Tb3+-doped borosilicate glass scintillators for high-resolution X-ray imaging

Wenjun Huang; Junyu Chen; Yi Li; Yueyue Wu; Lianjie Li; Liping Chen; Hai Guo

doi:10.3788/COL202321.071601

Abstract

Scintillators are the vital component in X-ray perspective image technology that is applied in medical imaging, industrial nondestructive testing, and safety testing. But the high cost and small size of single-crystal commercialized scintillators limit their practical application. Here, a series of Tb³⁺-doped borosilicate glass (BSG) scintillators with big production size, low cost, and high spatial resolution are designed and fabricated. The structural, photoluminescent, and scintillant properties are systematically investigated. Benefiting from excellent transmittance (87% at 600 nm), high interquantum efficiency (60.7%), and high X-ray excited luminescence (217% of Bi₄Ge₃O₁₂), the optimal sample shows superhigh spatial resolution (exceeding 20 lp/mm). This research suggests that Tb³⁺-doped BSG scintillators have potential applications in the static X-ray imaging field.

Keywords

borosilicate glass high spatial resolution scintillators Tb3+X-ray imaging

1. Introduction

Scintillators can absorb high-energy rays or particles (X-ray or $γ$ particles) and emit photons in the visible range^[1–4]. They are the vital component in X-ray perspective image technology that has been applied in medical imaging, industrial nondestructive testing, safety testing, and many other fields^[1–10]. At present, the commercial scintillators are mainly single-crystal scintillators with excellent scintillation performance, e.g., ${CdWO}_{4}$ (CWO), ${Bi}_{4} {Ge}_{3} O_{12}$ (BGO), CsI:Tl, and ${(Lu, Y)}_{2} {SiO}_{5} : Ce$ (LYSO)^[11]. However, the difficulties faced by single crystals in reducing the cost and achieving large scale and sizes production limit their further development^[12–16]. Low-cost preparation of large-sized scintillators with high spatial resolution is important and urgent. Therefore, many different materials have been widely investigated in scintillating fields^[17–21]. Glass scintillators stand out because of their mature process, excellent transmittance, high rare-earth ions (REIs) solubility, flexible plasticity, and low production cost^{[14–19,22–28]}.

Silicate glass has the advantages of high heat resistance, high mechanical strength, and excellent physical-chemical stability, but it also has a high melting temperature^[18,19,25]. The addition of boron oxide can effectively reduce the melting temperature, and the resulting phase separation phenomenon can be solved by adding sodium oxide. The addition of alumina can form aluminum–oxygen tetrahedrons in glass and bridge the glass network, which closes the glass structure and improves performance. The addition of barium oxide can effectively introduce heavy element barium to improve the radiation resistance and absorption of X rays. Such borosilicate glass (BSG), combined with a glass network intermediate (alumina) and glass network extras (sodium oxide and barium oxide), may present good scintillating properties in the X-ray imaging field after being activated by the appropriate REI.

Among the REI, ${Tb}^{3 +}$ is widely used in glass scintillators as an excellent doping activator^[16,29,30]. Benefiting from the stable luminescence at 542 nm that matched well with optical cameras (such as a charge-coupled device), ${Tb}^{3 +}$ -doped glass might be applied for static X-ray perspective image technology due to the millisecond lifetime of ${Tb}^{3 +}$ in a glass matrix^[16,24]. Nowadays, many ${Tb}^{3 +}$ -doped glass scintillators have been reported, and some of the relative integrated intensity of X-ray-excited luminescence (XEL) (compared with that of BGO crystals) are listed in Table 1. But there are few reports on X-ray imaging and spatial resolution. The improvement of XEL intensity and spatial resolution of ${Tb}^{3 +}$ -doped glass scintillators is therefore an area for further development.

