Effects of cooling rate during annealing procedure in BeO

The uncertainty associated with TL measurements has been related to the heating phase of the annealing and of the reading cycles, in particular, the need for stability of the ovens and the reproducibility of the heating rate during TL readout is pointed out. The cooling conditions are another element of importance in the annealing cycle of some TL materials. Therefore, sensitivity variations, in terms of cooling rate from high temperature to room temperature, have been investigated in BeO, LiF and CaF2:Dy loose detectors. Fast cooling in LiF but slow cooling in BeO markedly increases sensitivity, while CaF2:Dy is unaffected by cooling rate. The different behaviour of these TL materials could be explained by their different structures. The effect described is found not to be permanent, i.e. TL materials 'remember' only the last cooling cycle, since the crystalline structure of the detectors is not modified by thermal procedures carried out for annealing. To ensure reliability of the dosimetric data, extreme reproducibility of the entire annealing cycle is required.