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Terbium-activated lithium–lanthanum–aluminosilicate oxyfluoride scintillating glass and glass-ceramic
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 594 (2008) 215–219
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Terbium-activated lithium–lanthanum–aluminosilicate oxyfluoride scintillating glass and glass-ceramic Z. Pan a,, K. James a, Y. Cui a, A. Burger a, N. Cherepy b, S.A. Payne b, R. Mu a, S.H. Morgan a a b
Physics Department, Fisk University, Nashville, TN 37208, USA Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
a r t i c l e in fo
abstract
Article history: Received 5 May 2008 Received in revised form 27 June 2008 Accepted 28 June 2008 Available online 3 July 2008
Terbium-activated lithium–lanthanum–aluminosilicate oxyfluoride scintillating glasses, 55SiO2 6Al2O3 28Li2O 11LaF3 doped with different TbF3 concentrations, have been fabricated and investigated. By appropriate heat treatment of the as-prepared glasses above, transparent glass-ceramics were obtained. Differential scanning calorimetry, X-ray diffraction, optical absorption, and luminescence under both UV and beta-particle excitation have been investigated on as-prepared glasses and glassceramics. It has been found that these terbium-activated lithium–lanthanum–aluminosilicate oxyfluoride scintillating glasses exhibit good UV-excited luminescence and radioluminescence. The luminescence yield increases for glass-ceramics. The efficiency of beta-induced luminescence is comparable or nearly equal to that of the Schott IQI-301 product. & 2008 Elsevier B.V. All rights reserved.
Keywords: Scintillating Terbium-activated Luminescence Glass Glass-ceramic
1. Introduction Glass is an attractive scintillating material because of its lowcost, large-volume production possibility, and easy shaping of elements [1–3]. Glass, however, is a disordered material with significant point defects acting as traps which limit the free carrier transfer from the host matrix to the luminescence centers and provide additional multiphonon decay channels [1–3]. The pioneering work on scintillating glasses was performed during the late 1950s and early 1960s [4]. Cerium-activated lithium–aluminosilicate glasses were studied by Spowart et al. for use in neutron detectors [5–7], and during the 1990s, new terbium-activated lithium–aluminosilicate glasses were reported [8]. Terbiumdoped glass is considerably slower in response than its cerium-doped counterpart, but does not demonstrate the self-quenching phenomenon that occurs in cerium-doped glass when Ce3+ is oxidized to Ce4+ [8], probably because the oxidation of Tb3+ to Tb4+ is less thermodynamically favored. The radiation hardness of terbium scintillating glass is about 2–3 times better than cerium glass [9]. The maximum emission of Tb3+ ions is around 542 nm, convenient for a direct coupling with silicon detectors. Terbium-doped glass therefore is useful for thermal neutron detection and for applications in radiography [10–13]. It has been demonstrated that the constituent 6Li
Corresponding author. Tel.: +1 615 329 8622; fax: +1 615 329 8634.
E-mail address: zpan@fisk.edu (Z. Pan). 0168-9002/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.06.041
provides high neutron absorption and the aluminum oxide reduces the number of nonbridging oxygen defects in lithium– aluminosilicate glass matrix [14]. Oxyfluoride glass-ceramics have been developed to combine the particular optical properties of rare-earth ions in a nanocrystalline fluoride host with the chemical stability and mechanical property of oxide glasses [15]. The oxide glasses generally possess good mechanical strength, chemical durability, and thermal stability while the fluorides possess low phonon energy, resulting in an increased radiative emission rate of the incorporated rareearth ions [15]. Silicate oxyfluoride glass-ceramics containing Pb(Cd)F2 or LaF3 nanocrystals have been reported since 1990s [15–23]. Special attention has been paid to glass-ceramics containing LaF3 nanocrystals since Dejneka reported a new aluminosilicate glass-ceramic containing LaF3 in 1998 [16]. LaF3 is an excellent host material for rare-earth ions because it has a high solubility for rare-earth ions, low phonon energy (300–400 cm1), and desirable thermal and environmental stability [20,22]. In this work, terbium-doped lithium–lanthanum–aluminosilicate oxyfluoride glasses were successfully fabricated, and the corresponding glass-ceramics were obtained by appropriate heat treatment of the as-prepared glasses. X-ray diffraction (XRD), differential scanning calorimetry (DSC), optical absorption, and luminescence under both UV and beta-particle excitation have been investigated on as-prepared glasses and glass-ceramics. The beta-induced luminescence was compared with a standard scintillating glass Schott IQI-301 product [24].
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**
**
(b)
*
(a)
20
25
30
35 40 45 50 Diffraction angle, 2θ (deg.)
55
60
Fig. 2. XRD of LLASOF doped with 4.0 at% Tb: (a) as-prepared and (b) heat treated at 560 1C for 6 h.