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Sample	Ratio	Ref.
BGO	100%	[11]
B₂O₃-GeO₂-TeO₂-Gd₂O₃-Tb₂O₃	9%	[17]
B₂O₃-GeO₂-BaF₂-GdF₃-Gd₂O₃-Lu₂O₃-Tb₂O₃	27%	[29]
B₂O₃-GeO₂-Gd₂O₃-Tb₂O₃	30%	[24]
SiO₂-B₂O₃-Al₂O₃-Gd₂O₃-Tb₄O₇	40%	[31]
Al₂O₃-B₂O₃-SiO₂-Gd₂O₃-TbF₃	67%	[16]
SiO₂-BaF₂-BaO-AlF₃-Gd₂O₃-Sb₂O₃-Tb₂O₃-Ce₂O₃	83%	[25]
SiO₂-B₂O₃-Al₂O₃-CaO-Na₂O-TbCl₃	104%	[18]
Al₂O₃-B₂O₃-SiO₂-Na₂O-BaO-TbF₃	217%	This work

Table 1. Comparison of XEL Intensities of Tb³⁺-Doped Glass Scintillators with BGO

View all Tables

In this work, the BSG with the addition of alumina, sodium oxide, and barium oxide were designed and fabricated as the glass host for ${Tb}^{3 +}$ . These ${Tb}^{3 +}$ -doped glass specimens show excellent transmittance (87% at 600 nm). The sample doped with 6% (mole fraction) Tb³⁺ exhibits optimal performance in both photoluminescence (PL) and radioluminescence. The internal quantum efficiency (IQE) reaches 60.7%, its lifetime is 3.07 ms, and the integrated XEL intensity reaches 217% of BGO. More importantly, the optimal sample presents superhigh spatial resolution (exceeding 20 lp/mm). Hence, the results indicate that ${Tb}^{3 +}$ -doped BSG scintillators have a certain application value in the X-ray imaging field and are worthy of future research to improve their performance.

2. Experiment

2.1. Specimen preparation

Glass specimens were produced via conventional melt-quenching on the basis of a specific constituent of (mole fraction) $13 {Al}_{2} O_{3} - 11 B_{2} O_{3} - 46 {SiO}_{2} - 20 {Na}_{2} O - 10 {BaO - x Tb F}_{3}$ (named as G-host with $x = 0$ and G-xTb with $x = 3$ , 4, 5, 6, 7). ${Al}_{2} O_{3}$ (A.R.), ${SiO}_{2}$ (A.R.), ${Na}_{2} {CO}_{3}$ (A.R.), ${BaCO}_{3}$ (99%), $H_{3} {BO}_{3}$ (G.R.), and ${TbF}_{3}$ (99.99%) were selected as corresponding raw materials and weighed according to composition (total mass is 20 g). The mixture power was ground for 40 min and then transferred to a corundum crucible. After melting at 1400°C in air for 1 h, the melted liquid was dumped onto the preheated copper-steel (300°C) and gently cast with another heated copper-steel to fabricate glass specimens. The cooled glass specimens were annealed at 400°C for 4 h and then were cut and polished to 2 mm for further characterizations. The BGO crystal with a thickness of 2 mm was purchased from Shanghai Institute of Ceramics, Chinese Academy of Sciences.

2.2. Sample characterization

X-ray diffraction (XRD) patterns were analyzed utilizing an X-ray apparatus (MiniFlex/600, Rigaku) with a ${CuK}_{α 1}$ radiation source. Fourier transform infrared (FT-IR) spectra of G-host and G-6Tb were performed on an FT-IR spectrometer (NEXUS 670, Nicolet) with KBr as a diluent for tableting. Transmission spectra were obtained in an ultraviolet-visible spectrophotometer (U-3900, Hitachi). PL spectra, photoluminescence excitation (PLE) spectra, IQE, and decay curves were measured from a multifunction fluorescence spectrometer (Edinburgh FS5, Livingston). XEL spectra were obtained in an X-ray fluorescence spectrometer (OmniFluo960, Zolix). All measures were executed at room temperature.

