Development and optical property of Nd1%-doped Lu₂O₃ single crystal scintillator grown by the core heating method

Recently, development of oxide scintillators with the high stopping power have been needed for the remote-dosimetry system in the decommissioning of the Fukushima Daiichi Nuclear Power Plant. The emission wavelength of the oxide scintillators for the remote-dosimetry system should be red to near-infrared (NIR) emission in order to distinguish the scintillation signal from the backgrounds under high-dose condition more than 10 Sv/h. Nd³⁺ doped Lu₂O₃ is one of the potential materials for the remote-dosimetry system because it has a high density of 9.4 g/cm³, a high effective atomic number of 69 and the NIR emission deriving from Nd³⁺. However, Lu₂O₃ having the melting point of 2,500℃ is difficult to grow with the conventional crystal growth techniques using an iridium (Ir) crucible which has the melting point around 2,400℃. The novel crystal growth method called the 'Core Heating (CH) method' was developed and it enables to grow the material having the melting point over 2,400℃ in short time relative to the conventional crystal growth method. In this study, we attempted to grow Nd³⁺ 1%-doped Lu₂O₃ (Nd:Lu₂O₃) as the NIR emitting oxide scintillator for the remote-dosimetry system with the CH method, and Nd:Lu₂O₃ single crystal was successfully grown. The largest as-grown crystal was approximately 10 x 10 x 3 mm³ and it was cut and polished to a size of 5.0 x 5.0 x 1.8 (thickness) mm³. Phase identification of Nd:Lu₂O₃ crystal was performed by the powder X-ray diffraction measurement and the obtained diffraction patterns suggested single phase of Lu₂O₃. In the X-ray rocking curve (XRC) measurement, the clear sharp XRC was 126 arcsec in full width at half maximum. The transmittance, the X-ray excited radio luminescence (RL) and afterglow of Nd:Lu₂O₃ were measured for investigating the optical properties. Nd:Lu₂O₃ had the absorption peak of Nd³⁺ in the transmittance spectrum. The several peaks were observed in the RL spectrum from 870 to 974 nm originating from the ⁴F_3/2 → ⁴I_9/2 transitions of Nd³⁺ ions. The wavelength of the emission peak was 894 nm, and Nd:Lu₂O₃ crystal had a transmittance of 56% for 894 nm. The afterglow intensity at the X-ray-off for Nd:Lu₂O₃ decreased to 1% at 17 seconds and 0.1% at 150 seconds. Whereas more study regarding the scintillation properties for Nd:Lu₂O₃ was needed under the high-dose rate condition more than 10 Sv/h, we successfully developed NIR emitting oxide scintillator of Nd:Lu₂O₃ single crystal with the CH method. (author)