Undoped and Eu, Na co-doped LiCaAlF₆ scintillation crystals: Paramagnetic centers, charge trapping and energy transfer properties

Single crystals of LiCaAlF₆, undoped, Eu-doped and Na co-doped, were studied by electron paramagnetic resonance, radioluminescence and thermally stimulated luminescence techniques applied in a correlated manner. The undoped samples exposed to X-ray irradiation featured two hole-like charge trapping centers which were attributed to the perturbed F₂⁻ pseudomolecular ion and O⁻ (as a result of the contact with air). Their trap depths (E_t) and frequency factors (f₀) were determined as follows: E_tPerturbedF₂⁻=1.1±0.1eV, E_t(O⁻)=1.7±0.1eV, f₀PerturbedF₂⁻=f₀O⁻~10¹³s⁻¹, respectively. Annealing in air caused the appearance of another defect, a new electron-like center as well. It was found that the europium dominating charge state is 2 + in the LiCaAlF₆:Eu,Na samples, however, some amount of the Eu³⁺ is also present. Moreover, there were two Eu²⁺ centers: the dominating Eu₁²⁺=Eu_Ca²⁺ and the low-content Eu₂²⁺=Eu_Li²⁺. The amount of the latter is easily governed by the sodium admixture while the former is insensitive to the Na co-doping. The Eu and Na co-doping affected the defects distribution and incorporation in the LiCaAlF₆ host. Thermally stimulated luminescence yielded up to 8 peaks within the 100–600 K temperature range in glow curves of the undoped and doped crystals signalizing the presence of deep trapping states in the bandgaps. Along with the obvious degradation of the crystal surface in air by creation of new trapping centers this can contribute to the decreased light yield and downgrade of timing characteristics.