Lanthanide Doped Alkaline-Earth Metal Nanocrystalline as Ionising Radiation Storage Phosphors

Abstract

This work involved the preparation methods, structural and spectroscopic characterisations of lanthanide ions (Ln = Sm, Eu) doped nanocrystalline alkaline-earth metal fluorides MF2 (M = Ca, Sr) as ionising radiation storage phosphors for potential applications in the field of computed radiography, dosimetry, and optical data storage. The structural characterisation was conducted using powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and SEM and TEM energy dispersive spectroscopy (EDS). The storage mechanism of the phosphors was studied by photoluminescence spectroscopy (PL) and spectral hole- burning. The X-ray storage phosphor properties i.e. X-irradiation induced reduction of Sm3+ to Sm2+ and reverse photoionisation of Sm2+ to Sm3+, in CaF2:Sm3+ nanocrystals prepared by co-precipitation method were investigated by monitoring the PL intensities of both Sm3+ and Sm2+ ions. Both processes can be modelled by first-order dispersive kinetics. Besides, the Sm3+ to Sm2+ conversion upon X-irradiation in SrF2:Sm3+ were compared with CaF2:Sm3+. In addition, the X-ray storage phosphor properties in mechanochemically ball milled nanocrystalline CaF2:Sm3+ were explored in detail. The photoionisation of Sm2+ ions was also demonstrated by spectral hole-burning experiments of the X-irradiated nanocrystalline CaF2:Sm3+. The hole-burning rate decreased with the X-irradiation dose, while an increase was observed with an increase of the Sm concentration, manifesting the significance of the Sm3+ electron trap in the photoionisation of Sm2+ in CaF2. Furthermore, the Zeeman effects in magnetic fields up to 9 Tesla on the Sm2+ luminescence were investigated, in particular the splitting of the 7F1 ground state level and the quadratic dependence of the intensity of a forbidden transition. The photoinduced electron transfer between Eu2+ and Sm3+ in CaF2 nanocrystals prepared by a facile co-precipitation method was explored. The doping of divalent Eu was realised by reducing Eu3+ to Eu2+ in solution under nitrogen employing granular zinc. The forward and backward photoinduced electron transfer in CaF2:Eu2+,Sm3+ was investigated by monitoring the Eu2+ and Sm2+ luminescence signals, under ultraviolet (UV) light exposure at 310 and 340 nm. Importantly, this is the first report on the X-ray storage properties, spectral hole-burning, and photoinduced electron transfer phenomena of nanocrystalline CaF2:Sm3+, SrF2:Sm3+, and CaF2:Eu2+,Sm3+ powders