100
Transmittance (%)
Reagent grade anhydrous oxide powders of SiO2, Li2CO3, Al2O3, LaF3, and TbF3 were used to prepare glasses. The compositions of lithium–lanthanum–aluminosilicate oxyfluoride glass were 55SiO2 6Al2O3 28Li2O 11LaF2 (LLASOF). Three batches containing different terbium concentrations, 0.5, 2.0, and 4 mol% of TbF3, were prepared. Batches of 40 g were thoroughly mixed in an agate mortar, and melted in a platinum crucible at a temperature of 1400 1C. The melts were held for 50 min and then cast onto a copper plate and pressed by another copper plate from the top, forming a glass plate of about 3 mm in thickness. The glasses were subsequently annealed at near the glass transition temperatures for 20 min and then allowed to cool to room temperature in the furnace. Clear glasses were formed for all batches with different terbium concentrations. These glasses appear to be of very good optical quality, with no visual evidence of devitrification. DSC measurements were carried out using SDT 2960 from TA Instruments, under N2 atmosphere at a constant heating rate 20 1C/min. XRD measurements were performed using a Scintag X1 advanced diffraction system with a Cu Ka radiation (l ¼ 0.154 nm). UV–visible absorption was measured from 300 to 800 nm using a Cary dual beam spectrophotometer. Photoluminescence (PL) was measured using 325-nm laser excitation wavelength. An HeCd laser (KIMMON model IK320R-D) and an Oriel monochromator MS257TM with a TE cooled CCD detector were used. The incident laser power used is 5 mW. The laser light was focused into a dot about 50 mm in diameter on the sample surface. The luminescence was measured at room temperature. Radioluminescence spectra were acquired using a 90Sr/90Y source (average beta-energy 1 MeV) to provide a spectrum expected to be essentially equivalent to that produced by gamma excitation. Radioluminescence spectra were collected with a Princeton Instruments/Acton Spec 10 spectrograph coupled to a thermoelectrically cooled CCD camera.
Intensity (arb.unit)
2. Experiment
80
(a)
60 (b)
40 20 0 280
330
380
430
480
Wavelength (nm) Fig. 3. Optical transmittance spectra of LLASOF glasses doped with (a) 2.0 at% Tb and (b) 4.0 at% Tb.
3. Results 3.1. DSC, XRD results, and heat treatment
Heat Flow (mW)
Fig. 1 shows the DSC curve of an as-prepared LLASOF glass doped with 4 mol% of TbF3. The DSC curve showed a glass transition temperature Tg at 406 1C, and several exothermal peaks centered at 540, 667, and 711 1C, respectively. These exothermal peaks indicate different crystallization phases in glass-ceramic. The XRD measurements were performed on as-prepared glass and
Tg
Tc2
Tc3
Tc1
its corresponding glass-ceramic. In Fig. 2(a), the XRD pattern for as-prepared glass shows no diffraction peaks, indicating its amorphous structure in nature. In Fig. 2(b), sharp diffraction peaks appeared for the sample heat treated at 560 1C for 6 h. The marked peaks are matched with the diffraction peaks of crystalline LaF3 reported previously [17–20,22,23]. Additional diffraction peaks could indicate the coexistence of other crystalline phases in the glass-ceramic. Samples were heat treated for 6 h at two temperatures between the glass transition temperature and the first crystallization temperature. Samples doped with 2 mol% TbF3 were treated at 520 1C (T1) and 540 1C (T2); and samples doped with 4 mol% TbF3 were treated at 460 1C (T1) and 480 1C (T2). Samples heat treated at T1 show a slight reduction in optical transparency, while samples heat treated at T2 become milky and semitransparent. 3.2. Absorption and UV-excited PL
0
200
400 600 Temperature (C)
Fig. 1. DSC curve of LLASOF glass doped with 4.0 at% Tb.
800
Samples of 15 10 2 mm3 (L W T) with two parallel surfaces polished were used for optical measurements. The transmittance spectra were shown in Fig. 3(a) and (b) for the as-prepared glasses doped with 2 and 4 mol% TbF3, respectively. The transmittance spectra show absorption bands of Tb3+ ions centered at 350, 367, 376, 483 nm and a UV absorption edge at about 325 nm (T ¼ 50%) from the host glass.
ARTICLE IN PRESS Z. Pan et al. / Nuclear Instruments and Methods in Physics Research A 594 (2008) 215–219
3.3. . Beta-induced luminescence The beta-induced luminescence was measured at Lawrence Livermore National Laboratory and compared with the Schott IQI-301 product under the same experimental conditions. Fig. 7 shows the spectra of LLASOF glass and glass-ceramics doped with 2.0 mol% TbF3 and Fig. 8 shows the spectra of LLASOF glass and glass-ceramics doped with 4.0 mol% TbF3. These spectra indicate the following: (1) the radioluminescence intensity increases with Tb concentration from 2% to 4%; (2) the intensity increases with
Intensity (Arb.unit)
(c)
300
(b)
(a)
350
400
450 500 550 Wavelength (nm)
600
650
700
Fig. 5. UV-excited PL spectra of LLASOF doped with 2.0 at% Tb: (a) as-prepared, (b) 520 1C for 6 h, and (c) 540 1C for 6 h.
(c)
Intensity (Arb.Unit)
The room temperature PL was measured using 325-nm laser excitation. Fig. 4(a) shows a PL spectrum from a Tb-doped lead–cadmium–aluminosilicate oxyfluoride glass LCASOF (30SiO2 7.5Al2O3 22PbF2 29CdF2 3.5YF3 0.5TbF3) [15] for comparison and Fig. 4(b) shows PL spectra of three as-prepared LLASOF samples doped with 0.5, 2.0, and 4.0 mol% of TbF3, respectively. The excitation laser power used is 5 mW for LLASOF and 15 mW for LCASOF because the luminescence from LCASOF is very weak. The four major emission bands at 489, 542, 585, and 622 nm are attributed to transitions of 5D4-7Fi (i ¼ 6, 5, 4, and 3, respectively) of Tb3+ ions [1]. The dominant band is at 542 nm. The PL intensity is almost proportional to the Tb-doping concentration as shown Fig. 4(b), indicating no significant self-quenching effect. The background in spectra Fig. 4(b) is clean, indicating no significant luminescence from defects in LLASOF glass host. However, the PL spectrum (Fig. 4(a)) from a Tb-doped LCASOF glass shows a broad background emission extending from 360 to 650 nm, in addition to the weaker Tb3+ emission. This broad background emission is attributed to defects in glass host. Fig. 5 shows the heat-treatment effect on UV-excited PL spectra of LLASOF glass doped with 2.0 mol% TbF3. The PL intensity increases with increasing heat-treatment temperature. The glass-ceramics showed a higher luminescence yield compared to that of the as-prepared glass. The intensity increased 20% for the sample treated at T1 and 45% for the sample treated at T2. Spectra in Fig. 6 showed a similar heat-treatment effect on UV-excited PL of LLASOF doped with 4.0 mol% TbF3. The intensity increased 3% for the sample treated at T1 and 70% for the sample treated at T2.
