Effect of thermal vacancies on anomalous helium bubble growth in palladium: A positron annihilation study

Positron lifetime measurements on palladium implanted with 100 appm of helium are reported as a function of annealing time for three different temperatures. An anomalous bubble growth regime is identified from the variation of helium atom density inside bubbles with annealing time, as deduced from tau(2) In this growth regime, helium atom density is found to decrease sharply, indicating relaxation of pressure inside bubbles. This is explained as due to the availability of thermal vacancies which condense on to bubbles thereby reducing helium atom density. An activation energy of 0.88 eV is estimated from the experiment which matches with the reported vacancy migration energy. It is concluded that thermal vacancy migration to bubbles is the rate limiting process which triggers the anomalous bubble growth.