Tailoring Luminescence and Scintillation Properties of Tb³⁺-Doped LuYAGG Single Crystals for High-Performance Radiation Detection

Abstract

In this study, Lu_2.5Y_0.5(Al_2.5Ga_2.5)O₁₂ (LuYAGG) single-crystal scintillators doped with terbium ions (Tb³⁺) at concentrations of 0.5, 1, 5, and 10 mol% were successfully synthesized using the floating zone method. The structural, optical, photoluminescence (PL), and scintillation properties of the Tb³⁺-doped crystals were systematically investigated with a focus on their potential for high-performance scintillator applications. X-ray diffraction (XRD) confirmed the formation of a pure garnet phase without any secondary phases, indicating the successful incorporation of Tb³⁺ into the LuYAGG lattice. Optical transmittance spectra revealed high transparency in the visible range. Photoluminescence measurements showed characteristic Tb³⁺ emission peaks, with the strongest green emission observed from the ⁵D₄ → ⁷F₅ transition, particularly for the 5 mol% sample. The PL decay curves further confirmed that this concentration offers a favorable balance between radiative efficiency and minimal non-radiative losses. Under γ-ray excitation, the 5 mol% Tb³⁺-doped crystal exhibited the highest light yield, surpassing the performance of other concentrations and even outperforming Bi₄Ge₃O₁₂ (BGO) in relative comparison, with an estimated yield of approximately 60,000 photons/MeV. Scintillation decay time analysis revealed that the 5 mol% sample also possessed the fastest decay component, indicating its superior capability for radiation detection. Although 10 mol% Tb³⁺ still showed good performance, slight quenching effects were observed, while lower concentrations (0.5 and 1 mol%) suffered from longer decay and lower emission efficiency due to limited activator density. These findings clearly identify with 5 mol% Tb³⁺ as the optimal dopant level in LuYAGG single crystals, offering a synergistic combination of high light yield and excellent optical transparency. This work highlights the strong potential of LuYAGG:Tb³⁺ as a promising candidate for the next-generation scintillator materials used in medical imaging, security scanning, and high-energy physics applications.

1. Introduction

Scintillator materials are indispensable in a broad range of radiation detection applications, including medical imaging (e.g., positron emission tomography, PET), high-energy physics experiments, security screening, and environmental monitoring. The fundamental role of a scintillator is to absorb incoming high-energy photons or charged particles and re-emit the absorbed energy as visible or near-visible light. The ideal scintillator material therefore requires a high density (for effective stopping power), strong luminescence intensity (for good energy discrimination), and fast decay time (for high-count-rate applications) [1,2]. Moreover, excellent optical transparency, good mechanical properties, and chemical stability are crucial for practical deployment in large-area detectors and harsh operating environments [3,4]. Within the family of scintillator materials, rare-earth doped garnets stand out due to their robust crystal structure, high thermal and chemical stability, and favorable optical properties. Garnets of the general formula A₃B₅O₁₂ (where A is typically Y, Lu, or Gd, and B is Al or Ga) offer a tunable host lattice for accommodating various rare-earth activator ions [5,6,7]. By carefully selecting and tailoring the host’s composition, one can optimize critical scintillation characteristics such as the band gap, density, and energy transfer dynamics [8]. For instance, partially substituting Lu³⁺ with Y³⁺ can balance material density and cost, while replacing Al³⁺ with Ga³⁺ can modify the conduction and valence bands to enhance luminescence efficiency [9,10].

Although cerium (Ce³⁺) is widely employed in garnet scintillators (e.g., Ce:LuAG, Ce:GAGG) due to its fast emission (tens of nanoseconds), other rare-earth ions also exhibit promising properties. Terbium (Tb³⁺) has garnered increasing interest in recent years because of its characteristic green emission (⁵D₄-⁷F₅ at around 545 nm), relatively high light yield potential, and long-lived excited states. The specific electronic configuration of Tb³⁺ (4f⁸) gives rise to a series of intra-4f transitions that can be effectively excited by both UV photons and high-energy radiation, making Tb-doped materials suitable for certain imaging modalities or detection schemes where green luminescence is advantageous [3,11,12,13,14]. Moreover, Tb³⁺ can sometimes offer higher quantum yields under specific excitation conditions compared to other rare-earth dopants, depending on the host lattice environment and doping concentration. Despite these advantages, the scintillation performance of Tb-doped garnets can be sensitive to a range of factors, including dopant concentration, crystal growth conditions, and the presence of defects or charge-compensating ions [15,16]. At low Tb concentrations, luminescence intensity may be suboptimal due to insufficient activator centers. Conversely, at very high concentrations, concentration quenching and non-radiative energy transfer between Tb ions may degrade the light yield and lengthen decay times. Hence, a systematic investigation of Tb concentration is crucial to identify the doping level that maximizes light output without sacrificing decay time or transparency [15,17].

