Speaker
Description
Eu2+ doped LiCaAlF6 and LiSrAlF6 [Eu:LiCAF, Eu:LiSAF] single crystals have been investigated as a neutron scintillator for homeland security. The Eu:LiCAF and Eu:LiSAF single crystals indicated high light yield, ~30,000 photons/neutron, compared to present neutron scintillators. However, the segregation coefficient, keff, of Eu2+ ion in the Eu:LiCAF and Eu:LiSAF single crystals is extremely small, keff = 0.02~0.03, and the small segregation coefficient generated inhomogeneity in the bulk single crystals and decreased the yield rate of bulk single crystals. On these backgrounds, we grew Eu:LiCAF and Eu:LiSAF single crystals using Al metal as a starting material. Generally, EuF3 powder was used for starting material to grow the Eu:LiCAF and Eu:LiSAF single crystals. However, Eu ion is doped in LiCAF as Eu2+ ion, and Eu3+ ion has to be reduced before doping to LiCAF and LiSAF. Therefore, we tried to reduce the valence of Eu3+ ion in EuF3 by Al metal.
Eu:LiCAF and Eu:LiSAF single crystals were grown using Al metal powder as a starting material by the micro-pulling-down (µ-PD) method. Mixed powders with nominal compositions of Li(Ca1-xEux)(AlF1-yAlMy)F6 and Li(Sr1-xEux)(AlF1-yAlMy)F6 with x = 0.005~0.03 and y = 0, 0.01 were prepared from LiF (7.5% 6Li), CaF2, SrF2, AlF3, EuF3 and Al metal powders (> 4N purity). In the chemical formula, AlM and AlF are Al elements derived from AlF3 and Al metal, respectively. Rectangular specimens with the thickness of 1 mm were obtained from the grown crystals and they were polished for measurements of optical and scintillation properties.
Eu:LiCAF and Eu:LiSAF scintillator single crystals were grown using Al metal as a starting material to improve the segregation coefficient of Eu ion. All Eu:LiSAF single crystals using Al metal [Eu,AlM:LiSAF] indicated high transparency while Eu3%:LiSAF crystal without Al metal included milky parts in the crystal. The powder X-ray diffraction patterns indicated that all Eu,AlM:LiSAF single crystals were a single phase of colquiriite-type structure without impurity phase. Effective segregation coefficient, keff, of Eu2%,AlM1%:LiSAF single crystal was significantly improved to the value of 0.986 using the Al metal. In the transmittance spectra, all Eu,AlM:LiSAF single crystals indicated more than 70% transmittance and absorption peaks were observed around 200 and 300 nm. Light yield of Eu,AlM:LiSAF single crystals under thermal neutron irradiation increased with an increase of Eu concentration and the crystal with Eu2% indicated the maximum light yield, 10,000 photon/neutron. Details of crystal growth, optical and scintillation properties for Eu,AlM:LiCAF and Eu,AlM:LiSAF will be reported.``
