Thermoluminescence and isothermal decay of luminescence in x-irradiated urea and thiourea

Experiments have been performed on the thermoluminescence and the isothermal decay of luminescence of both urea and thiourea crystals excited by X-rays at low temperature. The analysis of the isothermal decay curves has been performed assuming that the light emission consists of a sum of exponential decays with different time constants τ_i. Taking into account the dependence of τ_i from temperature, the values of thermal activation energies of the trapping centres involved in the luminescence process are obtained. In addition, the analysis of the glow curve, that has been performed through three different methods, allowed to obtain the values of some parameters related to the thermoluminescence process, such as the thermal activation energy E, the frequency factor s and the ration A/B between the retrapping and recombination probabilities. These results agree well enough with those from the isothermal decay analysis. A comparison has been made between the results for urea and for thiorea. Some topics concerned with the nature of the defects responsible for the luminescence process in these substances are also investigated.