(Lu,Gd)₃(Al,Ga)₅O₁₂:Ce scintillator - a short history

Approximately ten years ago, Y₃Al₅O₁₂:Ce (YAG:Ce) and Lu₃Al₅O₁₂:Ce (LuAG:Ce) were the only relevant garnet scintillators. Advantages of garnets were decent light yield and energy resolution, excellent mechanical properties, high thermal conductivity and stability, and chemical stability, including non-hygroscopicity. On the other hand, YAG:Ce has low atomic number, therefore limited detection efficiency for photon radiation. Additionally, both materials suffer from high relative intensity of slow scintillation component, LuAG:Ce significantly more. Slow scintillation component is usually attributed to shallow traps connected to LuAl (YAl) antisite defects, which are introduced into the material due to high growth temperature, using e.g. Czochralski or Bridgman growth techniques.There are two approaches to solve shallow traps problem: 1/ production of material without traps, 2/ eliminating negative effects of traps. Lowering the growth temperature is adoption of the first approach. LuAG:Ce ceramics and epitaxial films are produced using temperatures several hundreds of K lower than necessary for melt growth. Indeed, these materials exhibit significantly lower intensity of slow scintillation component. However, light yield remained moderate.Second approach, so-called 'band-gap engineering' uses partial/complete substitution of Lu (Y) by Gd and partial substitution of Al by Ga. Ga addition lowers the bottom of conduction band. Using proper concentration, traps may be buried inside the conduction band, thus eliminating possibility of electron trapping. Unfortunately, ionization of Ce³⁺ excited state becomes more probable. On the other hand, Gd addition increases the gap between conduction band and Ce³⁺ excited state, thus decreasing the probability of excited state ionization. Indeed, optimization of growth condition and composition lead to increase of light yield (> 50 000 photons/MeV), FWHM improvement (± 5 %), and significant decrease of slow scintillation component intensity. Luckily, such success is not limited only to melt grown bulk crystals, but was reproduced using liquid phase epitaxy also. (Lu,Gd)3(Al,Ga)₅O₁₂:Ce scintillator has still only a short history and it is possible that it will be further improved by co-doping, growth condition optimization etc. (authors)