The static X-ray imaging system includes an X-ray source, imaging objects, scintillator sample, and optical camera. A Mini-X X-ray tube (Amptek) with Ag target was selected as the X-ray source (the input voltage is maintained at 50 kV, and the input current can be adjusted in the 5–79 µA range). An X ray partially absorbed by an imaging object (such as an encapsulated chip, ballpoint pen, or standard X-ray test pattern plate) was projected onto the scintillator glass; the resulting radiative luminescence is refracted through a prism and captured and recorded by an sCMOS camera (Teledyne Photometrics) to form X-ray images.

3. Results and Discussion

3.1. Structural properties

Figure 1(a) depicts the XRD patterns of all specimens. All samples only show diffraction humps without diffraction peaks, which means that all samples are glassy. Figure 1(b) exhibits the FT-IR spectra of G-host and G-6Tb specimens. The absorption at about $467 {cm}^{- 1}$ originates from the bending vibrations of Si-O-Si and Al-O-Si bonds^[32,33]. The bending vibrations of B-O-B bridge oxygen bonds give rise to the absorption at about 710 cm^-1[33,34]. The wide band at around $992 {cm}^{- 1}$ is caused by the superposition effect of the stretching vibrations of Si-O-Si, Al-O-Si, and B-O-B bonds^[32–35]. The asymmetric stretching vibrations of B-O result in the absorption at 1276 and 1396 cm^-1[32,33,35].

Figure 1.(a) XRD patterns of all specimens; (b) FT-IR spectra of G-host and G-6Tb specimens.

Density ( $ρ$ ) and effective atomic number ( $Z_{eff}$ ) are fundamental and important parameters for glass scintillation, and the $Z_{eff}$ values are estimated using the well-known empirical equation^[36], $Z_{eff} = \sqrt[2.94]{f_{1} {(Z_{1})}^{2.94} + f_{2} {(Z_{2})}^{2.94} + \dots + f_{i} {(Z_{i})}^{2.94}},$ (1)where $f_{i}$ is fraction of total number of electrons associated with each element, and $Z_{i}$ is the atomic number of each element. These parameters of G-xTb samples are estimated and listed in Table 2.

Specimen	G-3Tb	G-4Tb	G-5Tb	G-6Tb	G-7Tb
ρ	2.96	3.00	3.03	3.07	3.11
Z_eff	33.8	34.8	35.7	36.5	37.2
IQE (λ_ex = 378 nm)	45.0%	71.5%	72.5%	60.7%	59.4%
τ (ms)	3.52	3.34	3.13	3.07	3.03
XEL intensity	92%	150%	167%	217%	166%

Table 2. The Density (ρ), Effective Atomic Number (Z_eff), IQE Values, Average Lifetime (τ) of ⁵D₄ State of Tb³⁺ (λ_ex = 378 nm), and the XEL Intensity (Compared to Commercial BGO Crystal) of G-xTb Specimens

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The transmittance of the scintillator is an important factor that affects the spatial resolution of an X-ray imaging system. The transmittance spectra of all specimens are presented in Fig. 2(a). All specimens show good transmittance, whose value is about 87%@600 nm. The absorption peaks situated at 340, 352, 368, 378, and 484 correspond to the emblematic transition of ${Tb}^{3 +}$ from ${{}^{7}F}_{6}$ level to ${{}^{5}L}_{7}$ , ${{}^{5}L}_{9}$ , and ${{}^{5}D}_{2, 3, 4}$ levels, respectively^[24,29,37]. All absorption peaks become more obvious when boosting ${Tb}^{3 +}$ concentration. The bandgaps of G-host and G-6Tb specimens are estimated by the formula $α = B {(h ν - E_{g})}^{1 / 2}$ for direct band material, where $α$ and $B$ mean the absorption coefficient and a constant coefficient, respectively^[22], $h ν$ is photon energy, and $E_{g}$ stands for bandgap. $α$ can be obtained by converting transmittance data through $I = I_{0} e^{- a d}$ and $T = I / I_{0}$ , where $I$ , $I_{0}$ , $d$ , and $T$ mean incident intensity, transmitted intensity, thickness, and transmittance, respectively. And as presented in Fig. 2(b), $E_{g}$ values of G-host and G-6Tb specimens are 3.90 and 3.86 eV, respectively.

