The participation of different aggregated phases of the Eu impurity in the afterglow (AG) and thermoluminescence (TL) of KCl:Eu after UV-irradiation at 20 K is investigated. Several aggregated phases of Eu are formed by applying different thermal treatments and their formation is monitored by photoluminescence. It is shown that the treatments leading to high concentration of aggregates result in much higher AG and TL yields than the treatments leading to high concentration of dipoles. This is due to a more efficient AG and TL production for aggregates than for dipoles. Additionally, we observed a huge increase of the AG and TL production efficiency around dipoles after ageing at 200 °C. The results are discussed in view of a recently proposed mechanism for AG based on the migration of H centres through dislocation lines. The main conclusions are that dislocation lines trap dipoles and that precipitation occurs along dislocation lines causing a large fraction of aggregates to participate in the AG and TL.