Effects of Preparation Atmosphere and Doping Concentration on Scintillation and Photoluminescence Properties of Lu₂O₃:Eu Scintillation Single Crystals

Abstract

Lu₂O₃:1%Eu and Lu₂O₃:8%Eu series single crystals were grown by optical floating zone method under air, N₂, and H₂-Ar mixed atmosphere (5 vol%H₂ and 95 vol%Ar). The effects of preparation atmosphere and doping concentration on the scintillation and photoluminescence properties have been investigated and discussed. Correlated measurements of afterglow curves, X-ray excited luminescence spectra, photoluminescence excitation and photoluminescence, photoluminescence decay, and thermal stimulated luminescence curves were performed. Although Eu(S₆) luminescence can be significantly depressed under oxidizing atmosphere, Lu₂O₃:8%Eu grown under weak reducing atmosphere presents the lowest afterglow intensity. This phenomenon was explained by different interstitial oxygen atom concentrations in the samples.

1. Introduction

Scintillation materials can convert X-ray, gamma ray, and other high-energy particles into ultraviolet-visible photons [1]. They play an important role in ionizing radiation detection and are used in a wide range of applications in medical imaging, non-destructive testing, nuclear physics, oil well logging, and other fields [1,2,3,4,5]. In the application of scintillation materials, excellent performances, such as high density and high light yield, are required to obtain high X-ray stopping capability and high detection efficiency in X-ray imaging applications [6,7,8]. Europium-doped lutetium oxide, Lu₂O₃:Eu, has gained attention due to its excellent physical and scintillation properties [9,10,11,12,13,14]. It meets the demand for scintillator requirement for the high spatial resolution MeV X-ray imaging application with high absorption efficiency (density up to 9.42 g/cm)³, high light yield, and good physical and chemical stability [4]. The characteristic emission peak of Lu₂O₃:Eu scintillation single crystal is mainly located at 611 nm, and the emission wavelengths are well matched to the spectral sensitivity range of the CCD detector [6]. Another important performance parameter of Lu₂O₃:Eu is its characteristic decay time, and the relatively slow decay time on the order of microseconds is acceptable for radiation applications that do not involve high frame rates [13].

However, Lu₂O₃:Eu presents relatively strong afterglow [15]. In applications, involving repeated acquisition of imaging data at high frame rates, as well as strong afterglow, may limit the use of this scintillation material. The captured image may be obscured by the afterglow left by the previous frame if the afterglow does not dissipate in the time available between consecutive frames. Therefore, it is of great importance to understand and analyze the afterglow mechanism and impact factors of Lu₂O₃:Eu in its practical development and application. Eu³⁺ will occupy two lattice sites (C₂ and S₆) in its Lu₂O₃ host. It has been reported that afterglow mainly comes from Eu(S₆), where due to the centrosymmetric environment of S₆, only the magnetic dipole 4f-4f transitions of Eu³⁺can be recorded [16,17,18]. Thus, reducing the luminescence proportion of Eu(S₆) is one of the effective approaches to decrease the afterglow. According to our previous work, with the increase in Eu doping concentration, the proportion of Eu(S₆) in the afterglow decreases with the increase in Eu(S₆)→Eu(C₂) energy transfer efficiency, and the afterglow of Lu₂O₃:Eu gradually decreases [16]. In addition, the preparation atmosphere is also an important factor, affecting the scintillation performance. Zych investigated the effect of different atmospheres on the luminescence properties of Lu₂O₃:Eu ceramics [19]. There are no reports on the effect of growth atmosphere on the luminescence properties of Lu₂O₃:Eu crystals. Clarification of preparation atmosphere impact on photoluminescence and scintillation properties of Lu₂O₃:Eu crystals can contribute to the afterglow behavior optimization.

As reported, Lu₂O₃:Eu, with cubic structure, can be used both in single crystal and transparent polycrystalline ceramics. The advantages of ceramics are relatively low preparation temperature, preparation cost, high homogeneity, and large dimension samples [20,21,22]. Single crystal growth of Lu₂O₃ is a great challenge due to its super high 2400 °C melting point [23]. Several single crystal preparation methods are adopted to prepare the Lu₂O₃ single crystal, such as Czochralski (CZ) [23], the heat exchanger method (HEM) [24], the Bridgman method [25], laser-heated pedestal growth (LHPG) [26], the micro-pulling-down (m-PD) method [27], etc. The scintillation single crystal is the essential application form of a scintillator due to its higher transparency and excellent scintillation performance [28,29,30]. In scintillation properties study, a single crystal is preferred compared to ceramics because they eliminate the influence of the grain boundary [31]. In this work, Lu2O3:Eu single crystals were grown by the optical floating zone (FZ) method. The optical floating zone method, using optical heating, has advantages, such as having no crucible, no pollution, and fast growth rate, which show great superiority for some crystals that are difficult to grow (high melt point) or easy to contaminate.

