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0022-2313/$ - se

doi:10.1016/j.jlu

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Journal of Luminescence 128 (2008) 99–104

www.elsevier.com/locate/jlumin

Terbium-activated heavy scintillating glasses

Jie Fua,�, Masaaki Kobayashib, John M. Parkerc

aR&D Department, Ohara Inc., 1-15-30 Oyama, Sagamihara-shi, Kanagawa 229-1186, JapanbHigh Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan

cDepartment of Engineering Materials, The University of Sheffield, Mappin Street, Sheffield S1 3JD, UK

Received 19 February 2007; received in revised form 7 May 2007; accepted 25 May 2007

Available online 2 June 2007

Abstract

Tb-activated scintillating glasses with Ln2O3 (Ln ¼ Y, Gd, Lu) concentrations up to 40mol% have been prepared. The effects of Ln3+

ions on the density, thermal properties, transmission, and luminescence properties under both UV and X-ray excitation have been

investigated. Glasses containing Gd2O3 or Lu2O3 exhibit a density of more than 6.0 g/cm3. Energy transfer from Gd3+ to Tb3+ takes

place in Gd-containing glass and as a result the Gd-containing glass shows a light yield 2.5 times higher than the Y- or Lu-containing

glass. The effect of the substitution of fluorine for oxygen on the optical properties was also investigated.

r 2007 Elsevier B.V. All rights reserved.

Keywords: Terbium ions; Scintillating; High density; Glasses

1. Introduction

Glasses activated with rare-earth ions are promisingalternatives to single crystals and ceramic scintillators forapplications in high-energy physics, in X-ray CT forindustrial and medical imaging. The main advantages ofglassy scintillators lie in the possibility of low-cost produc-tion and in the ease of manufacture in different sizes andshapes, such as fibers. Among the properties required formost of the applications, e.g., for high-energy physics andX-ray CT, the density of the scintillators is particularlyimportant because high density results in an increased X-rayabsorption cross-section, thus significantly increasing theimage signal-to-noise ratio. Ce3+-activated heavy metalfluorohafnate glasses have high densities around 6 g/cm3 andwere suggested as promising candidates for use in electro-magnetic calorimeters for particle physics applications [1].However, these glasses exhibited relatively low light yieldand poor radiation hardness. Compared with fluorideglasses, oxide glasses have a relatively high light yield andare easier to manufacture. The disadvantage of existingoxide glass scintillators is their low density below 4 g/cm3

[2,3], which limits their applications. The introduction of

e front matter r 2007 Elsevier B.V. All rights reserved.

min.2007.05.006

ing author. Tel.: +8142 772 2101; fax: +81 42 772 2280.

ess: fu@ohara-inc.co.jp (J. Fu).

heavy components such as PbO, Bi2O3 allows the density ofthe glasses to be easily increased to more than 6.0 g/cm3,which is desirable for most applications, but is usuallyaccompanied by a dramatic decrease in the luminescenceresponse of rare-earth ions.Introduction of heavy metal oxides such as Y2O3, Gd2O3,

and Lu2O3 can also be expected to increase the density of theglasses. Moreover, these oxides are the main host compo-nents of some commercial scintillators such as Gd2SiO5:Ce,Lu2SiO5:Ce single crystals, and (Y, Gd)2O3:Eu, Gd2O2S:Tbceramics [4–7]. However, to the present authors’ knowledge,the effects of these oxides at concentrations of more than30mol% in glasses on the scintillating properties have notbeen reported. In this work, we report on terbium-activateddense scintillating glasses that contain concentrations ofLn2O3 (Ln ¼ Gd, Y, Lu) upto 40mol%. The effects of Ln3+

ions on the scintillating properties under X-ray excitationhave also been investigated. Furthermore, fluorine has beenintroduced into the glasses and its effect on scintillatingproperties has been studied.

