X-ray enhancement of CsI:Eu²⁺ radioluminescence

CsI:Eu²⁺ needle crystal layers were produced by Physical Vapour Deposition (PVD). The luminescence properties were studied before and after thermal anneal. It was shown by Electron Paramagnetic Resonance (EPR) that annealing produces Eu²⁺ monomer centres, probably starting from Eu clusters in the needle crystalline layers. These centres seem structurally very similar to the Eu²⁺ monomer centres in annealed CsBr:Eu²⁺ needle layers. They give rise to a narrow luminescence band peaking at about 450 nm (2.76 eV) at room temperature, both under X-ray and UV excitation. The luminescence intensity of as-deposited layers is low. Annealing enhances photo- and radioluminescence on average by a factor of 1.5–3. Surprisingly, further sensitization of radioluminescence with a factor of up to 7 is possible by exposing the annealed needle layers to X-rays or UV, while in contrast, UV excited luminescence is slightly reduced. X-ray enhancement thus leads to a structured scintillator with a conversion efficiency of about 42,000 photons/MeV upon X-ray excitation. A similar radioluminescence enhancement is observed for annealed CsI:Eu²⁺ fine powder, but not for large single crystals or conglomerated polycrystals. The measurements and observations are consistent with Eu²⁺ monomer centres stabilized by water molecules. These molecules are incorporated via annealing, and that this happens when the morphology is such that water can diffuse throughout the volume of the crystals. The presence of stabilized Eu²⁺ monomers is necessary to enhance radioluminescence by X-ray exposure. A mechanism is proposed for the phenomenon