In this paper, Lu₂O₃:1%Eu and Lu₂O₃:8%Eu crystals were prepared by the optical floating zone method under air, N₂, and H₂-Ar mixed atmosphere (5 vol%H₂ and 95 vol%Ar), respectively. The effects of the preparation atmosphere and Eu doping concentration on luminescence properties of Lu₂O₃:Eu scintillation single crystals have been studied. Their photoluminescence excitation (PLE), photoluminescence (PL), PL decay, X-ray excitation luminescence (XEL), and low temperature thermoluminescence (TSL) curves were also measured, and the underlying mechanism of the experimental phenomenon was discussed.

2. Experimental Procedure

2.1. Crystal Preparation

Lu₂O₃:1%Eu and Lu₂O₃:8%Eu crystals were prepared by the optical floating zone method under air, N₂, and H₂-Ar mixed atmosphere (5 vol%H₂ and 95 vol%Ar), respectively. The raw materials with 4N purity (Lu₂O₃ and Eu₂O₃) were weighted according to the nominal formula, sufficiently blended, and then shaped under the isostatic pressure of 200 MPa in the rubber tube. Polycrystalline rods were obtained after being sintered at 1700 °C for 8 h under air. A crystal growth process was carried out by a floating zone method in a vertical double-ellipsoid mirror furnace, equipped with 6500 W Xe arc lamp as heating source (SciDre HKZ, Dresden, Germany). The crystal preparation atmosphere is controlled by flowing gas into the quartz tube under atmospheric pressure. The light from a Xe arc lamp is focused through the two ellipsoid mirrors on the polycrystalline rods to heat and melt them. The crystal growth process is real-time monitored by a CCD camera, and the growth parameters, such as rotation and growth rates, are controlled by control software. The crystal growth rate is set to 5 mm/h, and the rotation speed of the polycrystalline rods is set to 5 r/min. At the end of crystal growth, the temperature is slowly reduced to room temperature by a cooling program. Due to the high melting point of rare earth sesquioxides (2400 °C), the process requires attention to adjust the growth parameters to keep the melt zone stable. Single crystals with a dimension of Φ5 mm × 1 mm were cut from the boules and polished on both sides, and the following measurements were carried out. The inset of Figure 1c shows the photos of Lu₂O₃:1%Eu and Lu₂O₃:8%Eu single crystal samples, where cracks in the samples are caused from the large thermal stress generated by the high temperature during crystal growth. The samples were analyzed by EDS with the line scan method, and the Eu content was found to be consistent with the theoretical value and homogeneously distributed. Lu₂O₃:1%Eu and Lu₂O₃:8%Eu samples prepared under air, N₂, and H₂-Ar mixed atmospheres are numbered as Lu-1-O, Lu-1-N, Lu-1-R, Lu-8-O, Lu-8-N, and Lu-8-R, respectively.

2.2. Measurements

Single crystal samples were grinded to powder to make a X-ray diffraction (XRD) measurement. The phase composition of samples was recorded utilizing Rigaku 18 KW D/MAX2500V+/PC, with Cu Kα1 radiation (λ = 1.54056 Å) in the 2θ reflection angle range of 10–90°, step 0.01°, speed 6°/min. The transmission spectrum over the range from 200 to 800 nm was measured using a Hitachi U-3900H spectrophotometer with a scanning speed of 600 nm/min and a step of 1 nm. A deuterium lamp was used as the light source, and the spectrometer was equipped with a photomultiplier tube (PMT) to detect signals in the UV-VIS wavelength region. The PLE and PL curves were measured on an Edinburg FLS 980 fluorescence spectrometer. The instrument employs time-correlated single photon counting (TCSPC) with a signal-to-noise ratio of 6000:1 and high spectral resolution, and the excitation source is a 450 W continuously operating Xe lamp. The PL decay curves, which were excited by a μF2 lamp under room temperature (RT), were recorded by using the Edinburg FLS 980 fluorescence spectrometer. Emission intensities are corrected according to the quantum efficiency curve of PMT. The XEL curves were recorded by using the XEL accessory in Edinburg FLS 920 fluorescence spectrometer, and the X-ray source is a tungsten X-ray tube operated at 40 kV and 40 μA. Afterglow curves were measured after 500 ms continuous X-ray irradiation provided by a tungsten X-ray tube operated at 15 kV and 15 μA, and the afterglow was collected by Hamamatsu R2059 PMT operated under -1200 V. Wavelength-resolved TSL measurements were performed in the 78–350 K range with a heating rate of 0.2 K/s after irradiation at 77 K (by a Philips 2274 X-ray tube operated at 70 kV and 1.5 mA), with a Jobin-Yvon Spectrum One 3000 CCD detector coupled to a Triax 180 Jobin-Yvon spectrometer operating in the 200–1100 nm interval. All the spectra were corrected for the detection response of the apparatus.

