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+ 4f(5)5d (A(1u)) -> 4f(6 7)F(1) (T-1g) luminescence at 725 nm that narrowed to an intense peak at 708 nm on cooling to similar 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 angstrom 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.