2. Experimental

Glasses with mole compositions 15SiO2–25B2O3–5P2O5–15Ga2O3–38Ln2O3–2Tb2O3 (Ln ¼ Gd, Y, Lu) and

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755

765

775

785

0.82 0.86 0.9 0.94

Ionic Radius (Å)

Tg (

°C)

120

140

160

180

200

Tx-

Tg (

°C)

Gd

Y

Lu

Fig. 1. Tg and (Tx�Tg) of 15SiO2–25B2O3–5P2O5–15Ga2O3–38Ln2O3–

2Tb2O3 (Ln ¼ Y, Gd, Lu) glasses.

J. Fu et al. / Journal of Luminescence 128 (2008) 99–104100

15SiO2–25B2O3–5P2O5–15Ga2O3–28Ln2O3–10GdF3–2Tb2O3

were prepared using the following procedure. Reagent-gradeSiO2, H3BO3, NH4H2PO4, Ga2O3, Gd2O3, Y2O3, Lu2O3,and GdF3 were used as starting materials. Fifty gramsbatches were placed in platinum crucibles and melted underan air atmosphere at 1520–1550 1C for 3h. The melts werepoured onto a preheated stainless-steel plate and pressed intoplates of 4mm thickness. The glasses were cut and polished to2mm thickness for various spectroscopic measurements.Density was measured by Archimedes’ method using distilledwater as an immersion liquid. Glass transition (Tg) andcrystallization (Tx) temperatures were obtained using differ-ential thermal analysis (DTA) at a heating rate of 10 1C/min.Ultraviolet/visible transmission spectra were recorded on adual-beam scanning spectrometer (Hitachi U-4000). Lumi-nescence under UV excitation using Xe lamp as excitationsource was recorded by a JASCO LP-750 spectrometer. Theradioluminescence (RL) spectra were measured for glassesand BGO crystal by using an X-ray generator (50kV, 30mA,Cu target) and a spectrometer (Hamamatsu, PhotonicMultichannel Analyzer PMA-11). The PMA-11 consists ofa grating monochromator, an image intensifier, and a linearCCD array, and is calibrated in the wavelength range of200–860nm. The RL was taken via a quartz optical fiberfrom the injected surface of the samples at 901 from the X-raydirection so that the sample thickness (transmission) shouldnot be important. The RL was integrated offline between 250and 750nm for glasses and between 200 and 700nm for BGOso that all the emission peaks can be covered. Relative lightyield of glass samples was estimated by comparing theintegrated RL intensity of the glasses with that of BGOcrystal. All the above measurements were performed at roomtemperature.

3. Results

3.1. Effects of Ln3+ ions on the physical properties

Table 1 lists the densities and refractive indices of the15SiO2–25B2O3–5P2O5–15Ga2O3–38Ln2O3–2Tb2O3 (Ln ¼ Y,Gd, Lu) glasses. As expected, the density increases in theorder of Y2O3oGd2O3oLu2O3. It is worth noting that theglasses containing Gd2O3 or Lu2O3 exhibit a density ofmore than 6.0 g/cm3, making them potential candidates forpractical applications in high-energy physics and medicine.The refractive index (nD) of these glasses is high, up to 1.83for Gd-containing glasses.

Table 1

Density and refractive index of 15SiO2–25B2O3–5P2O5–15Ga2O3–

38Ln2O3–2Tb2O3 glasses (Ln ¼ Y, Gd, Lu)

Ln Density (g/cm3) Refractive index (nD)

Y 4.52 1.793

Gd 5.96 1.832

Lu 6.56 1.796

The thermal properties of the above glasses based onDTA results are shown in Fig. 1. Tg decreases from 780 to760 1C and (Tx�Tg) decreases from 195 to 140 1C as theionic radius of the Ln3+ ions increases. The value of(Tx�Tg) is often used to judge the stability of a glassagainst crystallization, the higher the difference, the morestable the glass. The glass containing Lu exhibits thehighest stability but even the least stable glass can bemanufactured as 4-mm-thick samples.

