Influence of Dose Rate on TL Response of Heavily Irradiated Rock Salt

The effect of high absorbed doses of gamma radiation on rock salt has been studied, in the framework of the 4th R&D programme of the Commission of the European Communities 'Management and Storage of Radioactive Waste'. To fulfil the objectives of the aforementioned research, the Gamma Irradiation Facilities of the Joint Research Centre of the European Communities were used. These facilities operate with spent fuel which allows dose rates up to 200 kGy.h -1 . Several experiments have been performed so far, at dose rates of 4, 15 and 50 kGy.h -1 . Irradiations were performed under a controlled temperature of 100°C and total doses ranging from 20 kGy to 1223 MGy. The irradiated rock salt was studied by thermoluminescence (TL). As expected, there was an increase in the TL response with increasing dose. In addition, the temperature onset of the glow curve was shifted to higher temperatures with increasing dose. At doses around 800 MGy, the glow curve started above 430°C, hindering its evaluation with a standard commercial TL reader. For the same absorbed dose, the TL response was higher in samples subjected to lower dose rate irradiations. This phenomenon was also detected with optical absorption spectroscopy measurements performed in the same samples. The results agree with the predicted concentration of radiation-induced defects present in the rock salt obtained by modelling.