Introduction In non-destructive testing of large devices, industrial computed tomography (CT) with high-energy radiation sources requires scintillators with a high performance of light yield, short decay time, high effective atomic number, and strong radiation resistance to achieve high-resolution imaging. Eu3+ doped Gd2O3-Lu2O3 solid solution ceramics have a high density and an effective atomic number, having high X-ray stopping power and radiation resistance. The ceramics have a high light yield that is beneficial to improving imaging quality. Moreover, Eu3+ emits a light in the red region, which matches well with the spectral sensitivity of silicon photodiodes. EuxLu1.4—xGd0.6O3 transparent ceramics are one of scintillators with great application prospects in the field of X-ray imaging. A recentl study managed to prepare commercial products of EuxLu1.4—xGd0.6O3 transparent ceramics with a high transparency and a high light output. However, such a research is still at the initial stage, and the optical quality of ceramics needs to be improved to meet practical application needs. There is also a lack of research on component design. Especially, the effect of Eu3+ concentration on the structure and spectral properties of EuxLu1.4-xGd0.6O3 ceramics is still unclear. In this paper, EuxLu1.4-xGd0.6O3 (x=0, 0.02, 0.06, 0.10, 0.14, 0.18) ceramics with different Eu3+concentrations were prepared via coprecipitation and subsequent vacuum sintering/hot isostatic pressing (HIP), and their microstructure and optical properties were analyzed.Methods In the synthesis of nano-powder, the obtained precursor was calcined in air at 1 100 ℃ for 4 h to obtain EuxLu1.4-xGd0.6O3 (x=0, 0.02, 0.06, 0.10, 0.14, 0.18) powders. The synthesized powders were molded in a 18 mm mold at 40 MPa, and then the ceramic green bodies were obtained by cold isostatic pressing at 250 MPa, pre-sintered in vacuum at 1 600 ℃ for 2 h and then sintered in argon atmosphere under 176 MPa at 1 750 ℃ for 2 h. The fabricated EuxLu1.4-xGd0.6O3 ceramics were annealed in air at 1 100 ℃ for 20 h to remove oxygen vacancies introduced by sintering in a reducing atmosphere.The microstructure of the sample was characterized by a model SU9000 field emission scanning electron microscope (Hitachi Co., Japan). The XRD pattern was detected by a model D8/DISCOVER DAVINCIX X-ray diffractometer (Brooke Co., Germany). The photoluminescence spectra (PL) and photoluminescence excited spectra (PLE) of ceramics were tested by a model FLS-980 fluorescence spectrometer (Edinburgh Instruments Ltd., UK). The performance of scintillating ceramics was characterized by an X-ray excitation emission spectrometer. The emission signals of the test samples were analyzed by a model QE65000 spectrometer (Ocean Optics Co., USA) excited by an X-ray tube operating at a voltage of 70 kV and a current of 1.5 mA. The transmittance curve was tested by a model Cary-5000 ultraviolet visible near-infrared spectrophotometer (Varian Co., USA).Results and discussion The EuxLu1.4-xGd0.6O3 (x=0, 0.02, 0.06, 0.10, 0.14, 0.18) powders are all cubic crystalline, and the diffraction peak position shifts towards a lower angle as Eu3+ doping concentration increases because the ion radius of Eu3+(i.e., 0.950 ?) or Gd3+(i.e., 0.938 ?) is larger than that of Lu3+(i.e., 0.848 ?). According to Scherrer’s formula, the average grain size of the powders gradually decreases from 63.7 nm to 55.0 nm as the concentration of Eu3+increases. The EuxLu1.4-xGd0.6O3 ceramics have uniform grain sizes, no obvious pores occur, and there is no second phase. The grain size measured by a linear intercept method decreases from 1.0 μm to 0.9 μm as the Eu3+doping concentration increases. This is because the lattice distortion caused by Eu3+doping suppresses the growth of ceramic grains in sintering. Compared to the prepared ceramics, the grain sizes of the ceramics after HIP increase. The average grain size of the ceramics after HIP decreases from 76.5 μm to 30.1 μm as the Eu3+doping concentration increases from 0 to 9%.The in-line transmittance of ceramics firstly increases and then decreases with the increase of Eu3+concentration. When x=0.1, the in-line transmittance of ceramics reaches a maximum of 71.4% at 611 nm. When the concentration of Eu3+is high, the lattice distortion inside the ceramic intensifies, leading to a decrease in the in-line transmittance. In the XEL patterns of EuxLu1.4-xGd0.6O3 ceramics, the emission spectrum is composed of Eu3+characteristic emission peaks. The main emission peak is located at 611 nm, which is an intense red light emitted from the 5D0→7F2 transition. When the Eu3+concentration is low, the emission intensity of the ceramic gradually increases with the increase of the number of emission centers. When the concentration of Eu3+exceeds 3%, the distance between the emission centers decreases. Also, the energy transfer between Eu3+ increases and the transition probability between Eu3+and defects increases due to the high energy of the excitation source, resulting in a decrease in scintillation luminescence intensity. The ceramic exhibits an intense red light emission matching well with silicon photodiodes.Conclusions The maximum in-line transmittance of the ceramic could be obtained at the Eu3+doping concentration of 5%, reaching 71.4% at 611 nm. The photoluminescence and photoluminescence excited spectra of EuxLu1.4-xGd0.6O3 ceramics demonstrated that increasing the concentration of Eu3+ could enhance the absorption and emission peak intensities of the ceramics. At the concentration of Eu3+ of 3%, the ceramic exhibited the most intense red light emission under X-ray excitation with the main emission peak located at 611 nm, corresponding to the 5D0→F2 transition of Eu3+. High transmittance and high red light emission intensity were of great significance for achieving a high-resolution X-ray imaging.