Single-crystal lutetium yttrium oxyorthosilicate doped by cerium (LYSO:Ce) has been widely adopted as a scintillator material for radiation detection. The demands from nuclear medical imaging, as well as high-energy physics, are calling for ever-increasing scintillation properties. Here, Al 3+ codoping of LYSO:Ce is realized for the first time, and a series of LYSO:Ce,xAl (x = 0, 0.3, 0.6, and 1.0 atom %) crystals are successfully grown by a micro-pulling down (μ-PD) method. It is found that at x = 0.6 atom %, the overall scintillation properties are simultaneously improved. In particular, the slow component of rise time and the afterglow level are drastically reduced by 350% (from 304 to 86 ps) and an order of magnitude (from 0.2 to 0.02%), respectively. The energy resolution is also enhanced from 16.1 to 10.9%. These results are attributed to the fact that Al 3+ codoping significantly reduces the concentration of traps, accelerates the excited state lifetime of Ce 3+ ions and introduces a faster scintillating luminescence center, Ce 4+ ions. Our LYSO:Ce,0.6 atom % Al crystal offers a gain in the image signal-to-noise ratio of 1.4 over the non-codoped crystal, which will significantly benefit the next-generation time-of-flight positron emission tomography and enable more precise diagnosis of diseases.
In coaxial laser cladding, the quality and property of deposition products are greatly influenced by the powder flow, which is responsible to transport additive materials to the deposition point on a substrate precisely. The metallic powder flow in coaxial laser cladding is simulated by a numerical model based on the gas-solid flow theory. The characteristics of powder concentration distribution between coaxial nozzle and deposition point for a kind of nickel based alloy powder are studied by the proposed model. The relationship between the process parameters and powder flow characteristics, such as focus distance from the nozzle exit and maximum powder concentration, is analyzed to optimize the powder feeding process. In addition, the influence of substrate with different surface shapes on the powder flow is investigated. The results can be used as a guideline for the location of the substrate and the selection of proper processing parameters for coaxial laser cladding.
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