Crystal growth and scintillation properties of new ytterbium-activated scintillators Cs₄CaI₆:Yb and Cs₄SrI₆:Yb

Yb²⁺ was investigated as a potential alternative activator for Cs₄SrI₆ and Cs₄CaI₆ scintillators for the first time, as opposed to the previously studied Eu²⁺ activator. Single crystals with nominal Yb²⁺ concentrations of 0.5 mol%, 1 mol%, and 3 mol% were grown in Ø7 mm ampoules using the vertical Bridgman method. Luminescence and scintillation properties were evaluated as a function of Yb²⁺ concentration and host compound. The 5d → 4f electronic transition of Yb²⁺ was observed for both the Sr- and Ca-containing compositions. X-ray induced radioluminescence emission was centered between 449 nm and 463 nm depending on Yb²⁺ concentration and host compound. Both the spin-allowed and spin-forbidden transitions were observed in photoluminescence emission spectra and were centered at 446 nm and 476 nm, respectively, for both compounds. The best scintillation performance was achieved with Cs₄CaI₆:Yb 1 mol%, which had a 3.5% energy resolution at 662 keV and 43,000 ph/MeV light yield. To our knowledge this is the best energy resolution ever reported for a Yb²⁺-doped scintillator. Additional Cs₄CaI₆:Yb 1% crystals were grown in Ø12 mm ampoules to investigate the size dependence of scintillation properties, as well as crystal homogeneity.