Lutetium aluminum garnet (Lu₃Al₅O₁₂, LuAG) is investigated as a classical scintillation material. LuAG can be fabricated in the form of single crystals, optical ceramics, thin films and other morphologies to meet different scintillation application demands. Cerium-doped LuAG (Ce:LuAG) ceramics can be used as a promising candidate for detection material in shashlik calorimeter for high-energy physics (HEP) experiments due to their high density, fast decay, efficient scintillation and relatively low cost. For HEP experiments with a high event rate, however, a slow scintillation component of Ce:LuAG ceramics leads to a pileup effect that needs to be diminished as much as possible. Divalent, optically inactive, Ca²⁺ co-doping is an effective strategy for suppressing slow scintillation component according to the defect engineering theory. In addition, the coprecipitated nano-powder can be also used as a raw material of ceramics, and improving the consistency of structure, composition and luminescence within the ceramics is also an important strategy to optimize the performance of Ce:LuAG scintillation ceramics. In this paper, Ce,Ca:LuAG ceramics with different Ce concentrations were prepared with the coprecipitation nano-powders. The effect of activator ion concentration on the decay behavior, light yield (LY) at different shaping times and afterglow intensity of Ce,Ca:LuAG ceramics were investigated.

A series of Ce,Ca:LuAG ceramics with the chemical composition of (Ce_xCa_{0.001 5}Lu_{0.998 5-x%})₃Al₅O₁₂; x=0.1, 0.2, 0.3, 0.4, 0.5 were prepared with coprecipitated nano-powders through vacuum pre-sintering and hot isostatic pressed (HIP) post-treatment. In the process of nano-powder preparation, high purity Lu₂O₃ (99.995%), CeO₂ (99.999%), and CaCO₃ powders were dissolved in nitric acid solution, and Al(NO₃)₃·9H₂O (AR) powders were dissolved in deionized water to obtain various nitrate solutions. The metal nitrate solutions were mixed and further diluted to a specific concentration. Ammonium hydrogen carbonate (AHC, 99%) was dissolved in de-ionized water as a precipitant solution, and ammonium sulfate (99%) was added into the precipitant solution as a dispersant. The mixed metal nitrate solution was dripped into the AHC solution under mild agitation at room temperature (RT). The resulting suspension was aged for 1 h and then washed with deionized water and ethanol. The precursor was dried, sieved and calcined to obtain Ce,Ca:LuAG nano-powders with a pure garnet phase. The powders were uniaxially dry-pressed into pellets at 20 MPa and then cold-isostatically pressed at 250 MPa. The green bodies were vacuum pre-sintered at 1 825 ℃ for 10 h, and then HIP in argon atmosphere under 176 MPa at 1 600 ℃ for 3 h. The fabricated Ce,Ca:LuAG ceramics were annealed in air at 1 450 ℃ for 10 h to remove oxygen vacancies introduced via sintering in a reducing atmosphere. The final Ce,Ca:LuAG ceramics were polished on the both sides to the thickness of 1 mm for the coming characterization.

The XRD patterns of all the ceramic samples are in reasonable agreement with the standard XRD patterns of LuAG (PDF 73-1368), indicating that the main phase of the prepared ceramics is a LuAG garnet phase. The lattice parameters of Ce,Ca:LuAG ceramics with different Ce concentrations increase gradually with increasing doping concentration. The presence of Ce³⁺ can act as a sintering aid to accelerate the migration of grain boundaries during sintering of garnet ceramics and increase the average grain size. The absorption spectra of Ce,Ca:LuAG ceramics indicate that the amplitudes of 4f→5d₁ transition at 447 nm and the 4f→5d₂ transition at 345 nm of Ce³⁺ increase with the starting content of Ce³⁺ that evidences its entering into the garnet lattice. An intense absorption below 350 nm is due to the charge transfer absorption band induced by Ce⁴⁺ in the garnet lattice. The photoluminescence decay curves of the 5d₁→4f emission of Ce³⁺ (510 nm) under the 452 nm excitation show that the fast component intensity I₁ of all Ce,Ca:LuAG ceramics is > 95%. The main decay time shows an acceleration at the Ce concentration of > 0.3% because the formation of closely spaced (Ce⁴⁺—Ca²⁺) pairs can increase with the increase of Ce concentration. The scintillation decay under γ-excitation slightly slows down with increasing Ce concentration, since the proportion of induced Ce⁴⁺ decreases with increasing total Ce concentration, and the competition in accelerating scintillation process becomes progressively weaker. The X-ray excited luminescence (XEL) intensity of the Ce,Ca:LuAG ceramics firstly increases and then decreases with the Ce concentration, reaching the maximum value as x=0.3. The integral scintillation efficiency of Ce,Ca:LuAG ceramics is 146%-190% of a BGO crystal. The variation of the LY and the integral scintillation efficiency obtained from XEL have a similar pattern, i.e., increasing and then decreasing with the increase of Ce concentration. The optimum energy resolution (ER) is obtained as x=0.3. The normalized afterglow intensity after continuous X-ray irradiation monotonically decreases with the Ce concentration due to the more intense competition ability of cerium centers for charge carrier capture.

Ce,Ca:LuAG scintillation ceramics with different Ce concentrations were prepared with coprecipitation nano-powders via vacuum pre-sintering and HIP post-treatment. A small amount of Al-rich secondary phase existed in the ceramics due to the loss of lutetium during coprecipitation and washing, which could be avoided by component design in the future ceramic preparation. At the Ce concentration of 0.3%, Ce,Ca:LuAG ceramic had the optimum XEL integral efficiency, LY and ER. This study demonstrated that Ce doping concentration had a certain effect on the PL decay, scintillation decay and afterglow of Ce,Ca:LuAG ceramics, providing a guidance for the subsequent selection of doping concentration in practical applications.