Electron radiation damage near the threshold energy in aluminum

Foils of high-purity aluminum, both with and without preinjection of about 10 at. ppm helium, were electron irradiated in the electron microscope operating at voltages in the range 150 to 200 kV. In the helium-doped aluminum electron damage in the form of black spots was produced at 150 kV after about 90 min irradiation time. They showed contrast effects characteristic of dislocation loops. No such spotlike damage was observed at 150 kV in the uninjected aluminum for bombardment times over 4 hr. Damage was observed in both preinjected and uninjected specimens at 175 and 200 kV. Possible mechanisms for sub-threshold damage are examined to see which account for the electron damage observed below the accepted threshold of about 170 kV for aluminum.