A neutron-exposure unit for radiation-damage studies is derived which is proportional to gross-displacement production with little dependence on the reactor spectra or displacement model selected. The unit is applied to the analysis of irradiation-induced changes in nil-ductility temperature in steel. Data from a graphite reactor and from the core of a water-moderated test reactor were statistically distinct as originally reported. Through the application of this unit, these results could be reconciled to constitute a single body of data.