3. Results and Discussions

3.1. Sample Phase Identification

Figure 1a displays the XRD patterns of Lu₂O₃:1%Eu and Lu₂O₃:8%Eu single crystals grown under different atmospheres. The diffraction peaks of the samples match well compared to the standard Lu₂O₃ PDF card (PDF#12-0728). 2θ angle diffraction peaks around 20.88, 29.75, 34.49, 49.58, and 58.89 correspond to the Bragg diffraction of Lu₂O₃ at (211), (222), (400), (440), and (622) crystal faces, respectively. Impurities’ phases were not revealed at the XRD sensitivity limit. It can be demonstrated that the sample is a pure phase with a cubic structure and a space group of Ia3. Figure 1b presents the range of 28–31° for the sake of clarity. A slight shift to smaller 2θ angles for the samples with high doping concentrations of Eu³⁺ (the position of the highest peak changed from around 29.75 to around 29.63 degree) can be observed. Figure 1c shows the lattice constants of the samples calculated with the Bragg equation. The lattice constants are in the range of 10.39–10.40 Å for Lu₂O₃:1%Eu and 10.43–10.44 Å for Lu₂O₃:8%Eu. It is indicated that lattice expansion has occurred, which is due to the fact that Eu³⁺ ions have entered the host lattice, and the ionic radius of Eu³⁺ (0.947 Å) is larger than that of Lu³⁺ (0.861 Å), therefore more substitution of Eu³⁺ in the Lu³⁺ position in high concentration-doped samples results in a larger lattice constant.

Figure 1. (a) XRD patterns of Lu₂O₃:1%Eu and Lu₂O₃:8%Eu single crystals grown under different atmospheres. (b) Range of 28–31° for the sake of clarity. (c) Photograph of the samples and the calculated lattice constants a of the samples.

Figure 1. (a) XRD patterns of Lu₂O₃:1%Eu and Lu₂O₃:8%Eu single crystals grown under different atmospheres. (b) Range of 28–31° for the sake of clarity. (c) Photograph of the samples and the calculated lattice constants a of the samples.

Figure 2 represents the transmittance spectra of Lu₂O₃:1%Eu and Lu₂O₃:8%Eu single crystals grown under different atmospheres. The overall features are almost the same in all single crystals. The samples rise sharply between 300 nm and 400 nm, quickly reaching about 60–80% in most visible wavelength regions. The absorption features around 250 nm are due to the charge–transfer (CT) band of electrons transition from O²⁻ 2p orbital to the 4f orbital of Eu³⁺. The absorption peak of the sample at 350–70 nm may be attributed to the ⁷F_0,1→⁵D₄ transition of Eu³⁺ [13]. Absorption peaks at 466 nm and 535 nm are observed for Lu-8-O, Lu-8-N and Lu-8-R, which correspond to the ⁷F_0,1→⁵D₂ and ⁷F_0,1→⁵D₁ transitions of Eu³⁺, respectively. Lu-8-R exhibits the highest optical transmittance of 84% in the visible band, indicating its high crystal quality. The difference in transmittance of samples in the visible range is due to refraction and scattering caused by defects and cracks in the samples.