Figure 2.(a) Transmittance spectra of all samples; (b) relationship between α² and hν for G-host and G-6Tb samples.

The photographs of all samples are displayed in Fig. 3. In the sunlight, all samples appear very transparent. The high transparency of samples allows the printed font to be clearly seen through the sample. And under the excitation of 365 nm light, G-xTb specimens present typical green emission of ${Tb}^{3 +}$ , whereas the G-host sample dose not emit light (labeled in white dashed box). Among G-xTb samples, G-6Tb and G-7Tb show intense green emission.

Figure 3.Photos of all samples (a) under daylight and (b) under 365 nm light irradiation.

3.2. Photoluminescent properties

Figure 4(a) demonstrates the PL spectra ( $λ_{ex} = 378 nm$ ) of all specimens. The PL spectra comprise peaks at 487, 542, 583, and 621 nm, which are referred to as ${{}^{5}D}_{4}$ to ${{}^{7}F}_{6, 5, 4, 3}$ transition of ${Tb}^{3 +}$ ^[38–40]. By boosting ${Tb}^{3 +}$ contents, the emission intensities of these peaks are strengthened and then weakened. The PLE spectra ( $λ_{em} = 542 nm$ ) of all samples are shown in Fig. 4(b)^[23,41]. The excitation band (225–297 nm) is related to $4 f^{8}$ to $4 f^{7} {5 d}^{1}$ electron transition (f–d) of ${Tb}^{3 +}$ ^[16,25]. These excitation peaks at 302, 316, 339, 351, 369, 378, and 484 correspond to ${{}^{7}F}_{6}$ to ${{}^{5}H}_{6, 7}$ , ${{}^{5}L}_{7, 9}$ , and ${{}^{5}D}_{2, 3, 4}$ transition of ${Tb}^{3 +}$ , respectively, which are consistent with Fig. 2(b)^[13,25]. PL spectra and PLE spectra show that the optimal sample is the G-6Tb sample.

Figure 4.(a) PL spectra (λ_ex = 378 nm) of all samples; (b) PLE spectra (λ_em = 542 nm) of all samples.

IQE values of the G-xTb specimens were gauged and calculated, as demonstrated in Fig. 5(a). The values of IQE enumerated in Table 2 can be computed by utilizing the following formula^[41,42]: $IQE = \frac{\int L_{sample}}{\int E_{reference} - \int E_{sample}},$ (2)where $L_{sample}$ stands for the emission intensity of sample, and $E_{reference}$ and $E_{sample}$ mean the excitation intensity when a standard whiteboard or sample is used, respectively. G-5Tb shows the highest IQE value, which reaches 72.5%, and the G-6Tb sample also has a relatively high IQE value (60.7%). The relatively high IQE might be helpful to get high XEL intensity for X-ray imaging.

Figure 5.(a) IQE spectra of G-xTb specimens; (b) decay curves (λ_ex = 378 nm) of 542 nm emission of G-xTb specimens.

The luminescence decay curves ( $λ_{ex} = 378 nm$ ) of 542 nm emission of G-xTb specimens are revealed in Fig. 5(b). The τ values of ${{}^{5}D}_{4}$ state of ${Tb}^{3 +}$ in G-xTb specimens are calculated via the following formula^[16,43]: $τ = \int t I (t) d t / \int I (t) d t,$ (3)where $I (t)$ is the luminescence intensity at moment $t$ . As listed in Table 2, the τ values decrease continuously (from 3.52 to 2.98 ms) with the boost of ${Tb}^{3 +}$ concentration. The millisecond lifetimes of glass specimens are suitable for static X-ray imaging.

