Discovery and Development of Potassium-Based Metal Halide Scintillators for Radiation Detection Applications

Scintillators are important materials for radiation detection applications such as homeland security, geological exploration, and medical imaging. Scintillators for nuclear nonproliferation applications must have excellent energy resolution to distinguish the gamma-ray signatures of potentially dangerous radioactive sources from non-threat radioactive sources. There is an established need for scintillators with energy resolution in the 1-2 % (percent) range at 662 kiloelectron volts. However, there are challenges surrounding the development of this new generation of high light yield/high resolution scintillators; for example, the high cost of production due to low crystal yield and slow growth process, and crystal inhomogeneity. In this work we present the crystal growth, the physical and scintillation properties of four newly discovered europium doped scintillators; potassium strontium iodide (KSr₂I₅:Eu), potassium strontium bromide (KSr₂Br₅:Eu), potassium barium iodide (KBa₂I₅:Eu), potassium barium iodide (K₂BaI₄:Eu). These scintillators materials had excellent properties with light yields well above 50,000 photons per megaelectron volt and energy resolution between 2.4 – 2.9% (Percent) at 662 kiloelectron volts.Due to its ease of growth, Europium doped potassium strontium iodide was selected for further development. We focused our efforts optimizing the growth parameters required to grow one-inch diameter crystals at pulling rates up to 7 millimeters per hour. High performing, nearly crack free single crystals measuring up to one-inch diameter by six-inch in length were grown via the vertical Bridgman technique. The scintillation properties were evaluated using specimen that ranged ranging from 0.012 to 54 cubic centimeters.To further enhance the light yield and energy resolution in europium doped potassium strontium iodide, anion and cation substitution were investigated. We found that replacing 2% (percent) of the matrix iodine atoms with bromine increased the light yield from 84,000 to 97,900 ph/MeV (Photon per megaelectron volt), while maintaining it excellent energy resolution of 3.0% (percent) at 662 keV (Kiloelectron volts). We also found that by replacing 35% of strontium for barium not only increased the light yield increased from 84,000 to 120,000 ph/MeV (Photon per megaelectron volt) and but also improved the energy resolution from 3.0 to 2.3% (percent) at 662 keV (Kiloelectron volts) for 1 cm³ (cubic centimeter) crystals.