[en] We report on the influence of fast neutron irradiation on the critical currents in single crystalline YBa₂Cu₃O_7-δ and Bi₂Sr₂CaCu₂O_8+γ. From magnetization measurements and especially from an analysis of the irreversibility lines, models for the defect structure, effective for flux pinning in these systems, are derived. A comparison of fast neutron irradiation of YBa₂Cu₃O_7-δ at 5 K (and subsequent measurement without warming the sample above 95 K) and at ambient reactor temperature allows us to distinguish qualitatively between the contributions of defects of different size to flux pinning in these systems. (author)