Cerium-Doped Orthophosphate Scintillators
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A. J. WOJTOWICZ(1,2, 3), A. LEMPICKI(1, 2),4D. WISNIEWSKI (1,2,3) and L. A. BOATNER( ) (') Chemistry Dept., Boston University, 590 Commonwealth Ave., Boston, MA 02215, USA (2) ALEM Associates, 303A Commonwealth Ave., Boston, MA 02115, USA (3) Institute of Physics, N. Copernicus University, Grudziadzka 5, 87-100 Torun, Poland (4) Solid State Div., Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 3783 1,USA ABSTRACT Cerium-doped lutetium orthophosphate (LuPO 4:Ce) is one of the most promising new scintillator materials. In this paper we report on the effects of total or partial replacement of Lu by Yb. When not doped with Ce, crystals of YbxLul-xPO 4 scintillate rather poorly, the emission being due to the Yb charge transfer transition. Activation with Ce reduces the light output from Yb even that material's scintilfurther, while correspondingly the addition of Yb to LuPO 4:Ce also degrades lation performance. We suggest that the nonradiative decay of the (Yb2+-Ce 4+) charge transfer state is responsible for the mutual quenching of these ions. The differences in performance of LuPO 4:Yb and LuPO4 :Ce indicate that energy transfer mechanisms ordinarily associated with materials containing molecular anionic groups may not be quite sufficient to explain the results. Suggestions are presented that under y-excitation the lattice-to-ion energy transfer in orthophosphates is accomplished by sequential capture of charge carriers rather than the more conventional mechanism of exciton hopping between molecular groups. INTRODUCTION Cerium-activated lutetium orthosilicates (LSO) and orthophosphates (LOP) have recently been identified as outstanding scintillators [1,2]. Their remarkable performance has been ascribed to extremely fast and efficient transfer of energy from electronic excitations of the host to the activator
ion [3]. Phenomenologically, this transfer can be viewed as including all processes that occur between the generation of electron-hole pairs and the excitation of the Ce ions to one of the emitting 5d states. In the case of CeF 3, the value of the transfer efficiency S has been found to be substantially less than unity [3], indicating that competing nonradiative mechanisms such as electron-hole recombination prevent a significant fraction of the energy from ever reaching the activator ion. It is now becoming clear that the critical loss mechanism may occur not during the spatial migration of the energy, but rather during the step between the arrival of the energy at the emitting site and the excitation of the actual emitting state [4]. If after excitation by ionizing radiation the activator is capable of binding either electrons or holes, a new nonradiative path can be opened; thus, depending on the details of the higher electronic levels of the activator, a substantial loss mechanism can come into play even though the efficiencies of both the spatialtransfer and the near-UV photoluminescence are close to unity. We believe that this picture can explain both the relative inefficiency of CeF 3 scintill
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