The efficiency of void formation during annealing of irradiated molybdenum

Results are presented to demonstrate the high efficiency of vacancy retention that occurs when voids are formed by the annealing at 900°C of molybdenum samples neutron irradiated at 60°C. In the particular case examined the efficiency is shown to be some 50 times more than the theoretical maximum expected during the formation of voids under conventional high temperature neutron irradiation. Various aspects of this efficiency are discussed with particular reference to a recent position annihilation study of the processes taking place during annealing. A mechanism for the high vacancy retention is suggested that depends largely on the storage of vacancies in loop form during irradiation below the temperature of vacancy loop shrinkage and the transfer of vacancies to voids on annealing above this temperature. There seems to be only a small possibility that the mechanism could operate unintentionally in reactor
situations. On the other hand by ensuring void nucleation the high efficiency of void production by the irradiation-annealing method might be used to advantage in providing inexpensive void samples for experimental purposes. Finally, it is noticed in passing that the behaviour of the vacancy loops could provide a simple explanation of the emperical 0.3 T_m threshold for conventional void formation in metals.