TEMPERATURE DISTRIBUTION IN THERMOLUMINESCENCE EXPERIMENTS .2. SOME CALCULATIONAL MODELS

This is the second of a linked pair of papers on the subject of thermo luminescence experiments; it is primarily about calculational models, while the first (preceding) paper describes experimental results. In a typical thermoluminescence experiment, a sample rests on a metallic s trip which is heated in a controlled fashion so that the strip tempera ture rises linearly with time. Thermal contact is improved by the use of an inert exchange gas, usually argon. With this procedure, samples of interest emit light spectra of low intensity as electrons escape fr om traps. The technique is applied, for example, to dating of artefact s or geological materials, to radioactive dosimetry, and to the charac terization of optical materials. In this paper we consider some situat ions for which exact solutions of the heat conduction equation can be obtained. Horizontal temperature distributions in the heating strip ar e dealt with, as is, importantly, the time-dependence of the temperatu re within, or at the top of, a sample of finite thickness resting on t he heater strip. We show that, beyond a short initial transient, the t emperature variation caused at a point in the sample by a linear ramp of the strip is of the form beta(p)t - delta(p) where delta(p) represe nts a constant temperature lag and beta(p) is a rate which may be smal ler (but not greater) than the ramping rate of the strip. Delta(p) and beta(p) are calculated, for a plausible model, in terms of heat trans fer parameters defined in the paper, and these can be measured. These effects are important where data from different laboratories are compa red and we conclude that practitioners should routinely take note of t he following points. Firstly, ramping rates should be as low as possib le consistently with the optical sensitivity available (there is a tra de-off between the two, but temperature differences are likely to be i mportant where ramping rates exceed 5 K s-1). Secondly, it is advisabl e to use helium as exchange gas rather than the more usual argon, beca use it is a much better conductor of heat. Thirdly, users should do a few basic thermal experiments with their apparatus so that, with the a id of formulae given in this paper, they can make corrections where th ese are important.