurs in the instance that the average distance between luminescent centers shortens with increasing concentration of Tb3+, which may be responsible for the decay time variation[15]. Fig. 7 presents the XEL spectra of Tb3+-doped germanate glasses. Four emissions bands with peaks at 488, 543, 588, 622 nm, which correspond to the 5D4→7FJ (J=6, 5, 4, 3) transitions of Tb3+ respectively, were recorded. The strongest emission with the peak located around 543 nm, which is similar with the 376 nm light-excited emission spectra mentioned above. However, the intensities of four emissions increase with increasing Tb3+ concentration monotonously. This result is different from the PL spectra, which may be attributed to the discrepancy of excitation mechanism between ultraviolet light and X-ray initiated processes[16].

Fig. 7 XEL spectra of Tb3+ doped borogermanate glasses

3 Conclusions

Fig. 5 Excitation spectra of Tb3+ doped borogermanate glasses

Tb3+ doped high density borogermanate glasses were prepared by melt-quenching method. DTA result indicated the glass had good thermal stability. The densities of the glasses doped with Tb3+ were in the range from 5.690 to 6.086 g/cm3. Under UV and X-ray excitations, the glasses showed intense green emissions. The life-

790

JOURNAL OF RARE EARTHS, Vol. 35, No. 8, Aug. 2017

times of Tb3+ doped borogermanate glasses were in the range from 1.597 to 1.869 ms. The results indicated that Tb3+ doped borogermanate glasses could be a promising scintillator candidate in X-ray detection for slow event.

[11]

References:

[12]

[1] Zhang Q, Chen G R, Zhang G, Qiu J R, Chen D P. Infrared luminescence of Tm3+/Yb3+ codoped lanthanum aluminum germanate glasses. J. Appl. Phys., 2010, 107: 023102. [2] Salem Shaaban M, Shaltout I. Structure, electrical, and optical properties of ZnO-substituted PbO for the Er3+/Yb3+ co-doped Bi2O3-GeO2-Na2O glass system. J. Mater. Sci., 2010, 45(7): 1837. [3] Lakshminarayana G, Qiu J R. Photoluminescence of Pr3+, Sm3+ and Dy3+:SiO2-Al2O3-LiF-GdF3 glass ceramics and Sm3+, Dy3+:GeO2-B2O3-ZnO-LaF3 glasses. Physica B, 2009, 404(8): 1169. [4] Bensalem C, Mortier M, Vivien D, Diaf M. Thermal and optical investigation of EuF3-doped lead fluorogermanate glasses. J. Non-Cryst. Solids, 2010, 356(1): 56. [5] Sun X Y, Yang Q M, Xie P, Gao P, Wu H S. Effects of substitution of BaF2 for GdF3 on optical properties of dense oxyfluoride borogermanate scintillating glasses. J. Rare Earths, 2015, 33(8): 800. [6] Fan H Y, Gao G J, Wang G N, Hu J J, Hu L L. Tm3+ doped Bi2O3- GeO2-Na2O glasses for 1.8 μm fluorescence. Opt. Mater., 2010, 32(5): 627. [7] Lakshminarayana G, Yang H C, Qiu J R. White light emission from Tm3+/Dy3+ co-doped oxyfluoride germanate glasses under UV light excitation. J. Solid State Chem., 2009, 182(4): 669. [8] Zhang Q, Ding J, Shen Y L, Zhang G, Lin G, Qiu J R, Chen D P. Infrared emission properties and energy transfer between Tm3+ and Ho3+ in lanthanum aluminum germanate glasses. J. Opt. Soc. Am. B, 2010, 27(5): 975. [9] Fu J, Kobayashi M, Parker J M. Terbium-activated heavy scintillating glasses. J. Lumin., 2008, 128: 99. [10] Sun X Y, Zhang X, Chen H H, Hu Q L, Wang W F, Zhang

[13]

[14]

[15]

[16]

[17]

[18]

[19]

Z J, Zhao J T. Investigation on the luminescent properties of Eu3+-activated dense oxyfluoride borogermanate scintillating glasses. J. Non-Cryst. Solids, 2014, 404: 162. Jiang C, Deng P Z, Zhang J Z, Gan F X. Radioluminescence of Ce3+-doped B2O3-SiO2-Gd2O3-BaO glass. Phys. Lett. A, 2004, 323(3): 323. He W, Zhang Y P, Wang J H, Wang S X, Xia H P. Luminescence properties of terbium doped oxyfluoride tellurite glasses. Acta Phys. Sinica, 2011, 60(4):121. Chewpraditkul W, He X, Chen D P, Shen Y S, Sheng Q, Yu B, Nikl M, Kucerkova R, Beitlerova A, Wanarak C, Phunpueok A. Luminescence and scintillation of Ce3+-doped oxide glass with high Gd2O3 concentration. Phys. Status Solidi A, 2011, 208(12): 2830. Chen G, Yang Y X, Zhao D H, Xia F, Baccaro S, Cecilia A, Nikl M. Composition effects on optical properties of Tb3+-doped heavy germanate glasses. J. Am. Ceram. Soc., 2005, 88(2): 293. Sun X Y, Jiang D G, Chen S W, Zheng G T, Huang S M, Gu M, Zhang Z J, Zhao J T. Eu3+-activated borogermanate scintillating glass with a high Gd2O3 content. J. Am. Ceram. Soc., 2013, 96(5): 1483. Sun X Y, Yu X G, Wang W F, Li Y N, Zhang Z J, Zhao J T. Luminescent properties of Tb3+-activated B2O3-GeO2Gd2O3 scintillating glasses. J. Non-Cryst. Solids, 2013, 379: 127. Pisarska J, Kowal M, Kochanowicz M, Zmojda J, Dorosz J, Dorosz D, Pisarska W. Influence of BaF2 and activator concentration on broadband near-infrared luminescence of Pr3+ ions in gallo-germanate glasses. Opt. Express, 2016, 24(3): 2427. Pan Z, James K, Cui Y, Burger A, Cherepy N, Payne S A, Mu R, Morgan S H. Terbium-activated lithium-lanthanumaluminosilicate oxyfluoride scintillating glass and glassceramic. Nucl. Instrum. Methods Phys. Res., 2008, 594(2): 215. Li L, Lei X H, Ren L J, Feng Y A. Effect of glass matrix on luminescent properties of Tb3+ doped luminescence glass. Chin. J. Lumin., 2014, 35(4): 420.