3.2. Effects of Ln3+ ions on the optical properties

(Ln ¼ Y, Gd, Lu)

Fig. 2 shows the transmission spectra of 15SiO2–25B2O3–5P2O5–15Ga2O3–38Ln2O3–2Tb2O3 (Ln ¼ Y, Gd, Lu)glasses together with those of the Tb-free glasses forcomparison. Two weak absorption bands exist at 375 and480 nm in all Tb-doped glasses with an extra band at313 nm in Gd-containing glasses. The bands are due to f–ftransitions, the former two for Tb3+ and the latter onefor Gd3+. The Tb-doped glasses show almost the sametransmission regardless of the kind of Ln3+ ions, but theyexhibit a broad absorption band ranging from theabsorption edges to 580 nm in comparison with Tb-freeglasses, indicating that the broad absorption originatesfrom terbium ions.Fig. 3 shows the luminescence spectra of 15SiO2–

25B2O3–5P2O5–15Ga2O3–38Ln2O3–2Tb2O3 (Ln ¼ Y, Gd,Lu) glasses when excited with 276 nm light. All observedemission peaks are attributable to the 5D4–

7FJ transition ofTb3+. The Gd-containing glass has a luminescent responseat least twice that for the Y- or Lu-containing samples.Almost the same results are observed under X-rayexcitation as shown in Fig. 4. From the emission peaks,

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0

20

40

60

80

100

200 300 400 500 600 700

Wavelength (nm)

Tra

nsm

issi

on (

%)

Gd

YLu

(a): Tb-free(b): Tb-free

Fig. 2. Transmission spectra of 15SiO2–25B2O3–5P2O5–15Ga2O3–

38Ln2O3–2Tb2O3 (Ln ¼ Y, Gd, Lu) glasses, (a) 15SiO2–25B2O3–5P2O5–

15Ga2O3–40Gd2O3 glass and (b) 15SiO2–25B2O3–5P2O5–15Ga2O3–

40Lu2O3 glass. Thickness of all the glasses is 2mm.

0

200

400

600

450 500 550 600 650

Wavelength (nm)

Inte

nsi

ty (

a. u.)

Gd

Y

Lu

Fig. 3. Emission spectra for 15SiO2–25B2O3–5P2O5–15Ga2O3–38Ln2O3–

2Tb2O3 (Ln ¼ Y, Gd, Lu) glasses when excited with 276 nm.

0

1000

2000

3000

4000

450 500 550 600 650

Wavelength (nm)

Inte

nsi

ty (

a. u.)

Gd

Y

Lu

Fig. 4. Emission spectra for 15SiO2–25B2O3–5P2O5–15Ga2O3–38Ln2O3–

2Tb2O3 (Ln ¼ Y, Gd, Lu) glasses under X-ray excitation.

0

5

10

15

20

25

30

Gd Y Lu

Rela

tive L

ight Y

ield

(%

)

Fig. 5. Light yield from 15SiO2–25B2O3–5P2O5–15Ga2O3–38Ln2O3–

2Tb2O3 (Ln ¼ Y, Gd, Lu) glasses under X-ray excitation.

J. Fu et al. / Journal of Luminescence 128 (2008) 99–104 101

the light yield was estimated relative to a BGO singlecrystal and the results are shown in Fig. 5. The Gd-containing glass exhibits higher light yield 2.5 times that ofthe Y- or Lu-containing materials. From the decay curvesobtained by monitoring the intensity of the strongestemission peak at 545 nm, the decay time for all the glassesinvestigated above was estimated. Its value changed littlewith the type of Ln3+ and was around 1.8ms. This value is

too long to use for applications in high-energy physics andX-ray CT, but it is applicable in X-ray imaging forindustrial inspection systems [3].Dependence of the light yield on terbium concentra-

tion for the Gd-containing glasses is shown in Fig. 6. Thelight yield increases slightly with increasing terbiumconcentration.Fig. 7 shows the excitation spectra of 15SiO2–25B2O3–