3.3. Radioluminescent properties and X-ray imaging

To characterize the scintillating capability of G-xTb specimens in X-ray imaging, Fig. 6(a) shows the XEL spectra of commercial BGO crystal and G-xTb specimens. As the ${Tb}^{3 +}$ concentration increases, the XEL intensity of G-xTb samples enhances gradually, and abates afterwards. The G-6Tb sample presents the optimal scintillating performance.

Figure 6.(a) XEL spectra of BGO crystal and G-xTb specimens; (b) relationship between XEL intensity and X-ray dose rate.

The ratio of the integrated XEL intensity between G-xTb samples and BGO is calculated and listed in Table 2. Notably, the XEL intensity of the G-6Tb specimen reaches 217% of that of BGO. It suggests that the G-6Tb sample might have a potential application in X-ray imaging. The relationship between XEL intensity and the X-ray dose rate of the G-6Tb sample is displayed in Fig. 6(b). These experimental data points match well ( $R^{2} = 99.8 %$ ) with the fit line ( $y = 37.2 x + 157.5$ ), indicating that the radioluminescent intensity is critically dependent on the X-ray dose (which will create a high contrast in the X-ray imaging), suggesting their practical application^[44,45].

The X-ray imaging capacity was investigated by a suborbicular G-6Tb optimal sample with $Φ 35 mm \times 0.5 mm$ size and is presented in Fig. 7. The photo and X-ray-excited image of the G-6Tb specimen are displayed in Figs. 7(a) and 7(b). The homogeneity of the big glass samples can be obtained by stirring processing during the melting process. Figures 7(d) and 7(f) exhibit the X-ray images of the encapsulated chip and the ballpoint pen, and corresponding images are presented in Figs. 7(c) and 7(e). The covered content (such as wires, spring, and even ballpen ink) can be clearly observed.

Figure 7.Photos of G-6Tb sample for (a) X-ray imaging, (c) encapsulated chip, (e) ballpoint pen, and (g) standard X-ray test pattern plate; X-ray images of (b) sample, (d) encapsulated chip, (f) ballpoint pen, and (h) standard X-ray test pattern plate.

The spatial resolution is one of the most important parameters for X-ray imaging. It can be obtained in the image of a standard X-ray test pattern plate [photo in Fig. 7(g)], shown in Fig. 7(h). The image distinctly shows that six light lines can be resolved, even at densities over 20 lp/mm, indicating that the optimal sample presents superhigh spatial resolution (exceeding 20 lp/mm). Such high spatial resolution can meet the requirements of actual X-ray imaging. For a better comparison, the spatial resolution values of some different scintillating materials for X-ray imaging are listed in Table 3. The spatial resolution of the G-6Tb sample reaches the highest level. In brief, ${Tb}^{3 +}$ -doped BSG scintillators with big production size, low cost, and high spatial resolution might have latent applications in the field of X-ray detection and X-ray imaging.

Spatial Resolution (lp/mm)	Material	Ref.
5	Mn-based halides	[48]
7.5	CsCu₂I₃ film	[6]
9	Cs₂ZnBr₄:Cu film	[7]
10	SiO₂-AlF₃-Al₂O₃-Gd₂O₃-Sb₂O₃-CeO₂-Tb₄O₇ glass	[49]
12.5	polymer-ceramics	[8]
15	CsPbX₃ nanocrystals	[9]
20	Cs₄Mn_0.25Cd_0.75Bi₂Cl₁₂ nanoparticles	[45]
20	Tb³⁺-doped Ba₂LaF₇ glass ceramics	[10]
20	Al₂O₃-B₂O₃-SiO₂-Na₂O-BaO-TbF₃ glass	This work

Table 3. Spatial Resolution of Diverse Material

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Figure 8 illustrates the mechanism of ${Tb}^{3 +}$ -doped BSG scintillators. The conversion process happens first. The heavy atoms in glass generate a great number of high-energy particles (electrons and holes) after interacting with the X ray (photoelectric effect and Compton effect)^[46]. These particles are then thermalized to produce more secondary electrons and holes^[2,47]. During the subsequent transport process, the secondary electrons and holes are transported to ${Tb}^{3 +}$ ions in glass and pump these activator ions to higher excited levels. And in the final luminescence process, these excited ${Tb}^{3 +}$ ions make radioluminescence similar to PL. These pumped ${Tb}^{3 +}$ ions relax to ${{}^{5}D}_{4}$ level from various higher energy levels, and then jump to ${{}^{7}F}_{6, 5, 4, 3}$ levels with corresponding emission light^[5,16].

