Inorganic scintillation crystals are widely used in high-energy particle detection, medical imaging, nuclear physics, and other fields^[1]. Due to the development of high-energy physics and ultrafast pulsed radiation detection, the demand for ultrafast scintillators is becoming more and more urgent.

Ytterbium-doped yttrium aluminum garnet ($\mathrm{Yb}\text{:}{\mathrm{Y}}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$, Yb:YAG) is a traditional material with excellent comprehensive performance, such as good thermal conductivity, optical performance and excellent chemical stability. Up to now, Yb:YAG crystals have been widely used as high-power laser materials^[2–4].

Besides, Yb:YAG is also a very important inorganic scintillator since it possesses an ultrafast decay time (0.41 ns excited by 266 nm pulsed laser)^[5], which shows potential applications in pulsed radiation imaging, inertial confinement fusion (ICF) diagnosis, nuclear reaction kinetics diagnosis, and homeland security^[6–9].

The study of Yb:YAG scintillators started in 1997, when Raghavan et al. reported that 15% (mass fraction) Yb can be used to detect low-energy solar neutrinos^[10]. In 2000, van Pieterson et al. systematically studied the charge-transfer (CT) luminescence behavior of Yb ions in different compounds and estimated the thermal quenching temperature of Yb:YAG at $\mathit{T}=80\mathrm{K}$^[11]. In 2001, Guerassimova et al. reported the X-ray excited CT luminescence at $\mathit{T}=80\mathrm{K}$. The luminescence peaks at 333 nm and 500 nm belong to CT luminescence based on the luminescence spectrum^[12]. The CT state refers to the transfer process of electrons from oxygen ligands to rare earth ions (${\mathrm{Yb}}^{3+}$). CT luminescence refers to the energy transfer from the CT state to the two energy levels of ${{}^2\mathrm{F}}_{7/2}$ and ${{}^2\mathrm{F}}_{5/2}$. The Stokes shift of the CT state is $17,500{\mathrm{cm}}^{-1}$ reported by van Pieterson et al.^[11], while Antonini et al. reported on the dependency between light yield (LY) and temperature. In more detail, the maximum LY is $(13.5\pm 2.5)\times {10}^3\mathrm{ph}/\mathrm{MeV}$ at 140 K for 25% Yb:YAG, and the decay time $\tau$ is shorter than 4 ns under the $\mathrm{\gamma }$ source^[13].

Since Yb:YAG features extremely low LY in comparison with commercial scintillators like bismuth germanium oxide (BGO) and cesium-doped lutetium-yttrium oxyorthosilicate (Ce:LYSO), it is only suitable in the application of high-intensity pulsed gamma ray measurement. In previous studies, Chen et al.^[7] reported that the fluence rate linear response upper limit of the Yb:YAG crystal is about $9.1\times {10}^{18}\mathrm{MeV}·{\mathrm{cm}}^{-2}·{\mathrm{s}}^{-1}$, and the sensitivity of the Yb:YAG detector is $6.16\times {10}^{20}\mathrm{C}·{\mathrm{cm}}^{-2}·{\mathrm{MeV}}^{-1}$. In order to further improve the detecting capacity, the gamma ray cut-off ability and the radiation hardness of the Yb:YAG single crystal should be improved.

Preliminary studies have shown that doping of host elements can modulate the crystal scintillation properties. The Ce:LuAG has better energy resolution ($6.7\%\pm 0.3\%$) than Ce:YAG, while the scintillation decay time shows a longer slow component for the Ce-doped lutetium aluminum garnet (Ce:LuAG) with respect to Ce:YAG^[14]. Ce:LYSO has higher LY ($37,400\pm 3700\mathrm{ph}/\mathrm{MeV}$) but the worse energy resolution ($7.7\%\pm 0.2\%$) with respect to Ce-doped yttrium oxyorthosilicate (Ce:YSO)^[15]. Thus, we can conclude that Y-Lu solid solute plays a huge role in the improvement of scintillator properties. However, Yb-doped YAG-LuAG solid solute has not been investigated systematically.

