Abstract

In a fluorescent body (e.g., scintillation counter crystal for detecting nuclear radiation) some of the fluorescent light may be trapped within the body because it is totally internally reflected an indefinitely large number of times. This phenomenon occurs only in bodies whose shapes have high symmetry. Equations are derived for the trapping fractions of rectangular parallelepipeds, plane parallel sheets, and spheres. Illustrative numerical values are given for various values of refractive index. For rectangular parallelepipeds of index 1.225, 1.500, and 2.000, the trapping fractions are 0.0, 0.236, and 0.598 respectively. For a sphere of index 1.50, the trapping fraction is 0.414 or 0.264, depending on whether the index with respect to the exciting radiation is taken as 1.00 or 1.50, these alternatives corresponding essentially to excitation by gamma-radiation or by light, respectively.

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