Composition dependence of luminescence and scintillation properties of LuAG:Ce,Mg optical ceramics

Transcription

1 2016 Composition dependence of luminescence and scintillation properties of LuAG:Ce,Mg optical ceramics Shuping Liu 1, Jiří A. Mareš 2, Xiqi Feng 1, Martin Nikl 2, Yubai Pan 1, Huamin Kou 1, and Carmelo D Ambrosio 3 1 Shanghai Institute of Ceramics, Chinese Acad. Sci., Shanghai, China 2 Institute of Physics, AS CR, Cukrovarnická 10, 16200, Praha 6, Czech Republic 3 CERN, PH-LHB group, Geneva 23 CH1211, Switzerland

2 1. Background 2. Research work on Mg 2+ co-doped LuAG:Ce,Mg ceramics Fabrication process; Optical properties Ce 3+ /Ce 4+ contents; Luminescence and scintillation performance; Defect characterization- SEM, TSL measurements 3. Conclusion Outline

3 Research Background Ideal scintillators should have all these properties: High density High light yield Fast decay time Good energy resolution Radiation hardness Low cost T. Yanagida, et al. IEEE T Nucl Sci, 57 (2010) PEM Pr:LuAG Siemens PET/CT Ce:LSO

4 Why scintillation ceramic is proposed? Ceramics in our lab: Lu 3 Al 5 O 12 :Ce YAlO 3 :Ce Single crystals High melting point (~ C) Antisite defects No high doping concentrations High cost of growth Limited size of crystals Ceramics Preparation at lower T (~ C) High doping possible Composition tuning Bigger sizes and complex shapes are possible YAG/ Nd:YAG /YAG Pr:YAG / Yb:YAG PbWO 4 Ф100mm

5 Breakthrough in Ce doped LuAG:Ce ceramic Sample d(mm) L.O.(1μs)( p.e./mev) L.O.(1μs)( ph/mev) L.O.(10μs)( p.e./mev) L.O.(10μs)( ph/mev) LY 1μs /LY 10μs (%) LuAG:Ce SC pixel* LuAG:Ce SC ref LuAG:Ce,Mg ceramic * the highest values ever reported in literature so far.(j.a. Mares, et al. IEEE. T. Nucl. Sci. 59, (2012)2120) O 2- Ce 4+ Light yield of ph/mev has been achieved with short shaping time of 1μs. 22% higher than the highest values of LuAG:Ce single crystal pixel. 79% fast component contents What s the effect of Ce 4+ concentration in scintillation? S. Liu, et al., ISLNOM-6, October 20-23, 2013, Shanghai/ Phys. stat. sol., RRL 8, (2014) 105.

6 Main aim of this study: Mg 2+ co-doped LuAG:Ce,Mg optical ceramics Tuning of stable Ce 3+ /Ce 4+ ratios by Mg 2+ admixture; Optimization of scintillation performance; Comparison with the latest commercial LuAG:Ce single crystal.

7 Fabrication of LuAG:0.3%Ce,x%Mg ceramics Fabrication process: Solid-state reaction method & Air-annealing treatment (Lu 1-x-y Mg x Ce y ) 3 Al 5 O 12 : x=0.1~0.6%, y=0.3% Starting materials: α-al 2 O 3, Lu 2 O 3, CeO 2,MgO >99.99% purity Ball milling, Dry and Sieving Dry Pressing ~5MPa Cold Isostatic Pressing ~250MPa Vacuum Sintering ~1850ºC Non-annealed Air annealed ~1450ºC, 20h T%@550nm: >70% 2mm thick (i) The role of Mg 2+ : act as sintering aids, ensures to get high transmittance and optical quality ceramics.

