Luminescence and photoionization of X-ray generated Sm²⁺ in coprecipitated CaF₂ nanocrystals

We report photoluminescence and photoionization properties of Sm²⁺ ions generated by X-irradiation of nanocrystalline CaF₂:Sm³⁺ prepared by coprecipitation. The nanocrystals were of 46 nm average crystallite size with a distribution of ±20 nm and they were characterised by XRD, TEM and SEM-EDS. At room temperature, the X-irradiated sample displayed broad electric dipole allowed Sm²⁺ 4f⁵5d (A_1u) → 4f^{6 7}F₁ (T_1g) luminescence at 725 nm that narrowed to an intense peak at 708 nm on cooling to ∼30 K. The narrow f-f transitions of Sm³⁺ were also observed. The X-irradiation-induced reduction of Sm³⁺ + e^- → Sm²⁺ as a function of X-ray dose was investigated over a very wide dynamic range from 0.01 mGy to 850 Gy by monitoring the photoluminescence intensities of both Sm²⁺ and Sm³⁺ ions. The reverse Sm²⁺ → Sm³⁺ + e^- photoionization can be modelled by employing dispersive first-order kinetics and using a standard gamma distribution function, yielding an average separation of 13 Å between the Sm²⁺ ions and the hole traps (e.g. oxide ion impurities). The present results point towards potential applications of Sm doped CaF₂ nanocrystals in the fields of dosimetry and X-ray imaging.