Some effects of irradiation at 77°K on the physical properties of graphite

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Abstract
Samples of reactor graphite have been irradiated at 77°K with neutrons and earlier work with electrons has been extended to higher doses to enable comparison to be made. Changes in the electrical resistance, magneto-resistance, Hall constant and dynamic and static Young's moduli have been studied as a function of irradiation dose and annealing. It was found that the rate of change of the properties as a function of the number of displaced atoms differed for the two types of irradiation. Estimates of the number of trapped electrons and changes in the carrier mobility show that there are more trapped electrons per displaced atom for the neutron irradiations, but the scattering power of the traps is the same in both cases. Analysis of the changes in Young's moduli shows that neutron irradiation produces damage in which some of the interstitials are already clustered. Inverse annealing effects occur in the electrical properties in the range 100–160°k and in the mechanical properties in the range 200–280°k. The annealing behaviour of the neutron irradiated samples was more complex than that of the electron irradiated samples. It is shown that all the results are consistent with a model for neutron damage in which the displaced atoms are in small loosely coupled clusters, of about 10 interstitials, distributed along the paths of the primary knock-one. and for electron irradiation in which the damage is mainly randomly distributed simple Frenkel pairs.