3.2. PL and Scintillation Properties

Figure 3 presents normalized PLE spectra of Lu₂O₃:Eu single crystals grown under different atmospheres. Considering that Lu₂O₃ contains two kinds of cation sites (C₂ and S₆), the characteristic emission wavelength of Eu³⁺ is 611 nm for the C₂ site and 582 nm for the S₆ site [32]. Therefore, the excitation spectra of the single crystals were monitored at 582 nm and 611 nm, as shown in Figure 3. It is presented that the intensity, as well as the position of the excitation peaks, does not change obviously in growth atmospheres. The excitation peaks at 241 nm and 250 nm of Lu₂O₃:Eu can be clearly observed, which belong to the CT transition of Eu³⁺. All excitation peaks in the 300-550 nm are attributed to the 4f-4f transition of Eu³⁺. Peaks around 300, 325, 360, 387, 398, 410, 467, and 533 nm can be assigned to the ⁷F_0,1→⁵F₂, ⁷F_0,1→⁵H_J, ⁷F_0,1→⁵D₄,⁵G_J, ⁷F_0,1→⁵L₆, ⁷F_0,1→⁵D₃, ⁷F_0,1→⁵D₂, and ⁷F_0,1→⁵D₁ transitions of Eu³⁺, respectively. In addition, the relative intensity of 4f-4f excitation peaks for Eu³⁺ increase obviously at high Eu³⁺ doping concentration.

PL curves were measured, and it was found that the luminescence intensity of the sample with 8% Eu doping concentration was significantly higher than that of the sample with 1% Eu doping concentration. Figure 4 shows normalized PL spectra of Lu₂O₃:1%Eu and Lu₂O₃:8%Eu series single crystals under 241 nm and 250 nm excitation, respectively. The main emission peak at 611 nm belongs to the ⁵D₀→⁷F₂ transition in the C₂ lattice of Eu³⁺ [33]. A series of emission peaks in the range of 575–595 nm can be attributed to the ⁵D₀→⁷F_0,1 transition of Eu³⁺. The peaks around 650 nm and 720 nm are assigned to the ⁵D₀→⁷F₃ and ⁵D₀→⁷F₄ transition of Eu(C₂) [34]. As presented in the inserts of Figure 4, the peak around 582 nm belongs to the ⁵D₀→⁷F_1a transition of Eu(S₆), and the peak around 587 nm can be attributed to the ⁵D₀→⁷F_1a transition of Eu(C₂). The relative luminescence intensity is roughly similar for the samples with different preparation atmosphere at the same Eu doping concentrations. As shown in the illustrations of Figure 4c,d, the relative luminescence intensity of Eu(S₆) gradually decreases with the enhancement of oxidizing atmosphere. It is observed that the relative intensity of Eu(S₆) in Lu-8-O is obviously lower than that of Lu-8-R. The trend is caused by the effect of different preparation atmospheres on the defect state inside the crystal. Due to the oxygen-deficient fluorite structure, there are intrinsic oxygen vacancies around S₆ and C₂ lattice sites in Lu₂O₃, which makes it easier for the external oxygen atoms to enter the crystal lattice to form interstitial oxygen. The interstitial oxygen defects exist in the form of hole traps in the Lu₂O₃ host, and the dramatic temperature gradient during the crystal growth process tends to cause composition bias, which results in a deeper trap depth of hole traps [34,35]. These interstitial oxygen defects prefer to locate around S₆ site, considering that the energy level of the S₆ site is higher than that of C₂ [35]. Some of the holes are bound by the interstitial oxygen atoms during the electron transition, which decrease Eu³⁺ luminescence of the S₆ lattice site. This tendency will be obvious with increasing oxidizing atmosphere.

Figure 5 shows the decay time curves of Lu₂O₃:Eu single crystals. Figure 5a,c illustrates the characteristic decay curve of Eu(S₆). Figure 5b,d illustrates the characteristic decay curve of Eu(C₂). The decay curves of Eu(S₆) and Eu(C₂) are analyzed by mathematical fitting with the following Equations (1) and (2), respectively.

$$I=A_1\exp (-t/{\tau }_1)+A_2\mathit{exp}(-t/{\tau }_2)+A_3\mathit{exp}(-t/{\tau }_3)$$

(1)

$$I=A\exp (-t/\tau )$$

(2)

$$P=\frac{A_j{\tau }_j\left(j=1, 2, 3\right)}{{{\displaystyle \sum }}_{i=1}^3{A}_i{\tau }_i}$$

(3)

where A₁, A₂ and A₃ are the parameters, and τ₁, τ₂ and τ₃ are three components of the decay curves, respectively. The fitting results for the decay curves are shown in

Table 1. The decay time fitted results of Lu₂O₃:1%Eu and Lu₂O₃:8%Eu single crystals.

