Expanded phase stability of Gd-based garnet transparent ceramic scintillators
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Gadolinium-based transparent polycrystalline ceramic garnet scintillators are being developed for gamma spectroscopy detectors. The scintillator light yield and energy resolution depend on many of the ceramic characteristics, including composition, homogeneity, and presence of secondary phases. To investigate phase stability dependence on composition, three base compositions – Gd3Ga2.2Al2.8O12, Gd1.5Y1.5Ga2.2Al2.8O12, and Gd1.5Y1.5Ga2.5Al2.5O12 were studied, and for each composition the rare earth content was varied according to the formula (Gd,Y,Ce)3(YXGa1X)2(Ga,Al)3O12; where 0.01 , X , 0.05. We have found that yttrium and gallium help to stabilize the garnet crystal structure in the ceramics by allowing interionic substitution among the cationic garnet sites. Specifically, a composition of Gd1.49Y1.49Ce0.02Ga2.5Al2.5O12 can accommodate approximately 2 at.% excess rare earth ions from the perfect garnet stoichiometry and remain a phase pure transparent ceramic with optimal performance as a radiation detector. This expanded phase stability region helps to enable the fabrication of large transparent ceramics from powder with tolerance for flexibility in chemical stoichiometric precision.
Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2014.235
crystals that could be obtained were highly flawed.1 Generally speaking, single crystals can typically be grown with feedstock that is somewhat off-stoichiometry, since during solidification, the excess can be rejected from the as-formed crystals to the top of the boule, for certain melt-growth methods (such as Bridgman), or left behind in the crucible for others (such as Czochralski). In contrast, transparent ceramics require strict adherence to a stoichiometry that can provide the correct crystal phase for the entire volume. Any off-stoichiometric excess will lead to secondary phases and give rise to optically scattering secondary phase inclusions at grain boundaries. For example, the stoichiometry tolerances for YAG powders used in transparent ceramics fabrication are tight, as it is a line compound. Nonstoichiometric GGAG(Ce) phosphors described in Ref. 2 provided insight into the potential flexibility and relationship between the phase stability of Gd garnets and cationic substitution populations. In this paper, we describe our ongoing efforts to engineer a Gd-based garnet, amenable to transparent ceramics processing, by using the principle of intersubstitutional ions to enhance a broad stoichiometric range over which the garnet phase is stable. After our results published in 2010, demonstrating the energy resolution for GYGAG(Ce) of R (662 keV) ,5%,3 a breakthrough performance for oxide scintillators, many efforts have been made to explore compositions of Gd-based garnets containing intersubstitutional ions.4–6 Most reports involve single crystal growth, powder synthesis, and/or computational techniques, and are aimed at identifying high light yield compositions.
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Ó Mater
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