217
300
(b)
(a)
350
400
450 500 550 Wavelength (nm)
600
650
700
Fig. 6. UV-excited PL spectra of LLASOF doped with 4.0 at% Tb: (a) as-prepared, (b) 460 1C for 6 h, and (c) 480 1C for 6 h.
Intensity (Arb.unit)
(a)
(d)
Intensity (Arb.unit)
(b) 4.0 % Tb
2.0 % Tb
(c) (b) (a)
0.5 % Tb
300
350
400
500 450 550 Wavelength (nm)
600
650
700
Fig. 4. UV-excited PL spectra of (a) LCASOF glass doped with 0.5 at% Tb and (b) LLASOF glasses doped with 0.5 at%, 2.0 at%, and 4.0 at% Tb, respectively. The excitation is at 325 nm with a power of 15 mW for (a) and 5 mW for (b).
400
450
500 550 600 Wavelength (nm)
650
700
Fig. 7. Beta-induced luminescence of LLASOF doped with 2.0 at% Tb, compared with IQI-301: (a) as-prepared, (b) 460 1C for 6 h, (c) 480 1C for 6 h, and (d) IQI-301.
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Intensity (Arb.unit)
(d)
400
(c)
(b) (a)
450
500 550 600 Wavelength (nm)
650
700
Fig. 8. Beta-induced luminescence of LLASOF doped with 4.0 at% Tb, compared with IQI-301: (a) as-prepared, (b) 460 1C for 6 h, (c) 480 1C for 6 h, and (d) IQI-301.
Table 1 The relative efficiency of beta-induced luminescence of 4 mol% TbF3-doped lithium–lanthanum–aluminosilicate oxyfluoride scintillating glass and glassceramics compared with standard Schott IQI-301 product Beta-induced luminescence efficiency IQI-301 1.0
Glass 0.47
Glass-ceramic #1 0.52
Glass-ceramic #2 0.71
heat treatment; (3) the radioluminescence light yield for the LLASOF samples is comparable to that of Schott IQI-301 product. We also compared the beta-induced luminescence of Tb-doped LLASOF samples with a Tb-doped LCASOF glass; the radioluminescence intensity is more than one order of magnitude higher for Tb-doped LLASOF. Table 1 lists the values of relative efficiency of beta-induced luminescence of 4 mol% TbF3-doped LLASOF compared with standard Schott IQI-301 product, the highest value is 0.71. Considering that the wt% of the 4 mol% Tb-doped sample is 7.8 wt% while the Tb concentration of IQI-301 is 11.2 wt%, the obtained beta-induced luminescence yield from LLASOF glass and ceramics is encouraging [24].
4. Discussion The XRD pattern (Fig. 2) obtained from the heat-treated sample indicates that the glass-ceramic contains LaF3 and other nanocrystals. The first exothermal band in the DSC curve (Fig. 1) indicates the crystallization temperature of LaF3 nanocrystals [18,20]. The increased luminescence efficiency from the glassceramic compared to that of the as-prepared glass may be attributed to the formation of LaF3:Tb nanocrystals [17,22]. The LaF3 crystal has a very low phonon energy (300–400 cm1), compared to that of silicate-based host. The excited Tb3+ ions therefore have a reduced multiphonon decay rate, resulting in an increased radiative emission rate [15,19,21]. Our results indicate that the LLASOF-containing LaF3 nanocrystals is superior to LCASOF containing Pb(Cd)F2 nanocrystals for Tb-activated scintillating applications. The LaF3 crystal has a high solid solubility of rare-earth ions, owing to the similar crystal structure and the same valence as La3+. We successfully fabricated 4 mol% TbF3doped LLASOF glass with excellent quality. In contrast, the Tbdoping concentration in LCASOF is restricted to less than 1 mol% due to spontaneous devitrification. In addition, it has been
reported that the lead ions create nonbridging oxygen defects in the glass matrix [25,26]. These nonbridging oxygen defects in LCASOF are a strong source of charge traps, and may be responsible for the reduced UV- and beta-excited luminescence intensities we measure. The broad background emission shown in Fig. 4(a) is an evidence of the defects in glass-matrix. The observed luminescence from Tb-doped LLASOF is much stronger than that from Tb-doped LCASOF. There are three stages in a scintillating process: (1) the absorption of the incident radiation or particle by the host and conversion of the energy into thermalized electrons and holes, (2) transfer of some fraction of the excited electrons and holes to luminescence centers, and (3) the luminescence process [2]. The efficiency of a scintillating process is determined by the product of the efficiencies of each of these three stages. Compared to fully crystalline materials, glasses are expected to be less efficient in step (2), since more defects are present. Meanwhile, step (1) is degraded in materials with high phonon frequencies, but is not dependent on the crystallinity of the scintillator, and step (3) is principally determined by the quantum yield of Tb3+, which is high and not very hostdependent. UV excitation at 325 nm excites the LLASOF host above bandgap, because it exhibits significant absorption at this wavelength as shown in Fig. 3. Similarly, beta-particles or other high-energy excitation generate carriers that relax to the band-edge. Since both methods excite carriers above the band-edge, the relative light yields for PL and for radioluminescence are expected to agree. After above band-gap excitation, thermalized electrons and holes trap at Tb ions, with electrons being far more mobile than holes. Thus, electrons may trap and detrap multiple times in transit to a Tb4+ site where a hole has already trapped and the desired Tb3+* state may form and luminesce. This makes electron traps in scintillators predominantly detrimental to the light yield. The efficiency of radioluminescence is significantly higher in LLASOF host than in LCASOF host, likely because the number of nonbridging oxygen defects is expected to be much smaller in LLASOF than that in LCASOF. In addition to choosing the LLASOF glass host, thought to possess less intrinsic traps, another way to overcome the presence of non-radiative electron traps is to increase the concentration of emissive Tb ions, and indeed we observe the light yield is higher for the 4.0 mol% TbF3 samples compared to the 2% doped samples. As expected, the observed radioluminescence is consistent with the UV-excited luminescence; both show a similar Tb-concentration effect and heattreatment effect. The light yield for the best samples is comparable to that of Schott IQI-301 product under the same experimental conditions.