	λex = 241 nm − λem = 582 nm						λex = 250 nm − λem = 611 nm
Sample	A1	τ1/ms	A2	τ2/ms	A3	τ3/ms	A1	τ1/ms
Lu-1-O	24,284	0.07 (6%)	3646	1.04 (14%)	4356	5.51 (80%)	13,542	0.95
Lu-1-N	24,885	0.07 (6%)	4514	1.03 (15%)	3793	5.80 (79%)	13,802	0.97
Lu-1-R	27,400	0.08 (6%)	2251	1.09 (8%)	4664	5.96 (86%)	14,013	0.93
Lu-8-O	5070	0.07 (4%)	8942	1.07 (63%)	1333	3.79 (33%)	11,936	0.99
Lu-8-N	36,086	0.06 (9%)	5900	1.19 (34%)	2480	4.85 (57%)	11,203	1.01
Lu-8-R	31,073	0.07 (8%)	4155	1.27 (19%)	3642	5.532 (73%)	12,201	0.99

. It can be observed that the decay curves of Eu(S₆) are fitted to three components of decay time. All samples exhibit a fast decay time component, ranging from 60–80 µs in the characteristic decay curve of Eu(S₆), which is the fast relaxation of ⁵D₁→⁵D₀ energy level [36]. The decay time components in the range of 1 ms and 5 ms are the characteristic decay times of Eu(C₂) and Eu(S₆), respectively [37]. The relative proportions of the three decay time components were also calculated, using Equation (3), based on the fitting results and are listed in Table 1. It can be found that the samples with high Eu doping concentration have a higher proportion of Eu(C₂) decay time components and therefore exhibit faster decay time than the samples with low Eu doping concentration. This resulted from that the Eu(S₆)→Eu(C₂) energy transfer, which is enhanced after high concentration doping, which eventually leads to the decrease in the luminescence of the S₆ site and a shorter decay time. On the other hand, it can be obviously observed that the samples grown under oxidizing atmosphere show the shortest decay time, which is more significant at high doping concentrations of Eu³⁺. It corresponds to the above inference that the formation of interstitial oxygen defect can inhibit the luminescence of Eu(S₆). In addition, it can be observed that the PL decay curves for the Eu(C₂) emission can be fitted through single exponential function, and the decay times are very close to each other around 1 ms.

Figure 6 shows the XEL spectra of Lu₂O₃:Eu single crystals. It is shown that the emission peak patterns under X-ray excitation of Lu₂O₃:1%Eu and Lu₂O₃:8%Eu series samples are very similar to those in the PL spectra, and all of them belong to the 4f-4f transition peak of Eu³⁺, and the strongest peak appears at 611 nm, belonging to the intense red emission peaks of Eu(C₂) ⁵D₀→⁷F₂ transition. The illustrations of Figure 6 show the normalized intensity around 587 nm. It can be observed that the peaks of Eu(S₆) around 582 nm grown under oxidizing atmosphere have the lowest luminescence intensity, which is consistent with the results obtained from PL spectra. This phenomenon can be explained by the same mechanism in PL behavior of Lu₂O₃. When oxidizing a preparation atmosphere environment, an oxygen atom enters the lattice of Lu₂O₃, and thus interstitial oxygen atom defects are formed, which act as hole traps.

Figure 7 shows the afterglow curves of Lu₂O₃:1%Eu and Lu₂O₃:8%Eu single crystals grown under different atmospheres after 500 ms of X-ray irradiation at room temperature. It can be clearly observed that the afterglow level decreases significantly at high Eu doping concentrations. The decrease in the afterglow of Lu₂O₃:8%Eu is due to the fact that the afterglow mainly comes from the luminescence of Eu(S₆), and the energy transfer of Eu(S₆)→Eu(C₂) is concentration-dependent, and the high efficiency of energy transfer at high concentrations leads to a significant decrease in the afterglow, which is consistent with the results we obtained in our previous work. In addition, different growth atmospheres have an effect on the afterglow of Lu₂O₃:Eu single crystals. As shown in Figure 6, Lu₂O₃ single crystals grown under slightly reducing atmosphere present the lowest afterglow level for both high and low concentrations, and the samples grown under air atmosphere also showed the highest afterglow intensity. It is well known that afterglow behavior is closely related to the defect trap state of the sample. TSL curves for the Lu₂O₃:Eu single crystals grown under air, N2, and H2-Ar mixed atmosphere are recorded and are shown in Figure 8 to investigate the effect of growth atmosphere and europium doping concentration on the defect traps of the Lu₂O₃:Eu scintillation single crystals.

