Effects of increasing size and changing europium activator concentration in KCaI₃ scintillator crystals

KCaI₃:Eu crystals have been identified as very promising for use in spectroscopic detector applications related to nuclear nonproliferation and domestic security efforts. Initial studies have shown for small crystals a few mm³ in size with 3% europium dopant concentration, a high light yield of >70,000 ph/MeV and energy resolution of ≈3% at 662 keV is attainable which is comparable with the highest performance scintillators discovered. In this work, single crystals of KCaI₃ with a range of Eu²⁺ doping between 0 and 5 at% substituting for Ca²⁺ were grown at 22 mm diameter and their performance for gamma-ray spectroscopy studied. Comparisons among crystals approximately Ø22 mm×22 mm (8.4 cm³ or ≈0.5 in³) provide a more accurate understanding of how scintillation performance changes with Eu doping and increased crystal size. KCaI₃ in the undoped form is shown to be a highly efficient intrinsic scintillator with a defect-related emission at 404 nm which coexists with the Eu²⁺ 5d–4f emission in low dopant concentrations and is completely re-absorbed in more heavily doped crystals. For larger crystals, effects from self-absorption due to Eu activation become more evident by a near doubling of decay time for 0.5 in³ crystals as the activator is increased from 0.5 to 5.0 at% Eu. Comparisons of pulse-height spectra obtained for Ø22 mm×22 mm cylinders with varying Eu concentration suggests best performance is achieved using lower Eu additions closer to 0.5–1.0 at%. Using a modified crystal packaging featuring an offset reflector geometry, 0.5 in³ crystals of KCaI₃:Eu can attain under 4% energy resolution at 662 keV.