5. Conclusion New terbium-activated lithium–lanthanum–aluminosilicate oxyfluoride scintillating glasses and glass-ceramics have been fabricated and investigated. The inclusion of LaF3 in the glassmatrix is beneficial for a higher Tb-doping concentration and a higher light yield. We found that the terbium-activated lithium–lanthanum–aluminosilicate oxyfluoride glass exhibits good UV-excited photoluminescence and radioluminescence. The luminescence yield increases for glass-ceramics, exhibiting light yields comparable to that of the Schott IQI-301 product. This class of terbium-activated lithium–lanthanum–aluminosilicate oxyfluoride scintillating glasses and glass-ceramics is therefore promising scintillating materials for low-cost, large-volume production, and possibly also for the fabrication of scintillating fibers.
ARTICLE IN PRESS Z. Pan et al. / Nuclear Instruments and Methods in Physics Research A 594 (2008) 215–219
Acknowledgements This research is supported by the US National Science Foundation NSF-CREST-CA: HRD-0420516, NSF-STC CLiPS-Grant no. 0423914, and US Department of Defense (DOD)/ARO contracts: W911NF-05-1-0453 and W911NF-04-1-0400. References [1] S. Baccaro, A. Cecilia, A. Cemmi, G. Chen, E. Mihokova, N. Nikl, IEEE Trans. Nucl. Sci. NS-48 (2001) 360. [2] M.J. Weber, J. Lumin. 100 (2002) 35. [3] A.D. Bross, Nucl. Instr. and Meth. A 247 (1986) 319. [4] R.J. Ginther, J.H. Schulman, IRE Trans. Nucl. Sci. NS-5 (1958) 923. [5] A.R. Spowart, Nucl. Instr. and Meth. 135 (1976) 441. [6] A.R. Spowart, Nucl. Instr. and Meth. 140 (1977) 19. [7] E.J. Fairley, A.R. Spowart, Nucl. Instr. and Meth. 150 (1978) 159. [8] G.B. Spector, T. McCollum, A.R. Spowart, Nucl. Instr. and Meth. A 326 (1993) 526. [9] P. Pavan, G. Zanella, R. Zannoni, Nucl. Instr. and Meth. B 61 (1991) 487. [10] G. Zanella, R. Zannoni, R. Dall’Igna, P. Polato, M. Bettinelli, Nucl. Instr. and Meth. A 247 (1995) 547.
219
[11] S. Baccaro, R. Dall’Igna, P. Fabeni, M. Martini, J.A. Mares, F. Meinardi, M. Nikl, K. Nitsch, G.P. Pazzi, P. Polato, C. Susini, A. Vedda, G. Zanella, R. Zannoni, J. Lumin. 87–89 (2000) 673. [12] J.A. Mares, M. Nikl, K. Nitsch, N. Solovieva, A. Krasnikov, S. Zazubovich, J. Lumin. 87–89 (2001) 321. [13] J. Fu, M. Kobayashi, J.M. Parker, J. Lumin. 128 (2008) 99. [14] M. Bliss, R.A. Craig, P.L. Reeder, Nucl. Instr. and Meth. A 342 (1994) 357. [15] Z. Pan, A. Ueda, S.H. Morgan, R. Mu, J. Rare Earths 24 (2006) 699. [16] M.J. Dejneka, J. Non-Cryst. Solids 239 (1998) 149. [17] S. Tanabe, H. Hayashi, T. Hanada, N. Onodera, Opt. Mater. 19 (2002) 343. [18] Z. Hu, Y. Wang, F. Bao, W. Luo, J. Non-Cryst. Solids 351 (2005) 722. [19] Z. Pan, A. Ueda, M. Hays, R. Mu, S.H. Morgan, J. Non-Cryst. Solids 352 (2006) 801. [20] E. Ma, Z. Hu, Y. Wang, F. Bao, J. Lumin. 118 (2006) 131. [21] Z. Pan, A. Ueda, R. Mu, S.H. Morgan, J. Lumin. 126 (2007) 251. [22] A.C. Yanes, J. Del-Castillo, J. Me´ndez-Ramos, V.D. Rodrı´guez, M.E. Torres, J. Arbiol, Opt. Mater. 29 (2007) 999. [23] G.S. Yi, G.M. Chow, J. Mater. Chem. 15 (2005) 4460. [24] IQI-301 is a scintillating glass originally produced by Industrial Quality, Inc., of Gaithersburg, MD, and subsequently licensed to other companies, including Schott Glass. [25] Z. Pan, S.H. Morgan, D.O. Henderson, J. Chem. Phys. 101 (1994) 1767. [26] Z. Pan, B.H. Long, S.H. Morgan, J. Non-Cryst. Solids 185 (1995) 127.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Terbium-activated lithium–lanthanum–aluminosilicate oxyfluoride scintillating glass and glass-ceramic Z. Pan a,, K. James a, Y. Cui a, A. Burger a, N. Cherepy b, S.A. Payne b, R. Mu a, S.H. Morgan a a b
Physics Department, Fisk University, Nashville, TN 37208, USA Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
a r t i c l e in fo
abstract
Article history: Received 5 May 2008 Received in revised form 27 June 2008 Accepted 28 June 2008 Available online 3 July 2008
Terbium-activated lithium–lanthanum–aluminosilicate oxyfluoride scintillating glasses, 55SiO2 6Al2O3 28Li2O 11LaF3 doped with different TbF3 concentrations, have been fabricated and investigated. By appropriate heat treatment of the as-prepared glasses above, transparent glass-ceramics were obtained. Differential scanning calorimetry, X-ray diffraction, optical absorption, and luminescence under both UV and beta-particle excitation have been investigated on as-prepared glasses and glassceramics. It has been found that these terbium-activated lithium–lanthanum–aluminosilicate oxyfluoride scintillating glasses exhibit good UV-excited luminescence and radioluminescence. The luminescence yield increases for glass-ceramics. The efficiency of beta-induced luminescence is comparable or nearly equal to that of the Schott IQI-301 product. & 2008 Elsevier B.V. All rights reserved.
