CsBa₂I₅:Eu²⁺,Sm²⁺—The First High-Energy Resolution Black Scintillator for γ-Ray Spectroscopy

Scintillators are materials that absorb a high energy particle (α,β,γ radiation) and downconvert it into a short pulse of visible or near-visible light. As determined by photon detection statistics, the ultimate energy resolution for γ-photon detection can only be approached for materials that show a perfect proportional response with γ-energy. A large amount of research has resulted in the discovery of highly proportional materials, such as SrI₂:Eu²⁺ and CsBa₂I₅:Eu²⁺. However, the resolution is still limited because of unavoidable self-absorption of Eu²⁺ emission, especially when large-sized scintillators are to be used. By co-doping with Sm²⁺, the emission of Eu²⁺ can be efficiently shifted to the far-red by exploiting nonradiative energy transfer. Herein, this new idea is applied to CsBa₂I₅, and Sm co-doped CsBa₂I₅:Eu²⁺ can be considered as the first “black scintillator” with an emission wavelength around 755 nm, a remarkable high energy resolution of 3.2% at 662 keV gamma excitation, and a scintillation decay time of 2.1 μs. The proposed double-doping principle can be used to develop an entirely new class of near-infrared (NIR) scintillators.