Li+ incorporation and defect-creation processes imposed by X-ray and UV irradiation in Li-codoped Y3Al5O12:Ce scintillation crystals

Abstract

Cerium-doped rare-earth aluminum garnets represent an important class of scintillation and luminescent materials that are widely used in different technical fields, including medical imaging and solid-state white light sources, whose performances have to be further improved. One of the most effective tools for improving the light yield and timing characteristics of these materials is codoping with optically inactive ions. Herein, we investigate the Li⁺ incorporation, as well as defect formation processes imposed by X-ray and UV irradiation, in Li⁺-codoped Y₃Al₅O₁₂:Ce garnet scintillation crystals with the use of electron paramagnetic resonance (EPR), ⁷Li nuclear magnetic resonance, and thermally stimulated luminescence (TSL) methods. The EPR study showed that the Ce³⁺ content in the Li⁺-codoped crystals does not depend visibly on the Li concentration. The EPR spectra of X-ray-irradiated Li⁺-codoped YAG crystals revealed the presence of both isolated O⁻ centers (a hole trapped at the oxygen ion) and O⁻ centers stabilized by the neighboring Li⁺ ions. The latter centers are responsible for the TSL peak at 320 K. Our data also show that at low Li concentrations in the melt, the Li⁺ ions mainly substitute for Y³⁺ ions. However, at high Li⁺ ion concentrations (∼1 at% in melt), the Li⁺ ions also substitute for Al³⁺ ions. A strong increase in the oxygen vacancy concentration in the Li⁺-codoped crystals suggests that oxygen vacancies serve as effective compensators for the negative excess charge introduced into the crystal lattice by Li⁺ ions, thus suppressing the Ce³⁺ → Ce⁴⁺ conversion process, which plays an important role in the acceleration of the scintillation decay. The obtained data suggest that the decrease in a slow component in the scintillation decay of the Li⁺ ion-codoped crystals is caused, at least partly, by the redistribution of trapped charge carriers to more thermally stable centers.