Keywords: Scintillating Terbium-activated Luminescence Glass Glass-ceramic
1. Introduction Glass is an attractive scintillating material because of its lowcost, large-volume production possibility, and easy shaping of elements [1–3]. Glass, however, is a disordered material with significant point defects acting as traps which limit the free carrier transfer from the host matrix to the luminescence centers and provide additional multiphonon decay channels [1–3]. The pioneering work on scintillating glasses was performed during the late 1950s and early 1960s [4]. Cerium-activated lithium–aluminosilicate glasses were studied by Spowart et al. for use in neutron detectors [5–7], and during the 1990s, new terbium-activated lithium–aluminosilicate glasses were reported [8]. Terbiumdoped glass is considerably slower in response than its cerium-doped counterpart, but does not demonstrate the self-quenching phenomenon that occurs in cerium-doped glass when Ce3+ is oxidized to Ce4+ [8], probably because the oxidation of Tb3+ to Tb4+ is less thermodynamically favored. The radiation hardness of terbium scintillating glass is about 2–3 times better than cerium glass [9]. The maximum emission of Tb3+ ions is around 542 nm, convenient for a direct coupling with silicon detectors. Terbium-doped glass therefore is useful for thermal neutron detection and for applications in radiography [10–13]. It has been demonstrated that the constituent 6Li
Corresponding author. Tel.: +1 615 329 8622; fax: +1 615 329 8634.
E-mail address: zpan@fisk.edu (Z. Pan). 0168-9002/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.06.041
provides high neutron absorption and the aluminum oxide reduces the number of nonbridging oxygen defects in lithium– aluminosilicate glass matrix [14]. Oxyfluoride glass-ceramics have been developed to combine the particular optical properties of rare-earth ions in a nanocrystalline fluoride host with the chemical stability and mechanical property of oxide glasses [15]. The oxide glasses generally possess good mechanical strength, chemical durability, and thermal stability while the fluorides possess low phonon energy, resulting in an increased radiative emission rate of the incorporated rareearth ions [15]. Silicate oxyfluoride glass-ceramics containing Pb(Cd)F2 or LaF3 nanocrystals have been reported since 1990s [15–23]. Special attention has been paid to glass-ceramics containing LaF3 nanocrystals since Dejneka reported a new aluminosilicate glass-ceramic containing LaF3 in 1998 [16]. LaF3 is an excellent host material for rare-earth ions because it has a high solubility for rare-earth ions, low phonon energy (300–400 cm1), and desirable thermal and environmental stability [20,22]. In this work, terbium-doped lithium–lanthanum–aluminosilicate oxyfluoride glasses were successfully fabricated, and the corresponding glass-ceramics were obtained by appropriate heat treatment of the as-prepared glasses. X-ray diffraction (XRD), differential scanning calorimetry (DSC), optical absorption, and luminescence under both UV and beta-particle excitation have been investigated on as-prepared glasses and glass-ceramics. The beta-induced luminescence was compared with a standard scintillating glass Schott IQI-301 product [24].
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**
**
(b)
*
(a)
20
25
30
35 40 45 50 Diffraction angle, 2θ (deg.)
55
60
Fig. 2. XRD of LLASOF doped with 4.0 at% Tb: (a) as-prepared and (b) heat treated at 560 1C for 6 h.