In the present work, we focus on the synthesis and characterization of Tb-doped Lu_2.5Y_0.5(Al_2.5Ga_2.5)O₁₂ (LuYAGG) single crystals, aiming to explore the impact of Tb content (0.5, 1, 5, and 10 mol%) on the luminescent and scintillation properties. High-purity oxide precursors (Al₂O₃, Y₂O₃, Lu₂O₃, Ga₂O₃, and Tb₄O₇) were carefully weighed according to the stoichiometric ratios and subsequently processed to grow single crystals. The partial substitution of Lu by Y was chosen to balance material density and reduce production costs, while Al-Ga co-substitution is anticipated to fine-tune the crystal field and band structure, thereby influencing the energy transfer to Tb³⁺ centers [9]. This work provides a comprehensive evaluation of the synthesized Tb-doped garnets, examining their crystal structure, optical transparency, photoluminescence (PL), and scintillation performance (radioluminescence and decay times). By correlating these properties with different Tb doping levels, we aim to identify the optimal concentration for maximizing light yield and maintaining suitable decay characteristics. Furthermore, this study highlights the fundamental structure–property relationships in Tb-activated garnet systems, contributing valuable insights into the design of next-generation scintillators for high-resolution, high-speed radiation detection applications.

2. Experimental Details

High-purity precursor powders of Al₂O₃, Y₂O₃, Lu₂O₃, Ga₂O₃, and Tb₄O₇ (purity range: 99.99–99.999%) were employed in the preparation of single-crystal scintillators. These powders were precisely measured to achieve the desired LuYAGG composition, with Tb³⁺ incorporated at doping levels of 0.5, 1, 5, and 10 mol%. To prepare each sample, the precursor powders were thoroughly ground using an agate mortar to achieve uniform mixing. The resulting mixture was placed into an elastomeric mold and compacted into cylindrical rods via isostatic pressing. These green bodies were then sintered at 1200 °C for 10 h to improve their mechanical integrity and density. Following sintering, single crystals were grown using a dual halogen lamp floating-zone furnace (Canon Machinery FZD0192, Shiga, Japan), operating at a pulling speed of 4 mm/h and a rotation rate of 20 rpm. The grown crystals were subsequently sectioned and polished to obtain samples suitable for characterization.

The crystal structures of the samples were analyzed using X-ray diffraction (XRD) within the 2θ range of 10–60°, utilizing a Shimadzu XRD-6100 diffractometer with Cu-Kα radiation (40 kV, 15 mA). The resulting diffraction patterns were compared to standard reference data (PDF#63-0291) to verify phase purity and crystallinity. Optical transmission properties were assessed through diffuse transmission spectra, measured with a Shimadzu SolidSpec-3700 spectrophotometer. Photoluminescence (PL) emission behavior was examined using a spectrofluorophotometer(RF-5301PC, Shimadzu Corporation, Kyoto, Japan), while the photoluminescence (PL) contour map was measured with a Quantaurus-QY C11347 system (Hamamatsu Photonics, Shizuoka, Japan). PL decay time measurements were conducted using Quantaurus-τ C11367 (Hamamatsu Photonics, Shizuoka, Japan).

The scintillation behavior of the polished LuYAGG:Tb single crystals was assessed using a range of characterization techniques. Radioluminescence spectra were obtained using a custom-built system comprising a High Voltage Electronics Monoblock XRB80N100/CB X-ray generator (Spellman High Voltage Electronics Corporation, Hauppauge, NY, USA) and a DU-420-BU2 CCD (Andor-Oxford Instruments, Abingdon, Oxfordshire, England), which allowed for detailed spectral analysis under X-ray stimulation. The generator was operated at 40 kV and 1.2 mA [18]. During measurements, each sample was placed in direct contact with a fiber optic cable linked to the spectrometer to ensure optimal light collection. An X-ray dose of 1 Gy was applied, and emission spectra were collected over the 200–700 nm wavelength range.

