Effects of $\hbox {Ca}^{2+}$ Co-Doping on the Scintillation Properties of LSO:Ce

In addition to desirable physical properties including a density of 7.4 g/cm3 , an effective atomic number of 66, and no hygroscopicity, Lu2SiO5:Ce has well-known scintillation properties of ~30 900 photons/MeV, an emission peak near 420 nm, and a decay time of ~43 ns. These scintillation properties are achieved with Ce doping concentrations roughly in the range of 0.05 to 0.5 atomic percent relative to Lu. These properties make Lu2SiO5:Ce a widely used scintillator in positron emission tomography, in particular. We have found that both the light output and decay time may be improved by a combination of optimized crystal growth atmosphere and co-doping with divalent cations such as Ca. Scintillation light output of ~38 800 photons/MeV has been achieved as well as scintillation decay time as short as 31 ns with no long components. The relationship between growth conditions, dopant concentration, decay time, and light output is well defined, thus allowing one to reliably "tune" the crystal to the desired combination of light output and decay time. Possible explanations of the underlying mechanism are being explored and include compensation of oxygen vacancies, alteration of the relative occupancies of the cerium lattice sites, and suppression of trapping centers. In addition to higher count-rate capability and better coincidence timing, the improved decay time is expected to be particularly advantageous for time-of-flight positron emission tomography. Also, phoswich detectors comprising "standard" LSO (~43 ns decay time) and "fast" LSO (~31 ns decay time) become an attractive alternative to typical phoswich designs that often suffer from problems of mismatched light outputs and indices of refraction or the absorption of one scintillator's light by the other.