The Effect of Calcium Co-Doping on Praseodymium Doped LSO

Abstract

Cerium-doped lutetium oxyorthosilicate (LSO:Ce) is a well-known scintillator whose high density (7.4 g/cc), high light yield ( ~ 6 times that of BGO), and fast decay time ( ~ 43 ns) make it especially well suited for use in Positron Emission Tomography. Recent work suggests that divalent calcium co-doping eliminates many of the shallow electron traps, thus facilitating fast energy transfer to the Ce3+ luminescence centers and shortening the scintillation decay time to ~ 30 ns. Here we report preliminary observations of the effect of Ca2+ co-doping on the crystal growth, photoluminescence, and scintillation properties of LSO:Pr. Single crystals with Pr concentrations ranging from 0.05% to 0.2% and Ca2+ concentrations up to 0.2% were grown via the Czochralski technique. Absorption, emission, and excitation spectra revealed the numerous 5d-4f and 4f-4f transitions characteristic of trivalent Pr. The main optical absorption due to Ca2+ occurs in the far UV, similar to previous observations of LSO:Ce, but with a more intense tail extending through the visible region. Consequently, 4f to 5d optical excitation ( ~ 260 nm) is greatly reduced in Ca co-doped samples, and the related 5d to 4f emission ( ~ 400 nm) is largely absent. Optical excitation ( ~ 450-500 nm) of the red ( ~ 610 nm) emission appears to be unaffected, for the most part, by the presence of Ca2+. The scintillation decay of LSO:Pr is the sum of two exponential components with time constants of 7-8 and 26-29 ns, similar to previous reports. The addition of Ca2+ appears to relatively enhance the shorter component while reducing the longer one, although one or more additional slow components appear.