5P2O5–15Ga2O3–38Ln2O3–2Tb2O3 (Ln ¼ Y, Gd, Lu)glasses for the Tb3+ emission at 545 nm. The same

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0

10

20

30

40

0 4

x (mol%)

Rela

tive L

ight Y

ield

(%

)

62

Fig. 6. Dependence of the light yield on terbium concentration for

15SiO2–25B2O3–5P2O5–15Ga2O3–(40�x)Gd2O3–xTb2O3 glasses.

600

400

200

0450400350300250200

Gd

Y

Lu

Gd

Y

Lu

Inte

nsi

ty (

a. u.)

Wavelength (nm)

Fig. 7. Excitation spectra for 15SiO2–25B2O3–5P2O5–15Ga2O3–38Ln2O3–

2Tb2O3 (Ln ¼ Gd, Y, Lu) glasses for the Tb3+ emission at 545 nm.

0

1000

2000

3000

4000

5000

450 550 650

Wavelength (nm)

Inte

nsi

ty (

a. u.)

(a) F - free glass

(b) F - containing glass

Fig. 8. Emission spectra for (a) 15SiO2–25B2O3–5P2O5–15Ga2O3–

38Gd2O3–2Tb2O3 glass and (b) 15SiO2–25B2O3–5P2O5–15Ga2O3–

28Gd2O3–10GdF3–2Tb2O3 glass under X-ray excitation.

J. Fu et al. / Journal of Luminescence 128 (2008) 99–104102

excitation bands are observed for the Y- and Lu-containingglasses. The broad band peaking at 260 nm is due to the4f8–4f75d1 transition of Tb3+ and the remaining bands atlonger wavelength to the 4f–4f transitions of Tb3+. For theGd-containing glass, however, two extra, sharp, strongexcitation bands occur at 276 and 313 nm. These bands canbe attributed to the 8S7/2-

6IJ and8S7/2-

6P7/2 transitionsof the Gd3+ ion, respectively.

3.3. Effect of fluorine on the optical properties

To investigate the effect of fluorine on the properties,10mol% of the Gd2O3 in the 15SiO2–25B2O3–5P2O5–15Ga2O3–38Gd2O3–2Tb2O3 glass was replaced with anequivalent molar concentration of GdF3. Emission spectraunder UV excitation indicated that the introduction offluorine resulted in an increase in emission intensity, and asimilar result was also obtained under X-ray excitation asshown in Fig. 8. Transmission curves before and after theintroduction of fluorine are shown in Fig. 9; the broadabsorption band has clearly weakened significantly.

4. Discussion

Terbium in a glass exists mainly as Tb3+, usuallyaccompanied by a small amount of Tb4+. The ratio ofthese two ions depends on the melting atmosphere and thecompositions of the host. Tb3+ ion fluoresces, but Tb4+

does not. Tb3+ gives several sharp but weak absorptionpeaks due to f–f transition in the visible region as observedin Fig. 2 while Tb4+ has a broad absorption from theultraviolet to visible region peaking at 320–370 nm, due toa charge transfer transition [8]. The observed broadabsorption in the present Tb-doped glasses can therefore

be reasonably assigned to Tb4+ ions (Fig. 2). The similarityof the transmission for Y-, Gd-, and Lu-containing glassesimplies that the Tb3+/Tb4+ ratio is almost equal in theseglasses.

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The improvement of the transmission after the introduc-tion of fluorine indicates that the fluorine-containing glasshas a lower concentration of Tb4+ and hence a higher

0

20

40

60

80

100

200 300 400 500 600 700

Wavelength (nm)

Tra

smis

sion (

%)

(a) F - free glass

(b) F - containing glass

Fig. 9. Transmission spectra for (a) 15SiO2–25B2O3–5P2O5–15Ga2O3–

38Gd2O3–2Tb2O3 glass and (b) 15SiO2–25B2O3–5P2O5–15Ga2O3–

28Gd2O3–10GdF3–2Tb2O3 glass. Thickness of both glasses is 2mm.

