Radiation induced centers in irradiated SrB4O7 doped europium and their role in thermally stimulated reactions: Thermally stimulated luminescence, fluorescence and electron paramagnetic resonance studies

Polycrystalline samples of strontium tetraborate (SrB4O7: SBO) samples doped with different concentrations of europium (0%, 0.05%, 1% and 2%) were synthesized using solid state reaction in air at high temperature. These samples were characterized by X-ray diffraction (XRD), photoluminescence (PL), thermally stimulated luminescence (TSL) and electron paramagnetic resonance (EPR) techniques. X-ray diffraction studies indicated that SrB4O7 has orthorhombic structure with lattice parameters a=4.237±0.001, b=4.431±0.001, c=10.704±0.010, space group Pnm2i, Z=2. Thermally Stimulated Luminescence (TSL) investigations on the gamma irradiated sample showed an intense glow peak at 407 K and a weak glow peak at 500 K. The trap parameters namely activation energy (E), order of kinetics (b) and the frequency factor (s) for the main peak at 407 K were determined using the glow curve method. Fluorescence studies showed weak emission at ca 367 nm, which was attributed to f-d transition (4f65d-4f7) of Eu2+ ions in the host lattice, whereas an intense emission in the range 570-650 nm was assigned to 5D0→7FJ, J=0,1,2,3 and 4 transition of Eu3+ in the host lattice. The large value of I(R)/I(O) suggested that Eu3+ ions are situated at sites without center of inversion (lower symmetry) in SBO matrix. EPR studies suggested the formation of boron-oxygen hole trapped center viz, BOHC, electron trapped at oxygen vacancies and some impurity radicals like NO32− and CO2− radicals in SBO on irradiation. EPR temperature variation studies in the range 300-475 K on gamma irradiated europium doped samples, suggested a recombination process occurring between boronoxygen hole trapped species and electron trapped species and subsequently energy is transferred to Eu3+ ion, which on de-excitation yields thermally stimulated luminescence yielding an intense TSL glow peak at 407 K.