Figure 8.Mechanism of Tb³⁺-doped BSG scintillators.

In the measurement of XEL spectra, the color of the G-6Tb sample changes from colorless to slightly yellow, indicating that X-ray irradiation will cause damage to glass. Figure 9(a) shows the transmittance spectra of the G-6Tb sample irradiated by an X ray with different input powers for 5 min. It is clear that the transparency at 320–550 nm decreases with the increase of input power. For a better comparison and to explain the color change mechanism, the transmittance spectra of the G-6Tb sample before irradiation, after X-ray irradiation (10 W), and their difference are displayed in Fig. 9(b). A broad absorption band at around 320–550 nm is observed, which can corresponde to the charge change absorption of ${Tb}^{4 +}$ ions^[19,50]. ${Tb}^{4 +}$ ions are probably generated by the ionization of one electron of ${Tb}^{3 +}$ ions with the irradiation of high-energy X rays.

Figure 9.(a) Transmittance spectra of G-6Tb sample before radiation, after being irradiated by X rays with different input powers for 5 min, and after subsequent heat treatment; inset (i) is photo of G-6Tb sample after X-ray irradiation (10 W, A-I) and inset (ii) is photo of G-6Tb sample after subsequent heat treatment (H-T); (b) transmittance spectra of G-6Tb sample before irradiation, after X-ray irradiation (10 W), and their difference.

Inset (i) is the photo of the G-6Tb specimen after 10 W of X-ray irradiation. Inset (ii) displays the photograph of the G-6Tb specimen after subsequent heat treatment (300°C for 3 h). Combined with the transmittance curve after heat treatment in Fig. 9(a), it is interesting to note that the slightly yellow absorption from ${Tb}^{4 +}$ ions can be restored through heat treatment.

The effect of irradiation time on XEL intensity was investigated. The XEL spectra and relative intensity curve of the G-6Tb specimen with continuous long-time X-ray irradiation (7 W) are shown in Figs. 10(a) and 10(b). With increasing irradiation time, the G-6Tb sample exhibits good irradiation luminescence stability. Only slight changes occur. The corresponding transmittance spectra of the G-6Tb sample before irradiation, after 100 min of X-ray irradiation, and their difference are displayed in Fig. 10(c). Similar to Fig. 9(c), their difference curves also exhibit absorption bands from ${Tb}^{4 +}$ . So, the XEL intensity changes from 100% to 94% after 100 min of X-ray irradiation, which might be the result of the absorption of ${Tb}^{4 +}$ produced by X-ray irradiation. The recoverable transparency by heat treatment and the good irradiation stability indicate that the G-6Tb sample might be used for X-ray imaging.

Figure 10.(a) XEL spectra of G-6Tb specimen with different X-ray irradiation time (7 W); (b) relative XEL intensity of G-6Tb sample after different irradiation time; (c) transmittance spectra of G-6Tb sample before irradiation, after 100 min X-ray irradiation, and their difference.

4. Conclusions

A battery of highly transparent BSG scintillators for X-ray imaging were devised and manufactured by melt-quenching technology. The transmittance of all specimens is about 87% at 600 nm, and the $E_{g}$ values of samples are about 3.9 eV. The optimal G-6Tb sample presents a high IQE value (60.7%), high XEL intensity (217% of that of BGO), excellent spatial resolution (20 lp/mm), and good radiation tolerance and refreshable ability by heat treatment. The results suggest that ${Tb}^{3 +}$ -doped BSG scintillators with big production size, low cost, and high spatial resolution might have value for further optimization and applications in X-ray detection and X-ray imaging fields.

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