0

1000

2000

3000

4000

300 350 400 450

Wavelen

Inte

nsi

ty (

a.u

.)

x=4

x=8

x=16

x=38

Fig. 10. Emission spectra for 15SiO2–25B2O3–5P2O5–15Ga2O3–(3

concentration of Tb3+. This is consistent with theobservation that fluoride ions favor the lower oxidationstate of redox species.The appearance of Gd3+ excitation bands in the

excitation spectra of Tb3+ indicates that energy transferfrom Gd3+ to Tb3+ takes place in Gd2O3-containing glass.Energy transfer must be responsible for the much strongerluminescence response under either UV or X-ray excitationin Gd-containing glass than that in Y- or Lu-containingglasses. To investigate the optimum concentration of Gd3+

ion for energy transfer to Tb3+, Lu2O3 was systematicallysubstituted by Gd2O3 in a series of 15SiO2–25B2O3–5P2O5–15Ga2O3–(38�x)Lu2O3–xGd2O3–2Tb2O3 glasses;the emission spectra under X-ray excitation are shown inFig. 10. In addition to the emission peaks at longerwavelength beyond 350 nm due to Tb3+ ions, an emissionpeak at 313 nm due to the 6P7/2 level-

8S7/2 level transitionin Gd3+ ions is also observed. The intensity of the Tb3+

emission increases with increasing Gd3+ concentration, butthe intensity of the Gd3+ emission decreases significantlyand even disappears at concentrations of more than16mol%. The enhanced emission of Tb3+ ions with theincrease of Gd3+ concentration could be explained by thefollowing energy transfer processes. At low Gd3+ concen-tration, excited Gd3+ ions return to their ground state bytwo routes; one is by transferring its energy to Tb3+ andthe other by radiative decay from the 6P7/2 level to the8S7/2 level. At higher Gd3+ concentrations, however,besides the energy transfer from Gd3+ to Tb3+, energy

500 550 600 650

gth (nm)

8�x)Lu2O3–xGd2O3–2Tb2O3 glasses under X-ray excitation.

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transfer between Gd3+ ions becomes more probable thanthe radiative decay of the Gd3+ ions. Energy migratesamong Gd3+ ions and finally is trapped by Tb3+, henceresulting in a decrease in Gd3+ emission and an increase inTb3+ emission.

Fig. 8 reveals that fluorine is effective in improving thelight yield. This is probably due to the higher Tb3+

concentration in the fluorine-containing glass as discussedabove. The dependence of light yield on the terbiumconcentration (Fig. 6) supports this speculation. In ourprevious paper, we found that the substitution of fluorinefor oxygen in Ce3+-activated scintillating glasses had asimilar effect [9].

Finally, we should point out that the presence of Tb4+

may cause a decrease in Tb3+ emission since Tb4+ couldabsorb a part of energies generated by the X-ray irradiationdue to the charge transfer transition as observed in Fig. 2.

5. Conclusions

Tb-activated scintillating glasses containing high Ln2O3

concentration (Ln ¼ Y, Gd, Lu) have been studied. Theglasses are very heavy, having densities up to 6.0 g/cm3 for

Gd2O3-containing glasses and 6.6 g/cm3 for Lu2O3-con-taining glasses. Due to energy transfer from Gd3+ to Tb3+,Gd-containing glasses show a light yield 2.5 times higherthan for Y- or Lu-containing glasses. Partial substitution offluorine for oxygen is effective in improving the light yield.It is also very interesting to know how other luminescent

rare-earth ions such as cerium and europium ions scintillatein the present heavy host glasses. Studies relating to thesetwo ions are under way.

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