Monoenergetic neutron bursts in heavy moderators are being examined for application to the nondestructive assay of fissile material in fuel samples. The goal of these assays is the determination of fissile isotope content, generally in the presence of other fissile or fertile isotopes. The technique utilizes the relationship between neutron energy and the slowing down time, or time elapsed after the burst in the heavy moderator. The slowing down time spectrometer, a lead cube into which 14-Me V neutrons are pulsed, is used to determine the fissile isotope content. Differences in fission cross sections at specific energies are used to discriminate between fissile isotopes in the same sample.