Promising Alkaline Earth Halide Scintillators for Gamma-Ray Spectroscopy

Up to 1-in-diameter, Eu-doped CsBa₂I₅ and BaBrI crystals have been grown using the vertical Bridgman method. Scintillation properties of the crystals up to ~ 1 × 1 × 1 cm³ have been studied under gamma-ray irradiation. Systematic concentration studies have been performed for both these alkaline earth halides with Eu²⁺ doping levels ranging from 0% to 10% to optimize growth of large crystals. We have achieved excellent energy resolution of ~ 2.6% at 662 keV for a 2 × 2 × 1-mm³ sample of CsBa₂I₅:2%Eu. However, in this paper, the emphasis is on the larger crystals that can be used practically in nuclear nonproliferation applications. Our measurements of a 1.0 × 1.0 × 1.0-cm³ sample of CsBa₂I₅:3%Eu, processed from a 1-in-diameter crystal, shows very good energy resolution of 3.9% at 662 keV, high light output of ~ 80 000 photons/MeV, and a principal decay time of ~ 900 ns. The second investigated composition, BaBrI:5%Eu, 1.0-cm-diameter × 1.0-cm-thick sample shows high light output of 71 000 photons/MeV and energy resolution of 4.3% at 662 keV. A decay time of 480 ns was measured for this sample, which is shorter than most Eu²⁺ doped materials of this size. The emission in these materials is due to d → f transitions of Eu²⁺ and occurs over a single band between 400-500 nm. It peaks at ~ 430 nm for CsBa₂I₅:Eu and at ~ 413 nm for BaBrI:Eu. Since both compositions show high proportionality over a wide range of energies, we expect to see improvements in the energy resolution of large crystals, as better quality of 1-in-diameter crystals are grown.