Abstract
Results are presented which show that when voids are formed by annealing molybdenum at 900°C after neutron irradiation at 60°C, vacancies are retained with an efficiency about fifty times greater than the maximum possible when the material is irradiated at high temperatures. This high efficiency is considered in the light of recent positron annihilation studies of annealing in irradiated molybdenum by Peterson, Throne and Cotter ill (1973), and is thought to be due to the storage of vacancies in loop form below the temperature of vacancy loop shrinkage, and their transfer to voids on annealing above this temperature. The possible relevance to reactor situations and the 0·3 T m threshold are discussed.

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