Energy spectra of neutrons leaking from the core tank of the Massachusetts Institute of Technology heavy-water-moderated reactor have been measured with a “fast” neutron chopper. The energy range 2 × 10-3 eV to 2 × 105 eV was examined for three different fuel configurations. The spectra are fairly well described as the sum of a Maxwell-Boltzman distribution and a dE/E slowing-down distribution. The energy resolution, ΔE/E, is less than 5% at energies below 100 eV and varies as E½ above 100 eV, while the probable error in current, ΔJ/J, is less than 10% at all energies.