Lithium codoping
has emerged as an effective strategy to enhance the light yield of
oxide scintillators for radiation detection applications, but the
understanding of the actual role played by Li+ remains
unclear. In this work, we comprehensively study the effects of Li
codoping on optical and scintillation properties of Lu2SiO5:Ce (LSO:Ce) single crystals and reveal the critical
role of site occupancy of Li. High-quality LSO:Ce single crystals
codoped with 0.05, 0.1, and 0.3 at. % Li ions were grown by the Czochralski
method. The optical absorption spectra confirm nonconversion of stable
Ce3+ to Ce4+ in Li-codoped LSO:Ce regardless
of the Li codoping concentration. The photoluminescence decay kinetics
suggest an enhanced ionization of the excited 5d1 state
of Ce3+ centers in highly codoped samples. A simultaneous
improvement of scintillation light yield, decay time, and afterglow
is achieved in LSO:Ce codoped with low concentrations of Li. The preferential
occupation of Li at interstitial spaces and lutetium sites is proven
to rely on its codoping concentration by using the 7Li
nuclear magnetic resonance technique. The concentration-dependent
site occupancy of Li alters the defect structures of LSO:Ce, in particular
resulting in a distinct change in the number of cerium spatially correlated
oxygen vacancies confirmed by thermoluminescence and afterglow measurements.
Commercially available Lu2SiO5:Ce (LSO:Ce) scintillators for the nuclear medical imaging applications, such as positron electron tomography (PET), normally have a light yield of 30 000–32 000 photons/MeV and a scintillation decay time of 43–45 ns. We demonstrate a simultaneous improvement of light yield and decay time of LSO:Ce single crystal scintillators with lithium codoping. Li codoping significantly enhances the light yield of LSO:Ce from 32 500 to 39 000 photons/MeV, shortens the scintillation decay time from 45.4 to 42.1 ns, and reduces the room temperature afterglow by around one order of magnitude. The physical insights on the role of Li codopants in scintillation mechanism are provided by studying cerium oxidation state, luminescence properties of cerium activator centers, defect structure, and preferential occupation of lithium ions. The improved light yield and afterglow are explained in terms of the dissociation of spatially correlated oxygen vacancies (VO) and Ce centers or the suppression of VO formation, allowing a more efficient electron migration to Ce centers. We attribute the decay time shortening upon Li+ codoping to the reduction of slow Ce2 emissions via a non‐radiative energy transfer from six‐coordinated Ce2 to seven‐coordinated Ce1 centers.
Spin-orbital coupling effects and the underlying spin-dependent processes to achieve high-efficiency TADF are revealed based on magneto-optical studies.
Divalent cation codoping, such as with Ca 2+ and Mg 2+ , is beneficial for the scintillation performance enhancement of Czochralski-grown Lu 2 SiO 5 :Ce (LSO:Ce) single crystals for nuclear medical imaging applications, but with that benefit comes a tendency toward acentric growth due to the reduced surface tension of the melt. Here, we present a divalent Cu codoping strategy to achieve a simultaneous improvement of light yield, energy resolution, scintillation decay, and afterglow in LSO:Ce single crystals without destabilizing the solid−liquid interface or promoting acentric growth. High-quality 32-mm-diameter and 110mm-long LSO:Ce single crystals codoped with 0.1 and 0.3 atom % Cu 2+ ions in the melt were successfully grown using the Czochralski method. While the surface tension of the LSO melt does decrease with Cu 2+ codoping, analogous to the effect of Ca 2+ codoping, it is not reduced enough to affect the crystal growth stability or diameter control. With 0.1 atom % Cu 2+ codoping, the scintillation light yield of LSO:Ce can be significantly enhanced from 32 000 to 39 000 photons/MeV with an improved enegy resolution of 9% at 662 keV, and a reduced afterglow at room temperature. A continuous shortening of scintillation decay time with Cu 2+ codoping is ascribed to the combined effect of enhanced thermal ionization from the Ce 3+ 5d 1 state and a reduction of the emission contribution from Ce2 centers, i.e., in the site neighboring six oxygens. Thermoluminescence and afterglow measurements are utilized to study the defect structure and explain the variation in scintillation yield.
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