100
Transmittance (%)
Reagent grade anhydrous oxide powders of SiO2, Li2CO3, Al2O3, LaF3, and TbF3 were used to prepare glasses. The compositions of lithium–lanthanum–aluminosilicate oxyfluoride glass were 55SiO2 6Al2O3 28Li2O 11LaF2 (LLASOF). Three batches containing different terbium concentrations, 0.5, 2.0, and 4 mol% of TbF3, were prepared. Batches of 40 g were thoroughly mixed in an agate mortar, and melted in a platinum crucible at a temperature of 1400 1C. The melts were held for 50 min and then cast onto a copper plate and pressed by another copper plate from the top, forming a glass plate of about 3 mm in thickness. The glasses were subsequently annealed at near the glass transition temperatures for 20 min and then allowed to cool to room temperature in the furnace. Clear glasses were formed for all batches with different terbium concentrations. These glasses appear to be of very good optical quality, with no visual evidence of devitrification. DSC measurements were carried out using SDT 2960 from TA Instruments, under N2 atmosphere at a constant heating rate 20 1C/min. XRD measurements were performed using a Scintag X1 advanced diffraction system with a Cu Ka radiation (l ¼ 0.154 nm). UV–visible absorption was measured from 300 to 800 nm using a Cary dual beam spectrophotometer. Photoluminescence (PL) was measured using 325-nm laser excitation wavelength. An HeCd laser (KIMMON model IK320R-D) and an Oriel monochromator MS257TM with a TE cooled CCD detector were used. The incident laser power used is 5 mW. The laser light was focused into a dot about 50 mm in diameter on the sample surface. The luminescence was measured at room temperature. Radioluminescence spectra were acquired using a 90Sr/90Y source (average beta-energy 1 MeV) to provide a spectrum expected to be essentially equivalent to that produced by gamma excitation. Radioluminescence spectra were collected with a Princeton Instruments/Acton Spec 10 spectrograph coupled to a thermoelectrically cooled CCD camera.
Intensity (arb.unit)
2. Experiment
80
(a)
60 (b)
40 20 0 280
330
380
430
480
Wavelength (nm) Fig. 3. Optical transmittance spectra of LLASOF glasses doped with (a) 2.0 at% Tb and (b) 4.0 at% Tb.
3. Results 3.1. DSC, XRD results, and heat treatment
Heat Flow (mW)
Fig. 1 shows the DSC curve of an as-prepared LLASOF glass doped with 4 mol% of TbF3. The DSC curve showed a glass transition temperature Tg at 406 1C, and several exothermal peaks centered at 540, 667, and 711 1C, respectively. These exothermal peaks indicate different crystallization phases in glass-ceramic. The XRD measurements were performed on as-prepared glass and
Tg
Tc2
Tc3
Tc1
its corresponding glass-ceramic. In Fig. 2(a), the XRD pattern for as-prepared glass shows no diffraction peaks, indicating its amorphous structure in nature. In Fig. 2(b), sharp diffraction peaks appeared for the sample heat treated at 560 1C for 6 h. The marked peaks are matched with the diffraction peaks of crystalline LaF3 reported previously [17–20,22,23]. Additional diffraction peaks could indicate the coexistence of other crystalline phases in the glass-ceramic. Samples were heat treated for 6 h at two temperatures between the glass transition temperature and the first crystallization temperature. Samples doped with 2 mol% TbF3 were treated at 520 1C (T1) and 540 1C (T2); and samples doped with 4 mol% TbF3 were treated at 460 1C (T1) and 480 1C (T2). Samples heat treated at T1 show a slight reduction in optical transparency, while samples heat treated at T2 become milky and semitransparent. 3.2. Absorption and UV-excited PL
0
200
400 600 Temperature (C)
Fig. 1. DSC curve of LLASOF glass doped with 4.0 at% Tb.
800
Samples of 15 10 2 mm3 (L W T) with two parallel surfaces polished were used for optical measurements. The transmittance spectra were shown in Fig. 3(a) and (b) for the as-prepared glasses doped with 2 and 4 mol% TbF3, respectively. The transmittance spectra show absorption bands of Tb3+ ions centered at 350, 367, 376, 483 nm and a UV absorption edge at about 325 nm (T ¼ 50%) from the host glass.
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3.3. . Beta-induced luminescence The beta-induced luminescence was measured at Lawrence Livermore National Laboratory and compared with the Schott IQI-301 product under the same experimental conditions. Fig. 7 shows the spectra of LLASOF glass and glass-ceramics doped with 2.0 mol% TbF3 and Fig. 8 shows the spectra of LLASOF glass and glass-ceramics doped with 4.0 mol% TbF3. These spectra indicate the following: (1) the radioluminescence intensity increases with Tb concentration from 2% to 4%; (2) the intensity increases with
Intensity (Arb.unit)
(c)
300
(b)
(a)
350
400
450 500 550 Wavelength (nm)
600
650
700
Fig. 5. UV-excited PL spectra of LLASOF doped with 2.0 at% Tb: (a) as-prepared, (b) 520 1C for 6 h, and (c) 540 1C for 6 h.
(c)
Intensity (Arb.Unit)
The room temperature PL was measured using 325-nm laser excitation. Fig. 4(a) shows a PL spectrum from a Tb-doped lead–cadmium–aluminosilicate oxyfluoride glass LCASOF (30SiO2 7.5Al2O3 22PbF2 29CdF2 3.5YF3 0.5TbF3) [15] for comparison and Fig. 4(b) shows PL spectra of three as-prepared LLASOF samples doped with 0.5, 2.0, and 4.0 mol% of TbF3, respectively. The excitation laser power used is 5 mW for LLASOF and 15 mW for LCASOF because the luminescence from LCASOF is very weak. The four major emission bands at 489, 542, 585, and 622 nm are attributed to transitions of 5D4-7Fi (i ¼ 6, 5, 4, and 3, respectively) of Tb3+ ions [1]. The dominant band is at 542 nm. The PL intensity is almost proportional to the Tb-doping concentration as shown Fig. 4(b), indicating no significant self-quenching effect. The background in spectra Fig. 4(b) is clean, indicating no significant luminescence from defects in LLASOF glass host. However, the PL spectrum (Fig. 4(a)) from a Tb-doped LCASOF glass shows a broad background emission extending from 360 to 650 nm, in addition to the weaker Tb3+ emission. This broad background emission is attributed to defects in glass host. Fig. 5 shows the heat-treatment effect on UV-excited PL spectra of LLASOF glass doped with 2.0 mol% TbF3. The PL intensity increases with increasing heat-treatment temperature. The glass-ceramics showed a higher luminescence yield compared to that of the as-prepared glass. The intensity increased 20% for the sample treated at T1 and 45% for the sample treated at T2. Spectra in Fig. 6 showed a similar heat-treatment effect on UV-excited PL of LLASOF doped with 4.0 mol% TbF3. The intensity increased 3% for the sample treated at T1 and 70% for the sample treated at T2.