8 Fabrication of LuAG:0.3%Ce,x%Mg ceramics Microstructure of the starting materials and the ceramics Starting materials Lu 2 O 3 After ball milling Ceramics Al 2 O 3 CeO 2 Surface of Ceramics Fracture microstructure

9 Optical properties of LuAG:Ce,Mg ceramics - Absorption spectra Samples names: Hxx, here xx stands for Mg concentration (H00, H01 etc.) Absorbance nm Ce 3+ transition to 5d 3,4,5 5d 2 5d 1 H00-non-an H01-non-an H02-non-an H03-non-an H04-non-an H05-non-an H06-non-an Absorbance nm H00-0%Mg-an H01-0.1%Mg-an H02-0.2%Mg-an H03-0.3%Mg-an H04-0.4%Mg-an H05-0.5%Mg-an H06-0.6%Mg-an 1 Before annealing 0.5 (a) Wavelength (nm) 1 After annealing (b) Wavelength (nm) V O + 2Ce Lu + O 2 2 O o + 2 Ce Lu Ce 4+ Absorption below 350 nm (peaking at 242 nm) region enhances remarkably after Mg 2+ codoping and air-annealing process. (Ce 4+ CT absorption)

10 Ce 3+ /Ce 4+ content with various Mg X-ray absorption near edge structure (XANES) Normalized intensity (b) Ce 3+ Ce 4+ H00-non-an H01-non-an H02-non-an H03-non-an H04-non-an H05-non-an H06-non-an CeO2 CeF3 *linear combination fitting: Sample Ce 3+ fraction (%) Ce 4+ fraction(%) H00-0%Mg H01-0.1%Mg H02-0.2%Mg H03-0.3%Mg H04-0.4%Mg H05-0.5%Mg H06-0.6%Mg * Y. Wu, et al., APL Materials, 2 (2014) Ce 4+ content increases with Mg 2+ admixture. 0.2 (a) Energy (ev) But not linearly. Why?

11 Defect characterization Thermoluminescence (TSL) Mg codoping could induce various of defects; MgO 2Lu( Ce) 2MgO 2Lu V O 2Ce Lu Lu O 2Mg 2 Lu( Ce) 2O Mg ' Lu O ' Lu V O 2Ce Ce Lu Lu ( Ce ( Ce 4 ) 4 ) O - center At Mg % - 0.3%, Oxygen related defects increase rather than Ce 4+ ions Measured Above after RT x-ray irradiation at 10 K at 20 Measured after x-ray irradiation at 20 ºC at Below RT kv (20 ma) 30 kv (10 ma) TSL amplitude (arb. un.) H01-non-an H02-non-an H00-non-an H05-non-an H03-non-an H05-non-an 0.3%Mg 0 doped ceramic shows x 0.04 the lowest 10 TSL intensity at low T. 5 H05-an T ( C) TSL amplitude (arb. un.) %Mg doped ceramic shows the highest TSL intensity above RT. 100 Strong signal reduction can be % Mg codoped observed after Mg codoping and airannealing. H00-an H03-an H01-an H05-an T (K)

12 Radioluminescence spectra of LuAG:Ce,Mg ceramics without Mg with Mg doping Single crystal RL efficiency: LuAG:Ce ceramics > LuAG:Ce,Mg ceramics > LuAG:Ce SC No emission related to anti-site defect (AD) observed in LuAG:Ce,Mg ceramics; (ii) The role of Mg 2+ : act as an aliovalent ion impurity, directly affect the point defect structure and the RL efficiency of LuAG.

13 Scintillation performance of LuAG:Ce,Mg shaping time dependent Light yield ~110% ~70% ~15% Annealed % Mg 2+ ceramics show the highest LY values; Annealed LuAG:Ce,Mg ceramics show faster scintillation response than LuAG:Ce crystal and 0%Mg doped LuAG:Ce ceramic.

14 Energy resolution non-proportionality for the best LuAG:Ce,Mg ceramic compared with the best LuAG:Ce SC ~8.1% for crystal ~4.9 % for ceramic ER: Annealed LuAG:Ce,Mg ceramic is better than LuAG:Ce SC; non-proportionality: LuAG:Ce SC is better than LuAG:Ce,Mg ceramic

