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Journal Articles

Visualizing cation vacancies in Ce:Gd$$_{3}$$Al$$_{2}$$Ga$$_{3}$$O$$_{12}$$ scintillators by gamma-ray-induced positron annihilation lifetime spectroscopy

Fujimori, Kosuke*; Kitaura, Mamoru*; Taira, Yoshitaka*; Fujimoto, Masaki*; Zen, H.*; Watanabe, Shinta*; Kamada, Kei*; Okano, Yasuaki*; Kato, Masahiro*; Hosaka, Masahito*; et al.

Applied Physics Express, 13(8), p.085505_1 - 085505_4, 2020/08

 Times Cited Count:1 Percentile:24.67(Physics, Applied)

To clarify the existence of cation vacancies in Ce-doped Gd$$_{3}$$Al$$_{2}$$Ga$$_{3}$$O$$_{12}$$ (Ce:GAGG) scintillators, we performed gamma-ray-induced positron annihilation lifetime spectroscopy (GiPALS). GiPAL spectra of GAGG and Ce:GAGG comprised two exponential decay components, which were assigned to positron annihilation at bulk and defect states. By an analogy with Ce:Y$$_{3}$$Al$$_{5}$$O$$_{12}$$, the defect-related component was attributed to Al/Ga-O divacancy complexes. This component was weaker for Ce, Mg:GAGG, which correlated with the suppression of shallow electron traps responsible for phosphorescence. Oxygen vacancies were charge compensators for Al/Ga vacancies. The lifetime of the defect-related component was significantly changed by Mg co-doping. This was understood by considering aggregates of Mg$$^{2+}$$ ions at Al/Ga sites with oxygen vacancies, which resulted in the formation of vacancy clusters.

Oral presentation

Development of gamma-ray induced positron annihilation lifetime spectroscopy

Taira, Yoshitaka*; Fujimoto, Masaki*; Fujimori, Kosuke*; Kitaura, Mamoru*; Zen, H.*; Okano, Yasuaki*; Hosaka, Masahito*; Yamazaki, Junichiro*; Kato, Masahiro*; Hirade, Tetsuya; et al.

no journal, , 

For general positron sources, radioisotopes such as $$^{22}$$Na are often used. However, there is a problem that positrons cannot probe the deep region of metal materials with a thickness of 1 mm or more. Gamma-ray induced positron annihilation lifetime measurement (GiPALS) is a method for generating positrons in bulk samples with a thickness of several centimeters and samples placed in vessels such as high temperature and/or pressure furnaces. The annihilation lifetime of positrons is about 200 ps for metal materials, so it is important to use gamma rays with a shorter pulse width for GiPALS in order to accurately measure the positron lifetime. We have succeeded in the proof-of-principle experiment for GiPALS of ultra-short pulse gamma rays with a pulse width of 2 ps, which was originally developed using 90$$^{circ}$$ collision laser Compton scattering at UVSOR.

Oral presentation

Development of gamma-ray induced positron annihilation lifetime spectroscopy at UVSOR

Taira, Yoshitaka*; Fujimoto, Masaki*; Fujimori, Kosuke*; Kitaura, Mamoru*; Zen, H.*; Okano, Yasuaki*; Hosaka, Masahito*; Yamazaki, Junichiro*; Kato, Masahiro*; Hirade, Tetsuya; et al.

no journal, , 

For general positron sources, radioisotopes such as $$^{22}$$Na are often used. However, there is a problem that positrons cannot probe the deep region of metal materials with a thickness of 1 mm or more. Gamma-ray induced positron annihilation lifetime measurement (GiPALS) is a method for generating positrons in bulk samples with a thickness of several centimeters and samples placed in vessels such as high temperature and/or pressure furnaces. The annihilation lifetime of positrons is about 200 ps for metal materials, so it is important to use gamma rays with a shorter pulse width for GiPALS in order to accurately measure the positron lifetime. We have succeeded in the proof-of-principle experiment for GiPALS of ultra-short pulse gamma rays with a pulse width of 2 ps, which was originally developed using 90$$^{circ}$$ collision laser Compton scattering at UVSOR.

Oral presentation

Vacancy-type defects in garnet crystals revealed by gamma-ray-induced positron annihilation spectroscopy

Kitaura, Mamoru*; Fujimori, Kosuke*; Taira, Yoshitaka*; Fujimoto, Masaki*; Zen, H.*; Hirade, Tetsuya; Kamada, Kei*; Watanabe, Shinta*; Onishi, Akimasa*

no journal, , 

Positron annihilation spectroscopy is the only way to investigate the properties of cation vacancies because they are negatively charged. We generated high-energy pulsed gamma rays by the vertical collision of an ultrashort pulse laser and electron beam. In this study, we investigated the vacancy-type defects present in the crystals of GAGG(Gd$$_{3}$$Al$$_{2}$$Ga$$_{3}$$O$$_{12}$$), GAGG: Ce and GAGG: Ce, Mg by positron annihilation lifetime spectroscopy using the high-energy gamma rays. The lifetime of the defect-related component was significantly changed by Mg co-doping. This was understood by considering aggregates of Mg$$^{2+}$$ ions at Al/Ga sites with oxygen vacancies, which resulted in the formation of vacancy clusters.

Oral presentation

Origin of phosphorescence in Ce:Gd$$_{3}$$Al$$_{2}$$Ga$$_{3}$$O$$_{12}$$ crystals revealed by gamma-ray induced positron annihilation lifetime spectroscopy

Fujimori, Kosuke*; Kitaura, Mamoru*; Taira, Yoshitaka*; Fujimoto, Masaki*; Zen, H.*; Hirade, Tetsuya; Kamada, Kei*; Watanabe, Shinta*; Onishi, Akimasa*

no journal, , 

We generated high-energy pulsed gamma rays by the vertical collision of an ultrashort pulse laser and electron beam. In this study, we investigated the vacancy-type defects present in the crystals of GAGG(Gd$$_{3}$$Al$$_{2}$$Ga$$_{3}$$O$$_{12}$$), GAGG: Ce and GAGG: Ce, Mg by positron annihilation lifetime spectroscopy using the high-energy gamma rays. The lifetime of the defect-related component was significantly changed by Mg co-doping. This indicates that the Al/Ga vacancies disappear. This fact corresponds well with the suppression of the phosphorescence component and is an important result showing that the Mg co-doping is effective in suppressing the shallow electron capture center.

Oral presentation

Positron annihilation spectroscopy using ultra-short pulsed laser Thomson scattered gamma-rays

Taira, Yoshitaka*; Fujimoto, Masaki*; Okano, Yasuaki*; Kitaura, Mamoru*; Hirade, Tetsuya

no journal, , 

Laser Thomson/Compton scattering is a unique technique to generate gamma-rays with features such as quasi-monochromatic and tunable energy, highly polarized, low divergence angle, and low background. We have developed GiPALS using an ultra-short pulsed gamma-ray generated at the synchrotron radiation facility UVSOR-III. The gamma-rays are generated via laser Thomson scattering with 90 degree collisions between a 750-MeV electron beam and a Ti:Sa laser. The pulse width of the gamma-rays is calculated to be 5 ps (FWHM). The gamma-ray induced positron annihilation lifetime spectroscopy (GiPALS) is currently available for users. In addition to GiPALS, we develop gamma-ray induced age-momentum correlation (GiAMOC), which measures the lifetime and Doppler broadening of annihilation gamma-rays simultaneously. In this conference, we will present a generation method of the ultra-short pulsed gamma-rays and details of GiPALS and GiAMOC.

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