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 SrI 2 :Eu 2+ and CsBa 2 I 5 :Eu 2+ . However, the resolution is still limited because of unavoidable self‐absorption of Eu 2+ emission, especially when large‐sized scintillators are to be used. By co‐doping with Sm 2+ , the emission of Eu 2+ can be efficiently shifted to the far‐red by exploiting nonradiative energy transfer. Herein, this new idea is applied to CsBa 2 I 5 , and Sm co‐doped CsBa 2 I 5 :Eu 2+ 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.