A new damage function for predicting the effect of reactor irradiation on graphite in different neutron spectra

The damage function v (E2) is the number of displaced atoms produced by a recoil atom with energy E2. The assumptions made in previous derivations are examined and it is concluded that random cascade theories are insensitive to both the displacement model and the exact nature of the interatomic collision and the Kinchin and Pease function E2/2Ed is a good approximation. Recent measurements of the displacement energy Ed by Mitchell and Lucas 5) now make quantitative calculations possible. Focusing is not expected to effect v (E2) significantly and the question of channelling must bo left open for the present. Energy lost from the cascade by electron excitation is known to be an important effect, but previous attempts to include it in the theory have been very crude and based on scanty experimental justification. Measurements have recently been made of the energy loss of electrons from C+ ions in graphite below 1 MeV3,4). These data are incorporated into the theory of the damage function and a new function is derived which is used to predict the effect of changing the neutron spectrum on the damage rate in graphite. The new theory gives a small but significant improvement in the agreement between predicted and observed rates.