Therefore, in order to regulate the density and explore the relationship between the host atom substitution in the Yb:LuYAG system and spectral properties, we grow the single crystals of ${({\mathrm{Yb}}_{0.15}{\mathrm{Lu}}_{0.85x}{\mathrm{Y}}_{0.85-0.85x})}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$ ($x=0$, 0.25, 0.5, 1) by the Czochralski method. With the same valent state and similar atom size, ${\mathrm{Lu}}^{3+}$ can substitute ${\mathrm{Y}}^{3+}$ with an arbitrary value ranging from 0 to 100% (atomic fraction). The atom fraction of Yb is 15% in all the crystals. Due to the extremely low LY of Yb:YAG crystals, the spectrally integrated intensities of the X-ray excited luminescence (XEL) spectra are widely used to evaluate their LY at room temperature. Thus, the XEL properties and decay time of the crystals were characterized and analyzed in detail.

${({\mathrm{Yb}}_{0.15}{\mathrm{Lu}}_{0.85x}{\mathrm{Y}}_{0.85-0.85x})}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$ ($x=0$, 0.25, 0.5, 1) crystals were grown by the Czochralski method with a mid-frequency induction heating system. The raw materials were high-purity ${\mathrm{Yb}}_2{\mathrm{O}}_3$ (5N purity), ${\mathrm{Y}}_2{\mathrm{O}}_3$ (5N purity), ${\mathrm{Al}}_2{\mathrm{O}}_3$ (5N purity), and ${\mathrm{Lu}}_2{\mathrm{O}}_3$ (5N purity). The stoichiometry of the starting materials was weighed accurately under the formula (Yb_0.15Lu_0.85xY_0.85-0.85x)₃Al₅O₁₂ ($x=0$, 0.25, 0.5, 1). The crystals were grown with $\langle 111\rangle$ orientation in Ar atmosphere. The general cylindrical shape of the as-grown (Yb_0.15Lu_0.85xY_0.85-0.85x)₃Al₅O₁₂ ($x=0.25$, 0.5, 1) is shown in Fig. 1. The crystals grow in an oxygen-deficient environment with a high concentration of oxygen vacancies. Annealing in air can reduce the defect density and remove the thermal stress during growth. All crystals were annealed at 1200°C for 12 h. The as-grown crystals were blue and changed to colorless after the annealing process. The crystals were cut and polished for optical measurements, and the size of the sample is 10 mm × 10 mm × 1 mm (Fig. 2).

The density values of the four samples were measured using the Archimedes method. The optical properties were characterized by a PerkinElmer Lambda 1050 UV/VIS/NIR spectrometer (Massachusetts, USA). The XEL spectra and decay time profiles by pulse laser at 213 nm were recorded by a luminescence spectrometer (Edinburgh Instrument FLS1000, Edinburgh, UK). The X-ray source with Ag target operating at 50 kV and 15 µA was used as an excitation source. The pulse width of the pulse laser at 213 nm is 43.102 ps.

The density of single crystals for (Yb_0.15Lu_0.85xY_0.85-0.85x)₃Al₅O₁₂ ($x=0$, 0.25, 0.5, 1) is shown in Table 1. The density increases linearly with increasing of ${\mathrm{Lu}}^{3+}$ content, which is consistent with Eq. (1), and the adjusted ${\mathit{R}}^2=0.99668$: $$y=a+b·x,$$(1)where intercept $a=4.872$, and slope $b=1.907$.

