Bombardment with massive fast particles produces atomic displacements in a solid, and important changes in physical properties arise from the resultant atomic disorder. This paper is concerned with the calculation and measurement of the concentration of displaced atoms produced by nuclear radiations and, in particular, with a critical comparison of theory and experiment.

The theory of displacement production is outlined briefly. The number of atoms displaced in a solid by a given flux of bombarding particles of known energy can be calculated. It is assumed in the theory that the process can be described in terms of hard sphere collisions and that a definite threshold energy may be assigned to the displacement process.

The concentration of displaced atoms can be derived more or less directly from several types of experiments. The following are discussed: (1) neutron transmission studies of reactor-irradiated graphite as well as the numbers derivable from changes in electronic properties and the storage of energy in this material; (2) low temperature resistivity measurements on metals irradiated at low temperatures; (3) measurements of electronic properties of semiconductors after slight irradiation; and (4) optical studies of high melting oxides after slight irradiation. The discrepancy between theory and experiment is of the order of a factor of two for graphite. Such a discrepancy cannot be considered serious for such an intricate process and it is concluded that theory and experiment are in essential agreement in this case. For other types of solids agreement can be claimed only within an order of magnitude. The number of displaced atoms is apparently overestimated theoretically. At the present state of our knowledge it cannot be decided whether the discrepancy in these cases is due to inadequacies in the theory of displacement production or in the theories linking changes in physical properties to the concentration of crystalline defects.