Three sets of specimens were prepared in which thorium oxide contained as activators praseodymium, samarium and terbium in concentrations varying by known amounts. The specimens activated by praseodymium varied from 1 atom of praseodymium in 100 molecules of thorium oxide to 1 atom in 100,000 molecules; those activated by samarium from 1 atom in 15.5 molecules to 1 atom in 2000 molecules: those activated by terbium from 1 atom in 10 molecules to 1 atom in 40,000 molecules. The intensity of luminescence of specimens in each set was measured and the concentration of greatest brightness, the optimum concentration, determined for each of the following sources of excitation: cathode rays, ultraviolet light and hydrogen flame. The results show that the optimum concentration is different for each of the sources of excitation used. In all cases the concentration required for the optimum under cathode rays is less than that for the other methods of excitation used.
FRANCES G. WICK and CHARLOTTE G. THROOP, "Luminescence of Thorium Oxide Activated by Rare Earths. Optimum Concentration for Different Sources of Excitation," J. Opt. Soc. Am. 25, 57-58 (1935)