Efficient Channels of Energy Transfer in High Light Yield LuI 3 :Ce Scintillator
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1111-D08-09
Efficient Channels of Energy Transfer in High Light Yield LuI3:Ce Scintillator Andrey Knizhnik1, Andrey Vasil'ev1, Inna Iskandarova1, Andrey Scherbinin1, Igor Markov1, Alexander Bagatur’yants1, Boris Potapkin1, Alok Srivastava2, James Vartuli2 and Steven Duclos2 1
Kintech Lab Ltd, Moscow, Russian Federation
2
GE Global Research, Niskayuna, New York
ABSTRACT The extremely high scintillation efficiency of lutetium iodide doped by cerium is explained as a result of several factors controlling the energy transfer from the host matrix to activator, two of which are investigated in the present paper. The first one is the increase of the efficiency of energy transfer from self-trapped excitons to cerium ions in the row LuCl3-LuBr3-LuI3. The STE structure and the efficiency of STE to cerium energy transfer are verified by cluster ab initio calculations. We propose and theoretically validate the possibility of a new channel of energy transfer to excitons and directly to cerium, namely the Auger process when Lu 4f hole relaxes to the valence band hole with simultaneous creation of additional exciton or excitation of cerium. This process should be efficient in LuI3, and inefficient in LuCl3. In order to justify this channel we perform calculations of density of states using a periodic plane-wave density functional approach. The performed estimations theoretically justify the high LuI3:Ce3+ scintillator yield. INTRODUCTION It is known that the scintillation efficiency increases in the sequence of LuCl3 < LuBr3 < LuI3 matrices activated by cerium. Lutetium iodide crystals show to be extremely bright scintillators. The light yield of more than 100000 photons/MeV can be explained only if all the stages of the scintillation process are extremely efficient. It was suggested [1–3], that at room temperature, energy transfer by binary electron hole diffusion is the dominant mechanism in LuCl3:Ce3+ and LuBr3:Ce3+, while at lower temperatures, diffusion of self-trapped holes (STH, or Vk centers) becomes probably more important. However, the scintillation mechanism in LuI3:Ce3+ still remains unclear. We set up the following hypotheses for explanation the high efficiency of LuI3:Ce3+: (a) efficient energy transfer from self-trapped exciton (STE) to Ce3+ in LuI3; (b) additional channel for energy transfer due to relaxation of Lu 4f hole; (c) efficient capture of I 5p hole by Ce3+ in LuI3. Channels (a) and (b) are discussed in the present paper, while channel (c) will be discussed elsewhere. In order to explain the specific features of scintillation mechanisms in lutetium halides, it is necessary to understand the main physical mechanisms responsible for the efficiency and for its loss in these materials. An explanation of the yield trends must be done based on a complex analysis of differences in the scintillation mechanisms in the sequence LuCl3–LuBr3–LuI3. The paper is organized as following: (1) description of the ab initio cluster model used to calculate the STE lifetimes in the host LuX3 materials; (2) calculations of
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