Transmission spectra as a function of incident wavelength (200–1200 nm) for ${({\mathrm{Yb}}_{0.15}{\mathrm{Lu}}_{0.85x}{\mathrm{Y}}_{0.85-0.85x})}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$ ($x=0$, 0.25, 0.5, 1) single crystals annealed in air are shown in Fig. 3. It can be seen that the transmittance curves of these samples are similar in the range of 300–1200 nm, and the transmittance has been maintained around 80% in the range of 300–878 nm. The absorption bands of ${\mathrm{Yb}}^{3+}$ were centered at 940 nm corresponding to the 4f−4f transition. There is no significant difference in the absorption curves for the four samples near 940 nm. The absorption of the single crystals for ${({\mathrm{Yb}}_{0.15}{\mathrm{Lu}}_{0.85x}{\mathrm{Y}}_{0.85-0.85x})}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$ ($x=0$, 0.25, 0.5, 1) below 300 nm is related to the defects and impurity in the crystals^[16]. The absorption band near 255 nm, which is believed to arise from ${\mathrm{O}}^{2-}$ to ${\mathrm{Fe}}^{3+}$ CT^[17]. According to the research of Fagundes-Peters et al.^[18], ${\mathrm{Fe}}^{3+}$ impurities are usually unavoidable, which were caused by an iridium crucible and the growth environment. The ${\mathrm{Fe}}^{3+}$ ion will occupy the tetrahedral and octahedral ${\mathrm{Al}}^{3+}$ sites in the lattice^[19], and a small amount of ${\mathrm{Fe}}^{3+}$ can seriously affect the induced optical losses after ultraviolet irradiation^[20]. As shown in Fig. 3, (Yb_0.15Lu_0.2125Y_0.6375)₃Al₅O₁₂ has the highest transmittance near 255 nm, which needs further study.

The ${\mathrm{Lu}}^{3+}$ occupies the ${\mathrm{Y}}^{3+}$ site in the (Yb_0.15Lu_0.85xY_0.85-0.85x)₃Al₅O₁₂ crystal, and the radius of the ${\mathrm{Lu}}^{3+}$ ($\mathrm{radius}=0.083\mathrm{nm}$) is smaller than that of the ${\mathrm{Y}}^{3+}$ ($\mathrm{radius}=0.090\mathrm{nm}$), resulting in a smaller ligand size around the oxygen^[21,22]. Therefore, the energy of the electron transfer from the oxygen ligand to the rare earth ions increases, and the CT absorption band will appear as blue-shifted.

Figure 4 shows the XEL spectra of crystals for ${({\mathrm{Yb}}_{0.15}{\mathrm{Lu}}_{0.85x}{\mathrm{Y}}_{0.85-0.85x})}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$ ($x=0$, 0.25, 0.5, 1) at room temperature. The luminescence band near 330 nm and 500 nm corresponds to the transition from the CT state to the ${{}^2\mathrm{F}}_{7/2}$ ground state and to the ${{}^2\mathrm{F}}_{5/2}$ excited state. The result is consistent with the report of Guerassimova et al.^[12]. According to the position of the emission peaks in the XEL spectrum, the energy separation between ${{}^2\mathrm{F}}_{7/2}$ and ${{}^2\mathrm{F}}_{5/2}$ is $9761{\mathrm{cm}}^{-1}$ for all the samples. The energy separation is higher than $9300{\mathrm{cm}}^{-1}$ measured at 80 K reported by Nikl et al.^[23], possibly due to the increase of temperature.

According to Fig. 4, the luminescence intensity of the sample gradually increases with the increase of ${\mathrm{Y}}^{3+}$ concentration, which may be related to the concentration of the defect in the crystals^[24]. There is a non-stoichiometric growth phenomenon during the growth of the YAG single crystal^[25]. The existence of ${\mathrm{Y}}_{\mathrm{Al}}$ antisite defect is beneficial to the composition deviation to restore equilibrium. The antisite defects will lead to an increase in the volume of octahedral sites in the lattice^[26]. Therefore, the covalency of the lattice will increase. This leads to the splitting of energy levels at the bottom of the conduction band, resulting in localized energy levels in the band gap. These localized energy levels act as carrier traps, which adversely affect the scintillation process^[27]. The antisite defects concentration in the crystal is related to the radius of rare earth ions and melting point. The defects concentration increases with the decreasing rare earth ion radius^[17,28]. According to Zorenko et al., no antisite defects were observed in Ce:YAG single crystal films (SCFs) samples prepared at low temperatures^[29]. Therefore, the antisite defect concentration may increase with increasing crystal growth temperature. The melting point of the (Yb_0.15Lu_0.85xY_0.85-0.85x)₃Al₅O₁₂ increases with the ${\mathrm{Lu}}^{3+}$ concentration^[30]. Thus, the crystals with higher ${\mathrm{Lu}}^{3+}$ concentrations have higher concentrations of the antisite defects.

