AbstractNeutron leakage spectra through copper slab assemblies with thicknesses of 5.8 cm and 11.8 cm have been experimentally obtained on an absolute basis using proton recoil technique and differentiation unfolding procedure. The measured results have been compared with calculations employing MCNP transport code and continuous energy cross section data from obsolete ENDF/B-IV and state of the art ENDF/B-VI libraries. The comparisons show a disagreement of as much as 73% of the ENDF/B-IV calculations, and an overestimation of 16% in the 1–2 MeV energy range and an 18% underprediction of the forward emitted neutrons in the 5–10 MeV interval by the ENDF/B-VI calculations.