217
300
(b)
(a)
350
400
450 500 550 Wavelength (nm)
600
650
700
Fig. 6. UV-excited PL spectra of LLASOF doped with 4.0 at% Tb: (a) as-prepared, (b) 460 1C for 6 h, and (c) 480 1C for 6 h.
Intensity (Arb.unit)
(a)
(d)
Intensity (Arb.unit)
(b) 4.0 % Tb
2.0 % Tb
(c) (b) (a)
0.5 % Tb
300
350
400
500 450 550 Wavelength (nm)
600
650
700
Fig. 4. UV-excited PL spectra of (a) LCASOF glass doped with 0.5 at% Tb and (b) LLASOF glasses doped with 0.5 at%, 2.0 at%, and 4.0 at% Tb, respectively. The excitation is at 325 nm with a power of 15 mW for (a) and 5 mW for (b).
400
450
500 550 600 Wavelength (nm)
650
700
Fig. 7. Beta-induced luminescence of LLASOF doped with 2.0 at% Tb, compared with IQI-301: (a) as-prepared, (b) 460 1C for 6 h, (c) 480 1C for 6 h, and (d) IQI-301.
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Z. Pan et al. / Nuclear Instruments and Methods in Physics Research A 594 (2008) 215–219
Intensity (Arb.unit)
(d)
400
(c)
(b) (a)
450
500 550 600 Wavelength (nm)
650
700
Fig. 8. Beta-induced luminescence of LLASOF doped with 4.0 at% Tb, compared with IQI-301: (a) as-prepared, (b) 460 1C for 6 h, (c) 480 1C for 6 h, and (d) IQI-301.
Table 1 The relative efficiency of beta-induced luminescence of 4 mol% TbF3-doped lithium–lanthanum–aluminosilicate oxyfluoride scintillating glass and glassceramics compared with standard Schott IQI-301 product Beta-induced luminescence efficiency IQI-301 1.0
Glass 0.47
Glass-ceramic #1 0.52
Glass-ceramic #2 0.71
heat treatment; (3) the radioluminescence light yield for the LLASOF samples is comparable to that of Schott IQI-301 product. We also compared the beta-induced luminescence of Tb-doped LLASOF samples with a Tb-doped LCASOF glass; the radioluminescence intensity is more than one order of magnitude higher for Tb-doped LLASOF. Table 1 lists the values of relative efficiency of beta-induced luminescence of 4 mol% TbF3-doped LLASOF compared with standard Schott IQI-301 product, the highest value is 0.71. Considering that the wt% of the 4 mol% Tb-doped sample is 7.8 wt% while the Tb concentration of IQI-301 is 11.2 wt%, the obtained beta-induced luminescence yield from LLASOF glass and ceramics is encouraging [24].
4. Discussion The XRD pattern (Fig. 2) obtained from the heat-treated sample indicates that the glass-ceramic contains LaF3 and other nanocrystals. The first exothermal band in the DSC curve (Fig. 1) indicates the crystallization temperature of LaF3 nanocrystals [18,20]. The increased luminescence efficiency from the glassceramic compared to that of the as-prepared glass may be attributed to the formation of LaF3:Tb nanocrystals [17,22]. The LaF3 crystal has a very low phonon energy (300–400 cm1), compared to that of silicate-based host. The excited Tb3+ ions therefore have a reduced multiphonon decay rate, resulting in an increased radiative emission rate [15,19,21]. Our results indicate that the LLASOF-containing LaF3 nanocrystals is superior to LCASOF containing Pb(Cd)F2 nanocrystals for Tb-activated scintillating applications. The LaF3 crystal has a high solid solubility of rare-earth ions, owing to the similar crystal structure and the same valence as La3+. We successfully fabricated 4 mol% TbF3doped LLASOF glass with excellent quality. In contrast, the Tbdoping concentration in LCASOF is restricted to less than 1 mol% due to spontaneous devitrification. In addition, it has been
reported that the lead ions create nonbridging oxygen defects in the glass matrix [25,26]. These nonbridging oxygen defects in LCASOF are a strong source of charge traps, and may be responsible for the reduced UV- and beta-excited luminescence intensities we measure. The broad background emission shown in Fig. 4(a) is an evidence of the defects in glass-matrix. The observed luminescence from Tb-doped LLASOF is much stronger than that from Tb-doped LCASOF. There are three stages in a scintillating process: (1) the absorption of the incident radiation or particle by the host and conversion of the energy into thermalized electrons and holes, (2) transfer of some fraction of the excited electrons and holes to luminescence centers, and (3) the luminescence process [2]. The efficiency of a scintillating process is determined by the product of the efficiencies of each of these three stages. Compared to fully crystalline materials, glasses are expected to be less efficient in step (2), since more defects are present. Meanwhile, step (1) is degraded in materials with high phonon frequencies, but is not dependent on the crystallinity of the scintillator, and step (3) is principally determined by the quantum yield of Tb3+, which is high and not very hostdependent. UV excitation at 325 nm excites the LLASOF host above bandgap, because it exhibits significant absorption at this wavelength as shown in Fig. 3. Similarly, beta-particles or other high-energy excitation generate carriers that relax to the band-edge. Since both methods excite carriers above the band-edge, the relative light yields for PL and for radioluminescence are expected to agree. After above band-gap excitation, thermalized electrons and holes trap at Tb ions, with electrons being far more mobile than holes. Thus, electrons may trap and detrap multiple times in transit to a Tb4+ site where a hole has already trapped and the desired Tb3+* state may form and luminesce. This makes electron traps in scintillators predominantly detrimental to the light yield. The efficiency of radioluminescence is significantly higher in LLASOF host than in LCASOF host, likely because the number of nonbridging oxygen defects is expected to be much smaller in LLASOF than that in LCASOF. In addition to choosing the LLASOF glass host, thought to possess less intrinsic traps, another way to overcome the presence of non-radiative electron traps is to increase the concentration of emissive Tb ions, and indeed we observe the light yield is higher for the 4.0 mol% TbF3 samples compared to the 2% doped samples. As expected, the observed radioluminescence is consistent with the UV-excited luminescence; both show a similar Tb-concentration effect and heattreatment effect. The light yield for the best samples is comparable to that of Schott IQI-301 product under the same experimental conditions.
