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Growth of Eu:SrI2 Bulk Crystals by Modified μ-PD Method and the Scintillation Properties

Yuui Yokota 1Shunsuke Kurosawa 1,2Kei Nishimoto 2Kei Kamada 1Akira Yoshikawa 1,2,3

1. New Industry Creation Hatchery Center, Tohoku University, Aoba-yama 6-6-10, Aoba-ku, Sendai, Miyagi, Sendai 980-8579, Japan
2. Institute for Materials Research, Tohoku University (IMR), Sendai 980-8577, Japan
3. C and A corporation, Sendai 980-8579, Japan


  The micro-pulling-down (μ-PD) method has been used for the material research of functional single crystals due to the higher growth rate compared to the conventional methods such as Czochralski (Cz) and Bridgeman (BS) methods and many novel functional crystals were developed.  Recently, the halide scintillator crystals have been focused due to the high light yield and energy resolution originated from to the small band-gap.  Especially, the Eu doped SrI2 [Eu:SrI2] and Ce and LaBr3 [Ce:LaBr3] crystals have gathering attentions due to the greatly high light yield and less than 3% energy resolution.  However, most of halide materials have relatively high hygroscopicity and it is difficult to obtain their single crystals with high crystal quality.  Therefore, the development of growth method for halide scintillator crystals have been required.  Based on these background, we developed the modified μ-PD method for the growth of halide scintillator crystals with hygroscopicity.  The Eu:SrI2 and Ce:LaBr3 crystals were grown by the modified μ-PD method and their scintillation properties were investigated in our previous reports.  The modified μ-PD method can grow a fiber crystal at approximately ten times faster growth rate compared to the conventional methods.  However, the modified μ-PD method can grow only fiber crystal with diameter of 2~5 mm and bulk crystal with diameter of several inches can’t be obtained.  For the applications of radiation detectors, the size of halide scintillator crystals is required a minimum 1 inch in diameter and we developed the novel growth method of bulk crystal with diameter of 1 inch using the modified μ-PD method.  Then, the scintillation properties of grown bulk crystal were investigated.
  Eu:SrI2 bulk single crystals were grown by the modified μ-PD method using the special shaped carbon crucible.  Mixed powders were prepared from starting materials, SrI2(4N, Alfa Aesar) and EuI2(3N, Alfa Aesar) as nominal compositions of (Sr0.925Eu0.075)I2 in the glove box.  Mixed powders were set in the carbon crucible and the crucible was set in the removable chamber.  After the gate valve was closed, the chamber was taken our from the glove box and it was connected with the torbo morecular pomp and the inside was evacuated up to 10-4 Pa at 300°C to remove the moisture on the surface of starting materials, crucible, insulators and so on.  After the baking process for sseveral hours, the high-purity Ar gas (99.9999%) was introduced in the chamber and the crucible was heated up to the melting point of Eu:SrI2 by the high-frequency induction heating.  Then the crucible was pulled down at 0.6 mm/h.  After the crystal growth, the crucible was cooled to room temperature and the chamber was entered in the glove box again.  Finally, the grown crystal was obtained.  The grown bulk crystal was cut and polished in the glove box using 100% synthesis oil.  The structural phase was identified by X-ray diffraction measurement using a tight chamber.  Light yield and decay time under γ-ray irradiation were evaluated by the photomultiplier in the glove box using multichannel analyzer and ociloscope, respectively.
  Eu:SrI2 bulk crystal with diameter of 1 inch was grown by the modified μ-PD method as it is illustrated in Fig.1.  Obtained Eu:SrI2 bulk crystal had high transparency and almost all parts of the crystal were single crystal while it had several cracks in the crystal.  The light yield of Eu:SrI2 crystal irradiated under γ-ray from 137Cs radiation source was evaluated by the pulse-height spectrum.  Figure2 is the pulse-height spectra of Eu:SrI2 crystal with a thickness of 5 mm cut from the bulk crystal and Bi4Ge3O12 (BGO) as a reference.  The light yield was calculated by comparing the position of photo-peak position to that of BGO.  The light yield and energy resolution of Eu:SrI2 crystal was 85,000 ph/MeV and 5%, respectively.  The details of crystal growth and the scintillation properties of the Eu:SrI2 bulk crystal will be reported.


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Related papers

Presentation: Oral at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, General Session 5, by Yuui Yokota
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17

Submitted: 2013-04-14 19:28
Revised:   2013-07-19 19:58