(Invited) Laser Sintering of Polycrystalline Ceramic Scintillators: The Case Study of YAG:Ce
Authors: dos Santos Jerre C.A. , Trofimov Artem A. , Sampaio David V. , da Silva Ronaldo S. , Jacobsohn Luiz G. 
Published: February 2020

Scintillators are sensors used for the detection and measurement of ionizing radiation that find application in numerous strategic fields. Among the many possible scintillator forms, polycrystalline ceramics have received increasing attention due to advantages over single crystals, including faster and lower cost fabrication methods, higher homogeneity of the dopant, greater shape control, and easier fabrication of materials with high melting temperatures. Polycrystalline ceramic scintillators are commonly fabricated by high temperature vacuum sintering [1] and more recently by spark plasma sintering [2,3]. However, additional high-cost processing by means of hot isostatic pressing is many times necessary especially because it has been shown that the use of sintering aids is detrimental to scintillation performance [4]. On the other hand, despite the advantages of extremely short sintering times and high heating rates, laser sintering remains mostly unexplored as a viable method to fabricate these materials. To date, the only polycrystalline ceramic scintillator fabricated by this method was Bi4Ge3O12 (BGO) [5-7]. In this work, the fabrication of Y3Al5O12:Ce (YAG:Ce) polycrystalline ceramic scintillators by laser sintering is evaluated against a Czochralski-grown single crystal in terms of the microstructure characteristics as well as luminescent and scintillating properties.
YAG:Ce powders were prepared by a polymeric precursor method using Y(NO3)3×6H2O, AlCl3×6H2O, and CeH8O18N8 as metal precursors such that the concentration of Ce substituting for Y was 0.1 and 0.3 %. Metal precursors were dissolved in citric acid:distilled water solutions at 70 oC under continuous stirring followed by the addition of ethylene glycol. The final solution was heated up to 100 oC to eliminate water and promote polymerization. The resultant material was pre-calcined at 600 oC for 5 h followed by calcination at 1000 oC for 6 h, and then pressed into 4 mm dia. x 1.2 mm thick pellets. Laser sintering was executed using a Coherent GEM-100L CO2 laser generating a power density of ~3.3 W/mm2for 90 s. The microstructure was characterized by means of density, X-ray diffraction (XRD), scanning electron microscopy (SEM), attenuated total reflectance Fourier transform infrared spectroscopy (ATR FTIR), and X-ray absorption near edge structure (XANES) measurements. Optical transparency and luminescence were evaluated in terms of UV-visible transmittance, photoluminescence emission (PL) and excitation (PLE), radioluminescence (RL), and thermoluminescence (TL) measurements, while differential pulse height distribution measurements were used to characterize the scintillation response as a function of the gamma-ray energy.
Density measurements revealed the laser beam-induced densification yielded 97-99 % relative density. SEM results confirmed the presence of pores and an average grain size of 2 mm was extracted from image analysis. These results justified the optically opaque nature of the polycrystalline ceramics due to light scattering at the numerous internal interfaces. In addition to that, a higher degree of structural disorder in the polycrystalline ceramics that was revealed by inhomogeneous broadening of the optical absorption, PLE and absorption ATR FTIR bands when compared to the single crystal. XANES results of the YAG:Ce polycrystalline ceramics together with Ce(OH)CO3 and CeO2 commercial powders used as references for the 3+ and 4+ oxidation states of Ce, respectively, demonstrated the dominance of the +3 state. PL and RL confirmed luminescence to be dominated by the emission of Ce3+, and showed the presence of F+-related centers and antisite defects in both the polycrystalline ceramics and the single crystal. TL analysis revealed the presence of four first-order kinetics glow peaks, with the trap depths of the single crystal being similar or deeper than those of the polycrystalline ceramics. In terms of scintillation response, the relative light yield of the polycrystalline ceramics was significantly lower than that of the single crystal as expected from its optically opaque nature. Also, the polycrystalline ceramics presented a less linear energy response than the single crystal.

Acknowledgements

This material is based upon work supported by the National Science Foundation under Grant no. 1207080. This work has been supported by the Brazilian Synchrotron Light Laboratory (LNLS) beamline XAFS2 under proposals 15871 and 18854.

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