Akihiro Yamaji

Probing shallow electron traps in cerium-doped Gd3Al2Ga3O12 scintillators by UV-induced absorption spectroscopy

From measuring absorption spectra of cerium-doped Gd3Al2Ga3O12 (Ce:GAGG) and undoped GAGG crystals at low temperatures under UV-light irradiation, we find that they exhibit a broad band at around 12000 cm−1. This band is enhanced by high-temperature annealing under a hydrogen atmosphere. On the basis of present experimental results, the UV-induced band is assigned to shallow electron traps of defect complexes associated with oxygen vacancies. The UV-induced band completely disappears with Mg2+ codoping. We conclude that the Mg2+ codoping has the effect of inhibiting the formation of shallow electron traps, which realizes a faster scintillation response of Ce:GAGG.

Scintillation Properties of

Nd³⁺-doped Lu₂O₃ (Nd: Lu₂O₃) is a candidate for an infrared scintillator because (i) Lu₂O₃ has a high density of 9.5 g/cm³ and a high atomic number of 67 and (ii) Nd³⁺-doped materials emit in the infrared range and the emission lines from Nd³⁺ can be used in medical applications since human body has a transparency window between 600 and 1,100 nm. However, it is extremely difficult to fabricate Lu₂O₃ single crystals using conventional crystal growth methods because of the high melting point (2,510 ^° C). Using solid-state reactions, it is much easier to fabricate Lu₂O₃ into a ceramic structure. Therefore, Nd: Lu₂O₃ transparent ceramics were fabricated using a spark plasma sintering method. This technique is comparatively simple and consumes less time than other methods such as vacuum hot pressing. The scintillation properties and transmittance spectra of the as-produced ceramics were studied in both the visible and infrared regions. Radioluminescence spectra were measured in the range 800-1,200 nm. Nd³⁺ emission lines were observed in the transparency window of human body. Thus, these ceramic materials could be a candidate for medical imaging applications.

{\rm Nd}^{3+}

LiF/CaF2/LiBaF3 ternary eutectic scintillators were grown by the µ-PD method. In the solidified eutectic the phases were uniformly distributed in the transverse direction and aligned along the growth direction. For the Eu-doped samples, the expected emission peak observed at 425 nm was ascribed to Eu2+ 5d–4f transition from Eu:CaF2 under X-ray excitation. The LiF/CaF2/LiBaF3 ternary eutectic scintillators showed a light yield around 7,000 photons/neutron and decay time of 260 ns (73.6%) and 50 ns (26.4%).

-Doped

Ce and Ca co-doped LiGdF4-LiF eutectics were grown and their directionally solidified eutectic (DSE) system has been investigated. In these eutectics, apart from the expected 305 nm emission of the regular Ce3+, 310 nm emission of the Gd3+ and Ce3+-perturbed emission around 320-380 nm was observed. Ce3+-perturbed emission at 340 nm shows the intrinsic decay time of about 27 ns. In photoluminescence mechanism the observed decay time shortening of the regular Ce3+ and Gd3+ centers point to the energy transfer in the sequence the regular Ce3+→ (Gd3+)n→ the perturbed Ce3+ sites.

{\rm Lu}_{2}{{\rm O}_3}

Optical and scintillation properties of pure Cs2HfCl6 (CHC) single crystals were investigated. In particular, light output and energy resolution were measured using a Si avalanche photodiode (Si-APD), since the Si-APD has sufficient quantum efficiency of around 70 % at emission wavelength region of CHC around 420 nm. This CHC single crystal grown using the vertical Bridgeman method showed light output of 37,000± 2,000 photons/MeV . The FWHM energy resolution was determined to be 3.7± 0.5× (E/662 keV)−0.85± 0.03[%], where E [keV] is the gamma-ray energy. Moreover, the temperature dependence of the light output was stable from −5 to 30 oC, while the light output increased below −10 oC.

Ceramics in the Visible and Infrared Regions

We evaluated optical and scintillation properties of a Cr 0.5%:Y3Ga5O12 (YGG) and Cr 0.5%:Y3Al5O12 (YAG) crystals grown by the micro-pulling down method for infrared emission scintillators. In the photoluminescence and X-ray induced radioluminescence measurements, the emissions peaks of Cr:YGG were located in the longer wavelength than those of Cr:YAG. Radioluminescene spectrum of Cr:YGG had a broad peak at 716 nm related to Cr3+2E→4A2 and 4T→4A2 transitions.

LiF/CaF2/LiBaF3 ternary fluoride eutectic scintillator

We evaluated optical and scintillation properties of a 0.03% Cr-doped Gd₃Ga₅O₁₂ (GGG:Cr) single crystal grown by Czochralsk method for medical use. Since human body has low absorption region from approximately 700-1100 nm, near infrared scintillators would be applied to the real time dose monitor system. In the X-ray excited radioluminescence, the emissions related to ²E → ⁴ A₂ transitions of Cr³⁺ were observed at around 725 nm. The scintillation light yield of GGG: Cr 0.03% single crystal under 5.5 MeV alpha-ray excitation was determined to be 30% of BGO.

Growth and Luminescence Properties of Ce and Ca Co-Doped LiGdF4-LiF Eutectic Scintillator

In this study, we investigated the relation between crystallinity and scintillation properties of Pr:LuAG crystals grown by Czochralski (Cz) method. The crystallinity was determined as the dislocation density using the X-ray rocking curve technique. The scintillation pulse-height spectra were collected under 662 keV137Cs γ-ray excitation to evaluate the light yield and energy resolution. It was confirmed that both the light yield and energy resolution correlates with the crystalinity; however, the other factors also have an influence.

Cesium hafnium chloride scintillator coupled with an avalanche photodiode photodetector

Nonstoichiometric Lu3Al5+xO12 (x= 0.05, 0.15, 0.35, 0, -0.05, -0.15, -0.35) crystals were grown by the μ-PD method. Luminescence and scintillation properties such as absorption, excitation and emission spectra, light yield and decay time were evaluated. Expected anti-site defect related host emission have been observed in 250-420nm. Emission intensity was increased by increasing the nonstochiometry. The x=-0.35 sample showed the highest light yield of around 12000 photons/MeV and slowest scintillation decay time of 1.96μs.

Optical and Scintillation Properties of Cr Doped Y3Ga5O12 Crystal for Infra-Red Scintillators

Ce 0.1, 0.5, 1.0, and 2.0 mol%-doped K₂NaLuF₆ crystals with 2 mm in diameter and 30-50 mm in length were grown by micro-pulling-down method. The photoluminescence spectra under ultraviolet excitation showed intense emission ascribed to 5d-4f transitions of Ce³⁺ replacing Lu³⁺ isolated octahedral site. In the radioluminescenced spectra under alpha-ray irradiation, the emission band is due to the transition from the lowest 5d excited state of Ce³⁺ to ²<i>F</i>_5/2 ground state level is dominant. The light yields of Ce:K₂NaLuF₆ under 5.5-MeV alpha-ray excitation was estimated to be approximately 1.2 to 1.8% of those of BGO.