The scintillation light yield (LY) was determined using a pulse height analysis system under excitation with 662 keV γ-rays from a ¹³⁷Cs source. The experimental setup followed our original design for scintillators with slow decay times. A near-infrared sensitive photomultiplier tube (PMT; R7600-200; Hamamatsu Photonics) operated at −550 V was used as the photodetector [19]. The LY of the LuYAGG:Tb single crystals was estimated from the photoelectric absorption peak by comparison with that of a Bi₄Ge₃O₁₂ (BGO) single crystal. Scintillation decay characteristics were measured with a custom-built system featuring a pulsed X-ray source and a PMT (R7400P-06, Hamamatsu Photonics), operated in a time-correlated single-photon counting configuration [20]. Figure 1 shows images of Tb-doped Lu_2.5Y_0.5(Al_2.5Ga_2.5)O₁₂ single crystals under normal lighting and UV light source at wavelength 254 nm conditions.

3. Results and Discussion

3.1. Structural Analysis

Figure 2 shows the XRD patterns of all Tb-doped Lu_2.5Y_0.5(Al_2.5Ga_2.5)O₁₂ samples (with Tb concentrations of 0.5%, 1%, 5%, and 10%) that closely match the standard Y₃Al₅O₁₂ (PDF#63-0291) reference pattern, confirming the formation of a single-phase garnet structure without any detectable secondary phases or impurity peaks across the entire doping range. Despite increasing the Tb content up to 10 mol%, the major diffraction peak positions remain largely unchanged, and the peaks stay sharp and well-defined, indicating high crystallinity. This structural stability, even at higher Tb³⁺ doping levels, is attributed to the similar ionic radii of Tb³⁺ (0.92 Å), Lu³⁺ (0.86 Å), and Y³⁺ (0.90 Å), all with coordination number 6 [21,22], which minimizes lattice distortion. Maintaining a pure garnet phase is essential for optimal optical and scintillation performance, as secondary phases can reduce light yield and introduce undesirable scattering.

3.2. Optical and Photoluminescence Characteristics

Figure 3 presents the transmittance spectra (200–700 nm) for Lu_2.5Y_0.5(Al_2.5Ga_2.5)O₁₂ samples doped with 0.5, 1, 5, and 10 mol% Tb³⁺. All samples exhibited transmittance of about ~80–90% in the visible region (400–700 nm), confirming excellent optical transparency. Notably, there is no significant absorption near 545 nm, corresponding to the prominent Tb³⁺: ⁵D₄ → ⁷F₅ green emission, hence supporting the material’s effectiveness in scintillation applications. Distinct absorption bands were observed in the UV region, attributed to parity-forbidden f–f transitions of Tb³⁺ ions, which gain intensity through crystal field mixing. The primary absorption bands include ~220–240 nm (⁷F₆ → ⁵K₇), ~270–300 nm (⁷F₆ → ⁵K₉), and ~310–330 nm (⁷F₆ → ⁵H₇). As expected, absorption in these regions increases with higher Tb³⁺ concentrations due to a greater number of absorbing centers. However, the transitions remain relatively sharp, indicating a uniform crystal field environment around the Tb³⁺ ions and minimal defect-related scattering, both of which are crucial for preserving high optical quality [3,6,7,23].

High transmittance above 400 nm ensures the efficient escape of Tb³⁺ green emission (~545 nm), which is crucial for optimal scintillation performance. In addition, strong absorption in the UV range enhances the material’s suitability for excitation by UV or ionizing radiation, facilitating efficient energy transfer to Tb³⁺ ions. The observed decrease in UV transmittance with increasing doping also offers a qualitative means of estimating the Tb³⁺ concentration [13].

Figure 4a shows the PL excitation spectrum of LuYAGG doped with 5 mol% Tb³⁺, monitored at the characteristic green emission wavelength of 545 nm. The spectrum revealed two major excitation bands: the first, centered at ~236 nm, corresponds to the ⁷F₆ → ⁵K₈ transition of Tb³⁺, while the second strong band appeared near ~313 nm. These transitions arose from parity-forbidden intra-4f transitions that gained intensity through crystal field mixing, which was common in low-symmetry garnet hosts. Additional weaker bands were observed in the 320–380 nm range, attributed to higher-lying ⁵H and ⁵D states [24,25,26].

As shown in Figure 4b, the emission spectra in the 300–700 nm range were recorded under excitation at λ_ex = 236 nm (likely around 220–300 nm based on Tb³⁺ absorption) and displayed several sharp emission peaks corresponding to the characteristic 4f–4f transitions of Tb³⁺. In particular, the dominant green emission at ~545 nm (⁵D₄ → ⁷F₅) was accompanied by weaker peaks at ~490 nm (⁵D₄ → ⁷F₆), ~545 nm (⁵D₄ → ⁷F₅), ~585 nm (⁵D₄ → ⁷F₄), and ~620 nm (⁵D₄ → ⁷F₃). The emission intensity increases with Tb³⁺ concentration up to 5 mol%, beyond which it begins to saturate, suggesting the onset of concentration quenching. The spectra show no significant peak shifts or broadening, indicating that the crystal field environment remains stable across the doping range [24,25,26,27,28].

