A model for energy states in a typical crystal containing substitutional cations is reconsidered with emphasis on the behaviour of electrons normally occupying states in impurity levels in the forbidden band. The model is used to explain thermoluminescence after irradiation and field excitation, the fading of colour centres and the unusual type of luminescence kinetics for which the pre-exponential factor in the rate equation is inversely proportional to the dose. A method of finding activation energies from graphs of depolarisation current against temperature is used to find values for PbSO4:Sm. These and measurements from isothermal decays, glow curves, absorption and emission spectra are entered in an energy band diagram.
Experimental results presented by Lilley and McKeever (see ibid., vol.15, p.L39, 1983) to support their view that the kinetics of peaks 4 and 5 in the glow curves of LiF(TLD-100) are first order are re-analyzed to show that they are second order. Additional evidence that the temperatures at the peaks are independent of the dose implies that the frequency factors is inversely proportional to the dose, as for PbSO4:Sm.
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