The relationship between ${\mathrm{Lu}}^{3+}$ concentration and XEL integral intensity in the range of 280–400 nm is shown in Fig. 5. The black dot plot is the integral intensity with (Yb_0.15Lu_0.85xY_0.85-0.85x)₃Al₅O₁₂, and the red curve is the fitting result. The integral intensity decreases exponentially with increasing of ${\mathrm{Lu}}^{3+}$ content in the range of 280–400 nm, which is consistent with Eq. (2), and the adjusted ${\mathit{R}}^2=0.99922$: $$y=y_0+A·\exp (R_0·x).$$(2)

The ${({\mathrm{Yb}}_{0.15}{\mathrm{Lu}}_{0.85x}{\mathrm{Y}}_{0.85-0.85x})}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$ decay time is shown in Fig. 6. All decay curves follow the single-term exponential of Eq. (3): $$I(t)=I_0·\exp (-t/\tau ),$$(3)where $I_0$ is the intensity at zero time, and $\tau$ is decay time.

The decay time values of the single crystals for ${({\mathrm{Yb}}_{0.15}{\mathrm{Lu}}_{0.85x}{\mathrm{Y}}_{0.85-0.85x})}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$ ($x=0$, 0.25, 0.5, 1) at two wavelengths (330 nm, 500 nm) are listed in Table 2. All values were relatively close in the range of 0.78–1.34 ns, which are comparable to 0.41 ns excited by the 266 nm pulsed laser in Ref. [5]. However, Nikl et al.^[23] reported the 340 nm luminescence decay times of Yb:YAG and Yb:LuAG at 7 K, which were 75.8 ns and 51.4 ns, respectively, much longer than the fitting results in this paper. The reason is that the luminescence decay time shortened rapidly with increasing temperature, showing an obvious temperature quenching phenomenon^[31]. The decay times of each wavelength for the four samples are less than 2 ns, indicating that the ${({\mathrm{Yb}}_{0.15}{\mathrm{Lu}}_{0.85x}{\mathrm{Y}}_{0.85-0.85x})}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$ single crystals have a good application prospect in ultrafast scintillation.

Besides, the decay times of the crystals for (Yb_0.15Lu_0.85xY_0.85-0.85x)₃Al₅O₁₂ shorten with the increasing of ${\mathrm{Lu}}^{3+}$ concentration, which shows a strong relationship. Therefore, we depict the decay time at 330 nm with the concentration of ${\mathrm{Lu}}^{3+}$ in Fig. 7 and fit the curve with Eq. (4). The curve follows a quadratic function, and the adjusted ${\mathit{R}}^2=0.99995$. The trend of decay time is consistent with the XEL spectrum. The variation of these decay times may be related to the change of XEL intensity. Therefore, the crystal decay time and density can be smoothly tuned by ${\mathrm{Lu}}^{3+}$ concentration: $$y=\exp (0.11+6.73\times {10}^{-4}·x-4.3\times {10}^{-5}·x^2).$$(4)

The ${({\mathrm{Yb}}_{0.15}{\mathrm{Lu}}_{0.85x}{\mathrm{Y}}_{0.85-0.85x})}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$ ($x=0$, 0.25, 0.5, 1) crystals were obtained by the Czochralski method. The optical properties, XEL spectra, and luminescence decay time of the Yb-doped YAG-LuAG solid solute system were analyzed in detail. Compared with Yb:YAG, ${\mathrm{Lu}}^{3+}$ can effectively improve the effective atomic number and density of the crystal, which would have a better application prospect in the field of high-intensity detection^[32]. Besides, the XEL intensity and luminescence decay time can be accurately regulated by the Lu:Y ratio, which provides flexible choices for different application scenarios.