A. Andrew Carey

Thermoluminescence and scintillation properties of rare earth oxyorthosilicate scintillators

In recent years the scintillation properties of several cerium-doped rare earth oxyorthosilicate scintillators, Ln/sub 2/SiO/sub 5/: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)/sub 2/SiO/sub 5/:Ce, (Lu,Gd)/sub 2/SiO/sub 5/ and (Lu,Y,Gd)/sub 2/SiO/sub 5/:Ce have been reported. All of these crystals have either monoclinic P or C structures with characteristic SiO/sub 4/ 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 Ce/sup 3+/ is split into 3 observable levels with luminescence emission occurring only from the lowest 5d level to the 4f ground state (/spl sim/3 eV) with a Stokes shift of /spl sim/ 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.

Scintillation and optical properties of LuAP and LuYAP crystals

In this paper properties of LuAP (LuAlO/sub 3/:Ce), and LuYAP (Lu/sub 0.7/Y/sub 0.3/AlO/sub 3/:Ce) crystals are studied. The previously reported self-absorption has been confirmed, and a possible mechanism is discussed. The thermal stability of the material was evaluated, and both LuAP and LuYAP have been found to readily decompose when heated to a sufficiently high temperature. XRD studies were done to determine that a solid-solid phase transformation occurs under those conditions. In addition, thermoluminescence studies reveal the presence of additional traps in the LuYAP crystal, apparently the result of the yttrium addition to the crystal lattice.

Ce-Doped Lutetium Pyrosilicate Scintillators LPS and LYPS

In this paper we present scintillation, optical, and thermoluminescence properties of recently discovered scintillation material, LPS (Lu2Si2O7:Ce) and composition with yttrium LYPS ((Lu,Y)2Si2O7 :Ce). The latter was first grown at Siemens Medical Solutions Molecular Imaging, and had an yttrium concentration of 50%. Both have the thorveitite structure, with monoclinic symmetry, space group C2/m. XRD measurements confirmed the expected crystal structure in which there is a single crystallographic site for lutetium or yttrium ions, with six oxygen neighbors. The trivalent cerium activator ions are assumed to occupy the cation lattice site. The excited 5d state of Ce3+ is split into 2 observable levels with luminescence emission occurring only from the lowest 5d level to the 4f ground state with a Stokes shift of ~2250 cm-1. In this paper, we report on the scintillation properties of LPS and LYPS crystals. The difference in scintillation properties observed between samples is discussed. It was based on thermoluminescence data obtained in the temperature range from 30 to 600 K. These data show that the analyzed samples have surprisingly different sets of traps. Crystal growth and cutting issues of LPS and LYPS are also briefly discussed