Irradiation damage of nickel single crystals bombarded with 3 Mev electrons has been studied by Kossel line techniques. Changes of lattice parameter and x‐ray line breadth were determined for crystals bombarded with integrated dosages of 1016, 1017, and 1018electrons/cm2at liquid nitrogen temperature. It is concluded that the formation of Frenkel defects in the lattice due to the electron bombardment is consistent with the data obtained. Subsequent annealing studies support this conclusion.Using an electron microbeam probe to obtain the Kossel patterns, the absolute precision of the lattice parameter obtained is better than 1 part per 100 000 or ±0.00003 A. The limit of relative precision of change in lattice parameter of a given crystal is 3 parts per 1 000 000 or 0.00001 A. Basic principles of the formation of Kossel lines, experimental arrangements, various errors, and related models are presented.