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Light yield behavior in mixed oxide scintillation crystals
|Oleg Sidletskiy 1, Alexander Gektin 1, Boris Grinyov 1, Andrei Belsky 2|
1. Institute for scintillation materials of NAS of Ukraine (ISMA), Lenin avenue, 60, Kharkov 61158, Ukraine
A large variety of mixed scintillation crystals based on isovalent substitution of host atoms has been developed for a range of applications. Multicomponent crystals are usually avoided by technologists owing to complex character of components segregation leading to crystal inhomogeneity and, in many cases, to formation of cracks. Nevertheless, in some cases a variation of crystal host composition is a chance to modify functional properties of crystal and/or to simplify growth procedure. The current report is focused at development of fast, dense and bright new mixed oxide crystals doped with Ce3+. Primarily this trend of studies was pointed at simplification of crystal growth process (Lu2xY2-2xSiO5 (LYSO) instead of Lu2SiO5 (LSO)) by lowering of crystallization temperature and higher crystal production yield; no significant improvement in scintillation parameters was achieved. Later, it has been discovered that in many Ce-doped mixed crystals the behavior of light yield and other scintillation parameters is non-additive in respect to their constituents. For example, substantial improvement of light yield was achieved in Lu1-xYxAlO3:Ce , Lu1-xScxBO3:Ce , Lu2xGd2−2xSiO5 (LGSO:Ce) , RE3(Al1-xGax)5O12:Ce (RE=Y, Lu, or Gd) [4, 5]. All mentioned systems exhibit maximal light yield at x = 0.4 – 0.6, i.e., at component ratio near 1:1.Fig. 1. Light yield in Ce-doped Lu2xGd2−2xSiO5 (1), Lu2xY2−2xSiO5 (2), Gd3(Al1-xGax)5O12 (3), in accordance with [3, 4, 6]. Horizontal axis denotes formula units (x).
Mechanisms of the observed phenomenon may relate to different stages of scintillation process. In the present report we illustrate them on example of LGSO:Ce and YAGG:Ce crystals obtained by the Czochralski method and compared with the analogues. In garnet scintillators, substitution of lanthanide cation or Al/Ga is accompanied by the bandgap change and shift of Ce3+ energy levels in crystal field . Thereby tuning of the cation ratio provides choice of proper position of activator energy levels in the forbidden band and suppression of shallow electron traps which believed to dump carrier transport to activator. However, in the rest of mentioned systems, no evidences of energy structure modification were obtained. Crystalline structure in these solid solutions in long-range scale is linear dependent on component ratio in accordance with the Vegard’s law. In short-range scale, arising fluctuations of substituting atoms content influence electronic processes, for ex., conductivity, or magnetic properties. In case of scintillatorsthe increase in light yield can be called by limitation of electron-hole separation length due to interaction of thermalized carriers with charge fluctuations on defects called by inhomogeneities in mixed crystal . The inhomogeneity in the component ratio near 1:1 is supported by surprisingly high Ce3+ segregation coefficients, and variation of luminescent characteristics across LGSO:Ce crystal obtained by confocal microscopy. Besides this, shape of Ce3+ luminescence excitation spectra demonstrate increase in carrier multiplication efficiency in the same concentration range.
Feasibility of the mentioned mechanisms for certain mixed crystals is discussed. The effect of energy structure modification is more pronounced in crystals with strong crystal field such as rare-earth garnets, where splitting of Ce3+ 5d levels substantially depends on cation environment. A degree of inhomogeneity in crystal should increase with the difference between ionic radii of substituting cations. Maximal improvement in light yield evidently can be reached when different mechanisms are involved simultaneously.
The presented examples demonstrate the potential for significant improvement of scintillator parameters. This methodology can serve as the basis at engineering of new mixed crystals with optimized properties for optics and electronics.
The work is partially supported by the Project FP7-INCO-2011-6 (“SUCCESS”).
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Presentation: Oral at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, Topical Session 6, by Oleg Sidletskiy
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17
Submitted: 2013-03-28 14:47 Revised: 2013-07-18 20:12