5. Conclusions

The theoretical basis for positron annihilation in defects is best understood in metallic, covalent or ionic crystals, where it provides immediate access to defect free volume, whether the solid is single or polycrystalline. However, it can also be used to track free volume variations in molecular crystals or amorphous solids [87] . The method's potential has been illustrated by choosing fundamental problems where positron annihilation enables decisive progress to be made thanks to the specificity of its response. It can also easily be used for industrial testing. It can be used to monitor manufacturing processes for materials whose quality depends on defects at the atomic scale (semiconductors) or porosity at the submicroscopic scale (membranes). It can also be used to monitor the evolution and ageing of materials subjected...