Measurements of Low Energy Neutrons from Neutron-Induced Fission

The development of a comprehensive understanding of the process of neutron-induced fission of the actinides is of fundamental importance in many applied areas of nuclear technology, most notably in the design and operation of reactors and weapons, and in nuclear waste transmutation. While much is currently known about the basic fission process in these materials, important gaps remain in our knowledge of the details of fission. For example, it is known that high energy fission neutrons lead to the production of helium in reactors, yet the neutron yields near 10 MeV currently are not well measured. Various models are used to predict the distribution of fission neutron energies, but these must be tested against direct measurements. On the one hand, while the average neutron multiplicity is rather well known, it is much less certain how the neutrons are distributed in energy. And since a sizable fraction of the neutrons are emitted below 1 MeV where direct measurements do not currently exist, the average neutron multiplicity data cannot be used to constrain the high energy region. In short, our rather incomplete knowledge of the details of fission in uranium and plutonium limits our ability to understand processes which use these materials in technological application of importance to our national security and economy.