AbstractIn-core absolute neutron spectra are measured for high-enriched uranium (HEU) and low-enriched uranium (LEU) materials test reactor fuel using multiple foil activation. The measurements were made in special fuel elements configured to simulate a regular fuel element but that permitted reproducible insertion of a wide range of samples. The measured subcadmium LEU flux was 14% less than the HEU fuel, which is within the experimental uncertainty of the 19% calculated value. There was no observed difference between the fast spectra. A separate spectral unfolding of just the LEU fast flux yielded reasonable agreement with proton-recoil measurements.