Figure 4c presents the excitation (y-axis) versus emission (x-axis) wavelength map. The color gradient ranging from blue → green → red indicates increasing luminescence intensity, with red areas corresponding to the strongest emission. The photoluminescence (PL) contour map clearly revealed the characteristic ⁵D₄ → ⁷F_j transitions of Tb³⁺, which dominate at higher Tb³⁺ concentrations (5 and 10% Tb samples) due to efficient cross-relaxation processes. These transitions include emissions at 490 nm (⁵D₄ → ⁷F₆), 545 nm (⁵D₄ → ⁷F₅—the most intense green emission), 585 nm (⁵D₄ → ⁷F₄), and 620 nm (⁵D₄ → ⁷F₃) [27,29].

These emissions were effectively excited within the 250–330 nm wavelength range. The strongest emissions, particularly the prominent 545 nm peak, were most efficiently excited at ~270–300 nm. This excitation range corresponded to allowing for 4f⁸ → 4f⁷5d¹ transitions and charge transfer (CT) bands in Tb³⁺, indicating that the host matrix efficiently absorbed UV light and transferred energy to the Tb³⁺ ions. The broad excitation band (250–330 nm) made this material highly compatible with UV LEDs and suitable for X-ray scintillation. Furthermore, the intense green emission at 545 nm (⁵D₄ → ⁷F₅) is ideal for applications in solid-state lighting (e.g., white LEDs using blue chips with green phosphors), display technologies, and radiation detection through scintillator devices [26,27,28,29].

Figure 5 presents the photoluminescence (PL) decay of LuYAGG:Tb garnet crystals, at λ_ex = 340 nm and λ_em = 550 nm, corresponding to the green emission peak (~545 nm) from the ⁵D₄ → ⁷F₅ transition of Tb³⁺. The decay was recorded under pulsed excitation, with the time axis was displayed in milliseconds (ms), reflecting the typical ms-scale lifetimes of the ⁵D₄ excited states. The decay profiles fit well with a single-exponential decay function: I(t) = A exp (−t/τ) + background, where A is the initial amplitude, t is time, and τ represents the decay lifetime. For well-isolated Tb³⁺ ions, the decay was predominantly single exponential, corresponding to intrinsic ⁵D₄ → ⁷F_j transitions. However, at higher Tb³⁺ concentrations, non-radiative processes such as energy migration and cross-relaxation became more prominent, introducing faster decay components. This results in a decrease in τ, a characteristic feature of concentration quenching, where energy transfers non-radiatively between nearby Tb³⁺ ions and may migrate to quenching centers (e.g., defects or impurities), thereby shortening the effective lifetime [13,30,31]. The observed decay lifetimes (~2.9–3.4 ms) are characteristic of the ⁵D₄ → ⁷F_j transitions of Tb³⁺ and are sufficiently long to support a variety of optical applications. Such millisecond-scale luminescence is advantageous for time-gated bioimaging, optical security tags, and persistent phosphor technologies, where delayed or sustained emission is desirable for contrast enhancement, data encoding, or visual persistence [32,33,34]. The corresponding decay lifetimes for the LuYAGG:Tb samples are summarized in

Table 1. Photoluminescence decay time constants for Tb-doped LuYAGG single crystals.

Samples	τ1 (ms)
LuYAGG:Tb 0.5 mol%	3.25
LuYAGG:Tb 1 mol%	3.16
LuYAGG:Tb 5 mol%	3.03
LuYAGG:Tb 10 mol%	2.98

.

3.3. Scintillation Characteristics

Figure 6 presents the radioluminescence (RL) spectra of LuYAGG:Tb single crystals under X-ray excitation. The emission spans approximately 350 to 700 nm, corresponding to the characteristic 4f–4f transitions of Tb³⁺: ⁵D₃ → ⁷F_6,5,4 (~375–450 nm) and ⁵D₄ → ⁷F₆ (~490 nm), ⁷F₅ (~545 nm), ⁷F₄ (~585 nm), ⁷F₃ (~620 nm), consistent with typical Tb³⁺ scintillation behavior. The RL spectra of LuYAGG doped with 0.5, 1, 5, and 10 mol% Tb³⁺ revealed a clear trend in luminescence efficiency. The sample doped with 1 mol% Tb³⁺ exhibited the highest RL intensity, particularly at the ⁵D₄ → ⁷F₅ transition (~545 nm), indicating this as the optimal activator concentration. In contrast, the 0.5 mol% doped sample showed noticeably weaker emissions, likely due to an insufficient number of activator centers. At higher doping levels (5 mol% and 10 mol%), the RL intensity decreased significantly, attributed to concentration quenching, a result of enhanced cross-relaxation and non-radiative energy transfer to defect sites [12,15,24,27]. These findings suggest that 1 mol% Tb³⁺ offers the best compromise between adequate activator density and minimal non-radiative losses among all the samples.

