Optimization and Characterization of Luminescent Materials for the ESS Proton Beam Imaging System
Abstract: Spallation sources such as European Spallation Source (ESS) produces neutrons by bombarding high-energy protons onto the target wheel. To determine the location of the proton beam a Proton Beam Imaging (PBI) system will be installed. The PBI system at ESS relies on the properties of a luminescent coating that is sprayed onto the target wheel and the proton beam window. In this study, luminescent materials for the PBI system are investigated and evaluated on their luminescent properties using photoluminescence spectroscopy. The material in focus is the Cr-doped alumina, which has been investigated in two parts. Sprayed coatings, made from an existing precursor, are investigated to understand how the luminescent behavior of Cr-doped alumina is affected by various factors in the ESS environment. Thereafter, a new Cr-doped alumina precursor is developed and optimized to produce a coating with superior properties. The existing precursor from the sprayed sample consists of a mixture of alumina and chromia that reacts during spraying to form Cr-doped alumina. This study compared the luminescent properties of two spraying techniques: plasma-spray and flame-spray. The results show that plasma spraying produces brighter coatings than flame spraying due to lower presence of the undesired η-alumina phase in the finished coating. The coating thickness optimization showed that 100 μm gave the brightest coating. Pure chromia was found in all the sprayed samples, which indicates that full incorporation of chromia into alumina has been unsuccessful. In this study, a new Cr-doped alumina precursor is proposed in which the powder is sintered prior to thermal spraying. Pre-sintering of the precursor material is suggested to produce a more controlled, homogeneous and complete incorporation of the trivalent chromium ions into the alumina matrix, hence producing a coating with superior luminescent properties. Pink, bright luminescent Cr-doped alumina powders were successfully prepared via two synthesis routes, the solid-state and solution-based synthesis, and compared based on their luminescent properties after heat treatment at different temperatures. The solution-based samples exhibited superior luminescence at temperatures of 1100 °C and 1300 °C, which is due to earlier transition into the η and α-alumina phase. Sintering powders at 1550 °C gave equally bright materials from both syntheses. The optimal doping concentration of trivalent chromium ions was 1 wt% of chromia. To spray the precursors, the particle size needs to be in the range of 5-60 μm to give satisfactory results. Therefore, the effect of ball milling on luminescence was studied. It was discovered that high-energy ball milling reduced the luminescence intensity significantly due to the violent crushing of the alumina crystals. Grinding the particles with a mortar and pestle could reduce the particle size sufficiently without the material losing its luminescent properties. For the last part of this study, two alternative materials for the PBI system were investigated: Ti-doped alumina and Ce-doped YAG. The Ti-doped alumina powder showed no luminescent properties and the study continued with Ce-doped YAG. This material was successfully produced, which created a yellow powder that exhibited a broad emission from the green to yellow region upon blue light excitation. A doping concentration screening that was performed revealed that 1.6 mol% gave the brightest material.
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