Luminescence and Energy Transfer in Lu3Al5O12 Scintillators Co-Doped with Ce3+ and Tb3+

Lu3Al5O12 (LuAG) doped with Ce3+ is a promising scintillator material with a high density and a fast response time. The light output under X-ray or gamma-ray excitation is, however, well below the theoretical limit. In this paper the influence of codoping with Tb3+ is investigated with the aim to increase the light output. High resolution spectra of singly doped LuAG (with Ce3+ or Tb3+) are reported and provide insight into the energy level structure of the two ions in LuAG. For Ce3+ zero-phonon lines and vibronic structure are observed for the two lowest energy 5d bands and the Stokes' shift (2 350 cm(-1)) and Huang-Rhys coupling parameter (S = 9) have been determined. Tb3+ 4f-5d transitions to the high spin (HS) and low spin (LS) states are observed (including a zero-phonon line and vibrational structure for the high spin state). The HS-LS splitting of 5400 cm(-1) is smaller than usually observed and is explained by a reduction of the 5d-4f exchange coupling parameter J by covalency. Upon replacing the smaller Lu3+ ion with the larger Tb3+ ion, the crystal field splitting for the lowest 5d states increases, causing the lowest 5d state to shift below the D-5(4) state of Tb3+ and allowing for efficient energy transfer from Tb3+ to Ce3+ down to the lowest temperatures. Luminescence decay measurements confirm efficient energy transfer from Tb3+ to Ce3+ and provide a qualitative understanding of the energy transfer process. Co-doping with Tb3+ does not result in the desired increase in light output, and an explanation based on electron trapping in defects is discussed.