Abstract
Both single interstitials and single vacancies are absorbed at dislocation loops when both are mobile. The growth of existing loops during further irradiation is therefore a second-order effect, and it is shown to be small. This necessitates new assumptions to explain the loop size distribution observed at 300°K. A model is given in which it is assumed that loop formation takes place entirely within the displacement spike. Invariance of the loop size distribution with dose, which is observed experimentally, follows from this assumption. It is calculated that loop nucleation occurs by the collision of two or three defects within the spike.