The Role of Cerium Sites in the Scintillation Mechanism of LSO

The crystal structure of Lu_2(SiO_4)O:Ce (“LSO”) has two trivalent cation sites which may be occupied by Ce^(3+) to form luminescence centers. Previous investigations revealed the existence of two distinct sets of Ce^(3+) excitation and emission spectra and suggested that the differences in the spectra are due to differences in the crystal fields at the two lattice sites that shift the 5d levels of Ce^(3+). In the present report, we re-examine this issue and present new evidence which suggests a different interpretation. In particular, spectra measured at 13 K suggest that both lattice sites give rise to indistinguishable excitation and emission spectra while the second set of observed spectra arises from Ce^(3+) located in interstitial sites. The evidence for the interstitial sites is a disappearance of the doublet structure in the emission spectra and a large variation in the population of the sites as a function of total cerium concentration. In addition, the results indicate that the scintillation properties of LSO are influenced by the relative population of Ce in interstitial sites compared to lattice sites as well as by differences in the transfer efficiencies to different sites.