This paper compares various measurements of the absolute quantum efficiency of sodium salicylate and tries to explain the discrepancies between the different determinations that range from 25–99%. The sensitivity of a phototube is commonly assumed to be independent of the angle of incidence. An experiment described here has demonstrated that this is far from true. Since the photocathode angular sensitivity probably varies from tube to tube, great prudence should be shown when using sodium salicylate for absolute intensity determinations. The described photomultiplier-tube effect might also be of significant importance in scintillation counters and other fluorescence studies. The problems discussed here have been carefully avoided by Allison et al. We therefore conclude that their value of the quantum efficiency, which is equal to 99% for layer thicknesses 2–4 mg/cm2, is the most reliable one.
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