Characterization of imperfections in scintillator crystals using gamma-ray induced positron annihilation lifetime spectroscopy

Abstract

Gamma-ray-induced positron annihilation lifetime spectroscopy (GiPALS) was used to characterize pristine and codoped Lu3Al5O12 (LuAG) crystals grown using Czochralski (Cz) and micropulling-down (mPD) methods. GiPALS spectra were analyzed using a two-state trapping model. The bulk and defect lifetimes were theoretically calculated based on density functional theory (DFT) using the projector-augmented-wave (PAW) method. The experimentally measured and theoretically calculated bulk lifetimes almost agreed, thereby validating the experiment and simulation. The defect lifetimes almost coincided with positron annihilation lifetimes due to Al monovacancy at the 24d and 16a sites. Li+ and Mg2+ codoping weakened the vacancy-related component, indicating that codoping had effectively reduced the concentration of Al monovacancy. The Ce 5d–4f thermoluminescence (TL) glow curve revealed that Mg2+ codoping had annihilated the trapped electron centers. In contrast, Li+ codoping increased the concentration of interstitial Li+ ion associated trapped electron centers. Ce-L3 X-ray absorption near-edge structure (XANES) spectra demonstrated that Ce4+ ions at the 24c site had compensated for Mg2+ ion charges at the 24d and 16a site. The Ce4+ ions and oxygen vacancies were supposed to compensate for Li+ ion charges at the 24d and 16a sites.

Keywords

• Scintillator crystal
• Gamma-ray-induced positron annihilation lifetime spectroscopy
• Vacancy defect
• X-ray absorption near-edge structure spectroscopy
• Thermoluminescence
• Density functional theory