Relationship Between Microstructure and Efficiency of Scintillating Glasses
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M. BLISS*, R. A. CRAIG*, P. L. REEDER*, D. S. SUNBERG*, AND M. J. WEBER** *Pacific Northwest Laboratory, Box 999, Richland, WA 99352 "**Lawrence Livermore National Laboratory, Livermore, CA 94551 ABSTRACT Prior work has shown that there is a correlation between trap densities and scintillation efficiency of cerium-activated, lithium-aluminosilicate glasses. Raman spectroscopy has strongly suggested that phase separation may be playing an important role in governing the scintillation efficiency. In this study, we relate the thermoluminescence glow-curve data and microstructural analysis for a compositional series. The thermoluminescence data provide information about the traps in the neighborhood of the activator (Ce3+). The microscopy and crystallization of the glasses provide direct evidence of activator partitioning. INTRODUCTION Cerium-activated, lithium-silicate glasses constitute an important and interesting system of scintillators. They are widely used as thermal neutron detectors. Small changes in composition can have dramatic changes in scintillation efficiency. For example, adding magnesium to these glasses increases scintillation efficiency'. It is also known, that substituting calcium for magnesium decreases the scintillation efficiency. The change of network modifier from one alkaline earth to another would not be expected to cause a major change in the glass structure since there is no change in valence and only a small change in ionic radii. Yet, the substitution does have a dramatic effect on scintillation efficiency. The conventional model for scintillation involves three consecutive processes: ionization, energy
transfer, and luminescence. Ionization, the generation of electrons and holes in the matrix, is the result of interactions between the ionizing radiation and the matrix. The subsequent motion of these electrons and holes through the matrix to an activator (Ce 3+ in this case) constitutes the energy-transfer process. At the activator, the excitations recombine to produce luminescence. During the energy-transfer stage, the excitations can be trapped at charged defects in the matrix. This trapping will act to prevent or delay the excitation of and eventual recombination at the activator.
By examining light-generating mechanisms in a simple compositional series, it should be possible to discern if scintillation in cerium-activated glasses is dominated by any one of the three processes described above. Because the glasses are all chemically very similar (mostly silica), the initial conversion of the ionizing radiation in the matrix can be expected not to vary significantly. Changes in charge transport or electron and hole mobility through the matrix to the activator will be indicated by changes in time or energy dependent phenomena, such as scintillation lifetime and thermoluminescence. Direct excitation of fluorescence from the Ce3+ in the glass will indicate changes in the activator and its immediate environment. Examining the UV-Vis absorption of the glasses will indicate problems with scin
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