Abstract
Aluminium samples containing helium bubbles produced by 600 MeV proton irradiation at 430 degrees C were investigated by positron annihilation; both lifetime and angular correlation measurements were made. The angular correlation curves contain an unusually narrow component. This component is associated with positronium formation in the bubbles. The width of the component indicates localisation of the Ps atom, possibly in a surface physisorbed state. Sodium segregated at the surfaces of the bubbles is a likely cause of the positronium formation.