Material and Process Engineering for Bulk Single Crystal Growth of High Performance Scintillator Potassium Calcium Iodide

Protection against threats of nuclear terrorism relies on the deployment of an enormous number of radiation detection devices with energy resolution to differentiate the radiological signatures of special nuclear materials amongst naturally occurring radiation and other nuisance sources. The capabilities of these devices rely upon the availability of high performance scintillator and semiconductor materials which provide useful responses in the presence of radiation. So far, few materials have been developed to a level that can supplant the use of underperforming NaI:Tl [thallium doped sodium iodide] crystals in the field due to their high cost and/or low yields of production. KCaI3[potassium calcium tri-iodide] doped with divalent europium has shown significant promise as a high performance scintillator and development to explore its potential for large scale production is necessary. KCaI3 crystallizes into an orthorhombic symmetry and exhibits a moderate amount of anisotropy in thermal expansion between its melting point of 524°C [Celsius] and room temperature. Production of large single crystals up to 1.5” in diameter can be facilitated through the vertical Bridgman method of melt growth using pyrolytic carbon-coated ampoules which eliminate cracking upon cooling. Low dopant amounts of 0.5-1.0 at% of Eu can achieve excellent energy resolutions of 3-4% at 662 keV while minimizing deleterious self-absorption effects, making it comparable to the highest performing scintillators discovered. A purpose-built multi-ampoule growth station was constructed and through the course of the investigation, a process was developed whereby several high quality, uncracked KCaI3 crystals at 1” in diameter could be grown simultaneously, in parallel, using a randomly oriented self-seeded approach. The demonstration shows great promise as a potential pathway to reduce the costs of scintillator production that is limited to a large number of boules at the 1”- 2” diameter size. Furthermore, KCaI3 possesses internal radioactivity due to the presence of naturally abundant 40K [potassium – 40] and will restrict its utility to applications where an elevated background is not critical. A significant obstacle to production of KCaI3 crystals doped with europium is the uniform distribution of the activator which can result in concentration gradients in bulk crystals and degrade spectroscopic performance