welcome back to war! this is the list of radioactive sources on scintillation crystals emmiting amma radiation, and sealed in heavy lead!
As you see on the chemical formula , as (ce) cesium, or (ti) thalium or "eu" (europeon) , all have inside the disk, a very powerfull radioactive substance. (link on the comments)

http://www.epic-scintillator.com/NaI-scintillator-1.5inchx1.5inch

• GAGG(Ce) scintillation crystal (20)
• NaI(TI) scintillation crystal (16)
• NaI(Tl) scintillator detector (7)
• LaBr3(Ce) scintillation crystal (15)
• LYSO(Ce) scintillation crystal (37)
• CsI(TI) scintillation crystal (14)
• CdWO4 scintillation crystal (6)
• BGO scintillation crystal (21)
• CaF2(Eu) scintillation crystals (4)
• YAG(Ce), YAP(Ce) crystal (10)