15 Scintillation performance of LuAG:Ce,Mg - PL and scintillation decays no distinct changes in PL decay; relative intensity of fast component increases after Mg 2+ doping speed up the Scintillation response Sample PL decay τ 1 (ns)/ I 1 τ 2 (ns)/ I 2 (ns) (%) (%) LuAG:Ce SC - 70/ /66 H00-an 58 69/ /59 H06-nonan 58 54/ /53 H01-an 58 65/59 886/41 H03-an 59 64/58 981/42 H06-an 59 62/60 799/40 PL Amplitude (arb.un) H00-an H01-an H03-an H06-an I(t)= 6260 exp(-t/ 58.3 ns)+5 H06-non-an 1 exp fit I(t) Instr. response Time (ns)

16 Scintillation performance of LuAG:Ce,Mg - Afterglow Intensity(arb.units) H00-an H01-an H06-non-an H03-an LuAG:Ce SC-an H06-an BGO SC no Mg LuAG:Ce ceramics LuAG:Ce SC Time (ms) Annealed LuAG:Ce,Mg ceramics show much lower afterglow intensity when compared with that of LuAG:Ce single crystal; After x-ray cut-off: Sample Afterglow at 4 ms (%) / 400 ms (%) LuAG:Ce crystal / %Mg-an 6.78/ %Mg an / %Mg an / %Mg -an / %Mg non-an 2.62/0.117

17 Scintillation performance Afterglow optimization afterglow of non-stoichiometric LuAG:Ce,Mg ceramics afterglow of non-stoichiometric LuAG:Ce,Mg ceramics 1 1 Intensity (arb.units) L1-Lu3-1%AG:Ce,Mg L2-Lu3-2%AG:Ce,Mg L3-Lu3-3%AG:Ce,Mg L4-Lu3-4%AG:Ce,Mg LuAG:Ce SC-an BGO SC Intensity (arb.units) G1-Lu3+1%AG:Ce,0.2Mg G2-Lu3+4%AG:Ce,0.2Mg G3-Lu3+1%AG:Ce,0.6Mg G4-Lu3+4%AG:Ce,0.6Mg LuAG:Ce SC-an BGO SC Time (ms) Time (ms)

18 Why scintillation performance of LuAG:Ce,Mg ceramics is improved? 2 3 h (520nm) h (520nm) 3 CB Electron trap O Mg 2+ Ce 3+ Ce 4+ VB Electronic structure of O- center C. Hu, S. Liu, et al., Physica Status Solidi-RRL9 (2015) 245. (important) The effect of Mg 2+ : increase content of Ce 4+ centers, participate in scintillation,directly compete with electron traps for electron capture in step1; introduction of O - centers, providing an alternative fast channel for Ce 4+ centers to return back to its initial state efficiently in step 3.

19 Conclusions LuAG:0.3%Ce, x%mg ceramics with different Mg 2+ concentrations were developed. The content of Ce 4+ ions in LuAG:Ce,Mg ceramics increased nonlinearly with Mg 2+ concentrations. The highest LY was obtained when % Mg 2+ was codoped, which is comparable with the original Ce doping concentration. Compared with LuAG:Ce single crystal, LuAG:Ce,Mg ceramics are characterized by : Higher light yield, better ER & Faster scintillation response & Lower afterglow intensity better than that of the LuAG:Ce single crystal. The increase of Ce 4+ content and the introduction of O - centers could be the reasons for the improvement of scintillation properties of LuAG:Ce,Mg ceramics.

20 Acknowledgement We are grateful to the help of Prof. Anna Vedda and Prof. Mauro Fasoli from University of Milano-Bicocca for TSL measurements and analysis. The National Science Foundation of China (No , U ); Research Program of Shanghai Sciences and Technology Commission Foundation (No. 13JC ). Partial support of Czech Science Foundation project P204/12/0805. Thank you for your attention! Liu Shuping Ph.D student

21 SEM micrographs of the mirror-polished and thermal etched surfaces of LuAG:0.3%Ce,x%Mg ceramics. Mg 0.1 Mg 0.2 Mg um 10um 10um Mg 0.4 Mg 0.5 Mg um 10um 10um

22 X-ray flat panel detector imaging LuAG:Ce,Mg ceramics possess good imaging LuAG:Ce,Mg CsI resolutions; uniformity of luminescence still need to be improved;