Scintillators,　which　are　capable　of　converting　ionizing　radiation　into　visible　photons,　are　an　integral　part　of　medical,　security,　and　commercial　diagnostic　technologies　such　as　X-ray　imaging,　nuclear　cameras,　and　computed　tomography.　Conventional　scintillator　fabrication　typically　involves　high-temperature　sintering,　generating　agglomerated　powders　or　large　bulk　crystals,　which　pose　major　challenges　for　device　integration　and　processability.　On　the　other　hand,　colloidal　quantum　dot　scintillators　cannot　be　cast　into　compact　solid　films　with　the　necessary　thickness　required　for　most　X-ray　applications.　Here,　we　report　the　room-temperature　synthesis　of　a　colloidal　scintillator　comprising　CsPbBr　3　nanosheets　of　large　concentration　(up　to　150　mg/mL).　The　CsPbBr　3　colloid　exhibits　a　light　yield　(∼21000　photons/MeV)　higher　than　that　of　the　commercially　available　Ce:LuAG　single-crystal　scintillator　(∼18000　photons/MeV).　Scintillators　based　on　these　nanosheets　display　both　strong　radioluminescence　(RL)　and　long-term　stability　under　X-ray　illumination.　Importantly,　the　colloidal　scintillator　can　be　readily　cast　into　a　uniform　crack-free　large-area　film　(8.5　×　8.5　cm　2　in　area)　with　the　requisite　thickness　for　high-resolution　X-ray　imaging　applications.　We　showcase　prototype　applications　of　these　high-quality　scintillating　films　as　X-ray　imaging　screens　for　a　cellphone　panel　and　a　standard　central　processing　unit　chip.　Our　radiography　prototype　combines　large-area　processability　with　high　resolution　and　a　strong　penetration　ability　to　sheath　materials,　such　as　resin　and　silicon.　We　reveal　an　energy　transfer　process　inside　those　stacked　nanosheet　solids　that　is　responsible　for　their　superb　scintillation　performance.　Our　findings　demonstrate　a　large-area　solution-processed　scintillator　of　stable　and　efficient　RL　as　a　promising　approach　for　low-cost　radiography　and　X-ray　imaging　applications.　©　2019　American　Chemical　Society.