5. Conclusion New terbium-activated lithium–lanthanum–aluminosilicate oxyfluoride scintillating glasses and glass-ceramics have been fabricated and investigated. The inclusion of LaF3 in the glassmatrix is beneficial for a higher Tb-doping concentration and a higher light yield. We found that the terbium-activated lithium–lanthanum–aluminosilicate oxyfluoride glass exhibits good UV-excited photoluminescence and radioluminescence. The luminescence yield increases for glass-ceramics, exhibiting light yields comparable to that of the Schott IQI-301 product. This class of terbium-activated lithium–lanthanum–aluminosilicate oxyfluoride scintillating glasses and glass-ceramics is therefore promising scintillating materials for low-cost, large-volume production, and possibly also for the fabrication of scintillating fibers.
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Acknowledgements This research is supported by the US National Science Foundation NSF-CREST-CA: HRD-0420516, NSF-STC CLiPS-Grant no. 0423914, and US Department of Defense (DOD)/ARO contracts: W911NF-05-1-0453 and W911NF-04-1-0400. References [1] S. Baccaro, A. Cecilia, A. Cemmi, G. Chen, E. Mihokova, N. Nikl, IEEE Trans. Nucl. Sci. NS-48 (2001) 360. [2] M.J. Weber, J. Lumin. 100 (2002) 35. [3] A.D. Bross, Nucl. Instr. and Meth. A 247 (1986) 319. [4] R.J. Ginther, J.H. Schulman, IRE Trans. Nucl. Sci. NS-5 (1958) 923. [5] A.R. Spowart, Nucl. Instr. and Meth. 135 (1976) 441. [6] A.R. Spowart, Nucl. Instr. and Meth. 140 (1977) 19. [7] E.J. Fairley, A.R. Spowart, Nucl. Instr. and Meth. 150 (1978) 159. [8] G.B. Spector, T. McCollum, A.R. Spowart, Nucl. Instr. and Meth. A 326 (1993) 526. [9] P. Pavan, G. Zanella, R. Zannoni, Nucl. Instr. and Meth. B 61 (1991) 487. [10] G. Zanella, R. Zannoni, R. Dall’Igna, P. Polato, M. Bettinelli, Nucl. Instr. and Meth. A 247 (1995) 547.
219
[11] S. Baccaro, R. Dall’Igna, P. Fabeni, M. Martini, J.A. Mares, F. Meinardi, M. Nikl, K. Nitsch, G.P. Pazzi, P. Polato, C. Susini, A. Vedda, G. Zanella, R. Zannoni, J. Lumin. 87–89 (2000) 673. [12] J.A. Mares, M. Nikl, K. Nitsch, N. Solovieva, A. Krasnikov, S. Zazubovich, J. Lumin. 87–89 (2001) 321. [13] J. Fu, M. Kobayashi, J.M. Parker, J. Lumin. 128 (2008) 99. [14] M. Bliss, R.A. Craig, P.L. Reeder, Nucl. Instr. and Meth. A 342 (1994) 357. [15] Z. Pan, A. Ueda, S.H. Morgan, R. Mu, J. Rare Earths 24 (2006) 699. [16] M.J. Dejneka, J. Non-Cryst. Solids 239 (1998) 149. [17] S. Tanabe, H. Hayashi, T. Hanada, N. Onodera, Opt. Mater. 19 (2002) 343. [18] Z. Hu, Y. Wang, F. Bao, W. Luo, J. Non-Cryst. Solids 351 (2005) 722. [19] Z. Pan, A. Ueda, M. Hays, R. Mu, S.H. Morgan, J. Non-Cryst. Solids 352 (2006) 801. [20] E. Ma, Z. Hu, Y. Wang, F. Bao, J. Lumin. 118 (2006) 131. [21] Z. Pan, A. Ueda, R. Mu, S.H. Morgan, J. Lumin. 126 (2007) 251. [22] A.C. Yanes, J. Del-Castillo, J. Me´ndez-Ramos, V.D. Rodrı´guez, M.E. Torres, J. Arbiol, Opt. Mater. 29 (2007) 999. [23] G.S. Yi, G.M. Chow, J. Mater. Chem. 15 (2005) 4460. [24] IQI-301 is a scintillating glass originally produced by Industrial Quality, Inc., of Gaithersburg, MD, and subsequently licensed to other companies, including Schott Glass. [25] Z. Pan, S.H. Morgan, D.O. Henderson, J. Chem. Phys. 101 (1994) 1767. [26] Z. Pan, B.H. Long, S.H. Morgan, J. Non-Cryst. Solids 185 (1995) 127.