An experiment was performed to measure the energy-dependent damage function for the neutron irradiation induced increase in the lower yield strength of iron. Small tensile test specimens were exposed at low temperatures (<100°C) to 1017 - 1018 n/cm2 (E > 1 MeV) in seven reactor spectra while a high degree of consistency among all other variables known to be significant was maintained. The neutron flux spectra in all locations were measured using the multiple foil activation technique and the SAND-II code for the data analysis. The mechanical property and flux spectral data were then analyzed using SAND-II to unfold the damage functions. Estimates of errors induced in the solution by the iterative unfolding procedure were made considering possible contributions from both the lack of mathematical solution uniqueness and data error propagation.