The paper presents the results of a study of the build-up and decay of some 200 samples of silicate, tungstate, molybdate and sulphide phosphors, excited by Hg 2536A radiation. The normal silicates decay at first exponentially, with a time constant of the order of 10−2 sec. This initial rate is independent of the intensity and the duration of the excitation, and varies little among different samples of a given class (e.g., Zn2SiO4·Mn). The later decay is slower, and does vary markedly from sample to sample, and depends on the intensity and duration of the excitation. CaWO4 decays too rapidly to be measured with the apparatus used (<50 microseconds); the decay of MgWO4 lasts about 2×10−4 sec.; both these tungstates show a very faint long phosphorescence which can be enhanced slightly by addition of impurities. The red luminescence contributed by Sm in these and other “pure” phosphors decays exponentially with a time constant of about 7×10−4 sec. Sulphides have complicated decay characteristics, dependent on the intensity and duration of the excitation. Comparison is made with the results of a study of decay after electron bombardment, and with other published data.
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