CRITICAL HELIUM CONCENTRATION AND ITS EFFECT ON BUBBLE-GROWTH IN PALLADIUM

Helium bubble growth in palladium, implanted with different helium con centrations from 25 to 250 appm, has been investigated by positron lif etime spectroscopy during isochronal annealing. It is found that the b ubble growth behaviour for helium above a critical concentration of 10 0appm is different from that for doses below it. In palladium with < 1 00 appm helium, the bubble radius increases sharply with annealing tem perature. For helium concentrations greater-than-or-equal-to 100 appm, the bubble radius increases slowly upto 1200 K, above which the growt h rate picks up. These observations are explained on the basis of the build-up of over-pressure in bubbles and correlating the change in gro wth behaviour with pressure.