The influence of cobalt inclusions in ZnS : CdS ." Ag : Ni phosphors on their excitation and emission spectra, temperature-dependence of fluorescence and thermoluminescence has been studied. Increase in cobalt concentration seems to have no effect on the excitation and emission spectra. The temperature-dependence of fluorescence of phosphors containing 0~243 -0.065~243 cobalt has been measured over the temperature range of 77 ~ to 600 ~ The observed changes in the activation energy for the quenching process with increase in cobalt concentration are attributed to the presence of deep electron traps. Thermoluminescence curves show peaks identified as due to chlorine, cadmium, cobalt and nickel impurities. The corresponding energy depths are 0.26 eV, 0.52 eV, 0.53 eV and 0.78 eV. A thermoluminescence peak with ah energy depth of 1.09 eu has also been found. Ah attempt has been made to identify the origin of this peak.
IntroduetionImpurity-activated zinc sulphide type phosphors with additional nickel, cobalt or iron impurities as "lumineseence quenehers" have been investigated by many workers to study the fundamental luminescence meehanisms involved. It appears from the literature that most of the phosphors investigated contained only one type of queneher centre as the coactivator and no attempt was made to study the effect of introducing a second quencher centre together with the first. It is known that when cobalt, niekel or iron is included in zinc sulphide type phosphors characteristie trapping levels with high reeombination coefficients result. ARPIARIADI [7] and GERGELY [8] suggested that deep eleetron trapping states and/or, hole acceptor levels created by nickel of cobalt inclusions, are responsible for their quenching action, KRYLOVX [9] was the first to detect the thermal glow peak due to deep traps associated with cobalt impurity in zinc sulphide. This peak has a maximum at 383 ~ fora warming
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