From experiments on samples of different thickness the amount of stored light in a particular sample of the infra-red sensitive phosphor made by activating strontium sulfide with cerium and samarium is calculated to be 2.6 × 1016 quanta per cubic cm. This corresponds, for this sample, to one quantum of light for every 8 samarium atoms or every 60cerium atoms.
R. T. ELLICKSON, "Light Storage in Infra-Red Sensitive Phosphors," J. Opt. Soc. Am. 36, 501-502 (1946)