Evolution of Threshold Displacement Energy in Irradiated Graphite

Molecular-dynamics simulations are used to compute the threshold displacement energy (E-d) in a series of progressively damaged graphite structures. The E-d values are obtained by a statistically robust probabilistic method using a large number of primary-knock-on-atom events at energies up to 100 eV. No sharp threshold for E-d is observed, and a number of possible definitions are considered. For pristine graphite, the best estimate of E-d is 24 eV. E-d decreases with increasing irradiation damage, dropping by nearly a factor of 2 at a dose of one displacement per atom. For a fully disordered amorphous-carbon structure, E-d is around 5 eV. This evolution of E-d is an important missing ingredient in current estimates of radiation doses in nuclear reactors, which assume E-d is constant over the reactor lifetime, despite substantial structural evolution.