Investigation of cascade-induced point-defect fluctuations

Abstract High energy irradiation produces collision cascades, which occur randomly in space and time. Any given point in the material will therefore to subject to intermittent arrival of interstitials and vacancies, even under steady irradiation. The conventional rate theory formulation of the point-defect concentrations, which contains spatial and time averaging and a uniform production rate, may not always adequately describe processes such as void nucleation and irradiation creep. We develop in this paper a theoretical approach to describe the cascade-induced point-defect fluctuations by their moments. The first moment, giving the average point-defect concentration, is used in the traditional rate theory. However, the second moment provides the variance of the point defect concentration, and it becomes important for processes such as void nucleation, dislocation climb, and loop growth. An approximate analytical expression is given for the second moment. The obtained concentration variances compare well with numerical results. We demonstrate with the example of climb-induced dislocation glide how the moments can be used to extend rate theory results. Large enhancement in the creep rate due to point defect fluctuations can be obtained if the barrier size and the average climb rate are small and if the cascade size is large.