Bridgman growth of large SrI₂:Eu²⁺ single crystals: A high-performance scintillator for radiation detection applications

Single-crystal strontium iodide (Srl₂) doped with relatively high levels (e.g., 3-6%) of Eu²⁺ exhibits characteristics that make this material superior, in a number of respects, to other scintillators that are currently used for radiation detection. Specifically, SrI₂:Eu ²⁺ has a light yield that is significantly higher than LaBr ₃:Ce^{3 +}-a currently employed commercial high-performance scintillator. Additionally, SrI₂:Eu²⁺ is characterized by an energy resolution as high as 2.6% at the ¹³⁷Cs gamma-ray energy of 662 keV, and there is no radioactive component in SrI₂:Eu ²⁺-unlike LaBr₃:Ce³⁺ that contains ¹³⁸La. The Ce³⁺-doped LaBr₃ decay time is, however, faster (30 ns) than the 1.2 μs decay time of SrI₂:Eu ²⁺. Due to the relatively low melting point of strontium iodide (∼515 °C), crystal growth can be carried out in quartz crucibles by the vertical Bridgman technique. Materials-processing and crystal-growth techniques that are specific to the Bridgman growth of europium-doped strontium iodide scintillators are described here. These techniques include the use of a porous quartz frit to physically filter the molten salt from a quartz antechamber into the Bridgman growth crucible and the use of a "bent" or "bulb" grain selector design to suppress multiple grain growth. Single crystals of SrI₂:Eu²⁺ scintillators with good optical quality and scintillation characteristics have been grown in sizes up to 5.0 cm in diameter by applying these techniques. Other aspects of the SrI ₂:Eu²⁺ crystal-growth methods and of the still unresolved crystal-growth issues are described here.