Thermoluminescence and scintillation properties of rare earth oxyorthosilicate scintillators

In recent years the scintillation properties of several cerium-doped rare earth oxyorthosilicate scintillators, Ln₂SiO₅:Ce where Ln = Y, La - Lu, have been reported and, in some cases, extensively studied. In addition, binary and ternary compounds such as (Lu,Y)₂SiO₅:Ce, (Lu,Gd)₂SiO₅ and (Lu,Y,Gd)₂SiO₅:Ce have been reported. All of these crystals have either monoclinic P or C structures with characteristic SiO₄ tetrahedra and trivalent cations occupying two unique crystallographic positions. The trivalent cerium activator ions are assumed to occupy the cation lattice sites and possibly interstitial positions as well. The excited 5d state of Ce³⁺ is split into 3 observable levels with luminescence emission occurring only from the lowest 5d level to the 4f ground state (∼3 eV) with a Stokes shift of ∼ 0.5 eV. The band gap is about 6 eV, and the index of refraction is close to 1.8, with some variation according to crystallographic axes. Despite these similarities, important differences remain among the crystals including scintillation efficiency, decay time, rise time, and afterglow. In this paper, we report thermoluminescence measurements between 10K and 350K that allow the determination of trapping levels that may influence scintillation properties. The thermoluminescence data shows that the various scintillators compositions have surprisingly dissimilar sets of traps that may at least partially explain some of the differences in their scintillation properties.