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Recently discovered scintillators Cs 4 SrI 6 :Eu and Cs 4 CaI 6 :Eu have shown promising properties for gamma-ray detection in homeland security applications, with <4% energy resolution (at 662 keV) and light yields above 50,000 ph/MeV. However, a thorough investigation of the effects of europium concentration has not yet been conducted. In this work, Eu2+ concentration was varied from 0.5 mol% to 9 mol%, and Ø7 mm single crystals were grown from the melt via the vertical Bridgman method. Scintillation performance was evaluated as a function of Eu2+ concentration to determine the optimal amount. The observed trend was improved energy resolution and higher light yield with increasing Eu2+ concentration up to 7 mol%. The best energy resolution achieved was 3.2% for Cs 4 SrI 6 :Eu 7% and 3.6% for Cs 4 CaI 6 :Eu 7%. Their respective light yields were 71,000 ph/MeV and 69,000 ph/MeV. • Photoluminescence and radioluminescence emission show weak dependence on europium concentration in Cs4SrI6:Eu and Cs4CaI6:Eu. • A maximum light yield of 78,000 ph/MeV is obtained with 9 mol% europium in Cs4SrI6:Eu. • An energy resolution of 3.2% at 662 keV is achieved with 7 mol% europium in Cs4SrI6:Eu. [ABSTRACT FROM AUTHOR]