Figure 7 shows the pulse height spectra of LuYAGG:Tb single-crystal samples compared with a commercial Bi₄Ge₃O₁₂ (BGO) crystal, used as a reference, under 662 keV γ-ray excitation from a ¹³⁷Cs source. The corresponding light yield (LY) values are summarized in

Table 2. LY_γ at 662 keV γ rays for the LuYAGG:Tb single-crystal samples.

Sample	LYγ (ph/MeV)	LYγ (% of BGO)	PMT’s Quantum Efficiency (%)
BGO	8200	100	24.7
LuYAGG:Tb 0.5 mol%	30,000	366	10.7
LuYAGG:Tb 1 mol%	42,000	512	10.7
LuYAGG:Tb 5 mol%	60,000	732	10.7
LuYAGG:Tb 10 mol%	50,000	610	10.7

. Among all the samples, LuYAGG:Tb³⁺ with 5 mol% doping exhibited the highest absolute light yield of 60,000 ph/MeV, which is 732% of the BGO output. After correcting for the lower quantum efficiency of the PMT used with LuYAGG:Tb³⁺ (10.7% compared to 24.7% for BGO), the actual light output was estimated to exceed 138,000 ph/MeV, underscoring its strong potential as a next-generation scintillator.

The highest LY was observed for the LuYAGG:Tb³⁺ crystal doped with 5 mol% Tb³⁺, outperforming all other compositions when compared against the BGO reference. This suggests that 5 mol% represents the optimal balance between sufficient activator concentration for efficient radiative transitions and minimal non-radiative quenching. Although 10 mol% still exhibited a characteristic of ⁵D₄ → ⁷F_j transitions of Tb³⁺, its slightly reduced light yield implies the onset of concentration quenching mechanisms, such as cross-relaxation or energy migration to non-luminescent sites. Conversely, samples with 0.5 and 1 mol% Tb³⁺ showed weaker emission due to limited radiative centers and inefficient energy transfer [7,12,15,24].

Figure 8 displays the scintillation decay curves of LuYAGG single crystals doped with Tb³⁺ at concentrations of 0.5, 1, 5, and 10 mol%. The corresponding decay parameters are summarized in

Table 3. Scintillation decay time constants for Tb³⁺-doped LuYAGG single crystals.

Samples	τ1 (ms)	τ2 (ms)
LuYAGG:Tb 0.5 mol%	0.9	3.6
LuYAGG:Tb 1 mol%	0.9	3.3
LuYAGG:Tb 5 mol%	0.4	2.8
LuYAGG:Tb 10 mol%	0.5	2.8

. The X-ray scintillation decay profiles exhibit a bi-exponential behavior, comprising a fast radiative recombination component and a slower delayed emission, both of which are significantly influenced by Tb³⁺ concentration. At 0.5 mol%, the decay profile was well-balanced, showing moderate speed and minimal non-radiative losses. Increasing the doping to 1 mol% could enhance the prompt decay component, indicating efficient energy transfer and strong luminescent output. Interestingly, the 5 mol% Tb³⁺ sample demonstrated the fastest overall decay response, dominated by a rapid component with minimal tailing, suggesting a highly efficient radiative recombination at this concentration. The 10 mol% sample retained relatively characteristic ⁵D₄ → ⁷F_j transitions of Tb³⁺, with a slight broadening in the tail, signaling the onset of concentration quenching effects, likely due to cross-relaxation or energy migration among Tb³⁺ ions [15,24,30].

4. Conclusions

In this study, high-quality single crystals of Tb³⁺-doped Lu_2.5Y_0.5(Al_2.5Ga_2.5)O₁₂ were successfully grown and systematically evaluated for their optical and scintillation properties. All samples exhibited excellent phase purity, high transmittance in the visible range, and sharp emission features characteristic of Tb³⁺. The optimal scintillation performance was observed at 5 mol% doping, with a light yield exceeding 60,000 ph/MeV and favorable decay characteristics. These findings highlight the potential of Tb³⁺-doped LuYAGG as a promising green-emitting scintillator material for radiation detection and imaging applications. Further improvements may be achieved through co-doping or host matrix engineering to enhance energy transfer and suppress quenching effects.