TSL curves of Lu₂O₃:Eu single crystals are reported in Figure 8. The general-order kinetics model was adopted to fit the TSL curves (intensity I as a function of temperature T) by using the following simplified equation, Equation (4) [37]:

$$\begin{array}{c}I\left(T\right)=s{n}_0\mathit{exp}(-\frac{E_t}{k_{B}T})\\ \times {\left\{\frac{\left(l-1\right)s}{\beta }\times T\times \mathit{exp}\left(-\frac{E_t}{k_{B}T}\right)\times \left[\left(\frac{k_{B}T}{E_t}\right)-2{\left(\frac{k_{B}T}{E_t}\right)}^2+6{\left(\frac{k_{B}T}{E_t}\right)}^3\right]+1\right\}}^{l/\left(l-1\right)}\end{array}$$

(4)

where n₀ is the concentration of trapped charges at t = 0, E_t is the energy level of the trap, k_B is the Boltzmann constant, l is the kinetic order, s is the frequency factor, and β is the heating rate (0.2 K/s in this measurement). The fitting results of the TSL curves are presented in

Table 2. The TSL fitted results of Lu₂O₃:1%Eu and Lu₂O₃:8%Eu single crystals.

	Lu-1-O		Lu-1-N		Lu-1-R
No.	Tmax(K)	Etrap(eV)	Tmax(K)	Etrap(eV)	Tmax(K)	Etrap(eV)
1	103	0.31	106	0.33	93	0.22
2	116	0.37	148	0.44	107	0.31
3	136	0.39	178	0.51	125	0.36
4	176	0.50	193	0.53	186	0.51
5	193	0.53	251	0.72	206	0.56
6	258	0.72
7	294	0.84
	Lu-8-O		Lu-8-N		Lu-8-R
No.	Tmax(K)	Etrap(eV)	Tmax(K)	Etrap(eV)	Tmax(K)	Etrap(eV)
1	167	0.34	103	0.30	97	0.23
2	186	0.53	171	0.47	169	0.46
3	218	0.69	186	0.52	187	0.53
4			247	0.73	247	0.73

. For the three single crystal samples of Lu₂O₃:1%Eu, a total of five to seven TSL peaks were obtained, with trap depths in the range of 0.22–0.84 eV. For the three single crystal samples of Lu₂O₃:8%Eu, a total of three to four TSL peaks were obtained, with trap depths in the range of 0.23–0.73 eV. TSL peaks in the ranges of 100–150, 150–225, and 225–300 K are attributed to cation Frenkel, oxygen Frenkel defect, and Schottky defect, respectively [16]. The TSL peak around 243 K of three Lu₂O₃:8%Eu sample may be due to the defect caused by relatively large lattice expansion under high Eu doping concentration. It is generally accepted that the afterglow of the scintillator mainly comes from the carrier trap, corresponding to TSL peaks around room temperature [16]. As shown in Figure 8a–c, the intensity of TSL peaks around 225–350 K gradually decreases, corresponding to decreasing afterglow intensity of the Lu₂O₃:1%Eu samples. As for the Lu₂O₃:8%Eu samples, no peaks in the range of 225–350 K were observed in the TSL plots, which may be related to the lower Schottky defect concentration, corresponding to the lower afterglow intensities compared with Lu₂O₃:1%Eu samples.

Overall, the decrease afterglow intensity of Lu₂O₃:1%Eu and Lu₂O₃:8%Eu samples from oxidizing to weak reducing atmosphere is related to fewer intrinsic defect concentrations. In addition, the afterglow level has been shown to be influenced by the Eu(S₆)→Eu(C₂) energy transfer, and this effect is also one reason for the different afterglow intensities between Lu₂O₃ doped with low and high europium concentration.

4. Conclusions

In conclusion, Lu₂O₃:Eu scintillation single crystals with high(8%) and low(1%) Eu doping concentration under air, N₂, and H₂-Ar mixed atmosphere were grown by the optical floating zone method. The relative luminescence intensity of Eu(S₆) gradually decreases with the enhancement of the oxidizing atmosphere. The characteristic decay time of Eu(C₂) is about 1ms. The characteristic decay time of Eu(S₆) decreases with increasing oxidizing atmosphere. The Lu₂O₃:8%Eu sample grown under reducing atmosphere exhibited the lowest afterglow intensity. The high concentration of Eu³⁺ doping can improve the energy transfer efficiency of Eu(S₆)→Eu(C₂) to achieve the purpose of afterglow suppression. In addition, the afterglow intensity of Lu₂O₃:Eu is mainly influenced by internal defect traps. Weakly reducing atmosphere can inhibit the formation of oxygen atom interstitial defects inside the crystal. From the TSL test, it was found that the introduction of reducing atmosphere changed the trap state inside the sample and reduced the afterglow intensity.