Scintillator crystals have been widely applied in various fields such as medical imaging, homeland security and gamma-ray astronomy.  The high scintillation light output and energy resolution are required for most of the applications.  Among scintillator crystals, halide scintillator crystals with the small band-gap indicate relatively high light yield and energy resolution and especially Eu:SrI₂ single crystals have attracted attention recently [1].  However, the Eu:SrI₂ crystals is strongly hygroscopic and it is difficult to grow the single crystal of a high quality.

Therefore, we have developed the novel growth method for halide crystals which is called the atmosphere controled  micro-pulling-down (m-PD) method [2].  One of its advantages is that the crystal can be grown at approximately ten times faster growth rate when compared to conventional methods.  We have already reported the growth and scintillation properties of Eu:SrI₂ crystals by the m-PD method.  However, the light yield and energy resolution were smaller than reported in the literature.

In this paper, we tried to improve the light yield and energy resolution by codoping the Eu:SrI₂ single crystals grown by the m-PD method.  The emission at 430nm originates from 5d-4f transition of Eu²⁺ ion in the Eu:SrI₂ crystal.  However, if the crystal included O^2- impurity at I^- site or cation vacancies, Eu²⁺ ion changes to the Eu³⁺ ion which does not have radiative 5d-4f transition.  Therefore, the light yield could be increased by increasing the Eu²⁺ content in the crystal, which we tried to achieve by co-doping with the trivalent cation.  The scintillation properties of the grown co-doped crystals were investigated and the results were compared to the Eu:SrI₂ crystals without co-doping.

Figure 1 is the schematic of the modified m-PD furnace with the removable chamber which can be moved in the glove box filled with Ar gas.  Mixed powders with the nominal compositions of (Sr_1-x-yEu_xA_y)I₂ x = 0.05, 0.75, y = 0.01, 0.05, 0.1, A = La, Gd, or Lu, were prepared with the starting materials of SrI₂ (4N), EuI₂(3N) and Co-dopants, LaI₃, GdI₃, LuI₃ (3N).  The mixed powders were set into the carbon crucible with a f2 mm hole at the bottom in the glove box.  The crucible  together with the quartz insulator was placed in the center of chamber which was taken out from the glove box after the gate valve was closed.  The chamber was connected with the turbo molecular pump and vacuumed up to 10^-4 Pa. After the introduction of high purity Ar gas (99.9999%), the crucible was heated by the high-frequency induction coil up to the melting point of Eu:SrI₂.  Crystal growth was performed by pulling-down from the melt using the Pt wire as a seed at 0.05~0.1mm/min growth rate.  The grown crystals were cut and polished in the glove box for the measurements of optical and scintillation properties.  The structural phases and the lattice parameters were measured by the powder X-ray diffraction using the tight chamber.  Radioluminescence spectra were measured with the spectrometer and CCD camera using X-ray as the excitation source.  To determine the light yield, the polished crystals were optically coupled with the PMT by optical grease in the glove box and pulse-height spectra under g-ray from ¹³⁷Cs radiation source were evaluated.  At the same time, the decay time was also measured using the oscilloscope.

Figure 2 shows the La, Gd and Lu1% co-doped Eu5%:SrI₂ crystals grown by the m-PD method.  All the crystals had f2 mm diameter and several centimeters length.  All the polished crystals indicated high transparency and there were no visible cracks and inclusions in the crystals.  The powder XRD patterns indicated these crystals were single phase of SrI₂ crystal structure. Figure 3 shows the pulse-hight spectrum of the La, Gd and Lu1% co-doped Eu5%:SrI₂.  All the co-doped crytsals showed higher light yield than Eu5%:SrI₂ crystal without co-dopant.  The other scintillation properties such as radioluminessence and decay time under g-ray irradiation will be reported.

Fig 1.  Schematic of the modified m-PD furnace

[1] E.V.van Loef, C. M. Wilson, K. S. Shah, et.al., IEEE Trans.Nucl. Sci. 56, 869,(2009)

[2] Y. Yokota, A. Yoshikawa, et.al., J. Cryst. Growth 318 (2011) 908-911

1. POSTER: Effects of codoping on Scintillation Properties of Eu:SrI_{2} Single Crystals, JPEG image data, JFIF standard 1.01, 0.1MB
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3. POSTER: Effects of codoping on Scintillation Properties of Eu:SrI_{2} Single Crystals, PNG image data, 815 x 254, 8-bit/color RGBA, non-interlaced, 0.4MB
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