Physics of Delayed Neutrons—Recent Experimental Results

In connection with the U.S. Atomic Energy Commission’s nuclear safeguards research and development (R&D) program, measurements of absolute delayed-neutron yields as a function of energy of the neutron inducing fission have been carried out at the Los Alamos Scientific Laboratory on ²³⁹Pu, ²³³U, ²³⁵U, ²³⁸U, ²³²Th, and ²⁴²Pu. The observed constancy of delayed-neutron yield for neutron energies up to ∼4 MeV brings important advantages to the practical application of delayed-neutron techniques for quantitative assay of fissionable materials. Also reported are delayed-neutron half-lives and group abundances in 14.7-MeV fission of ²³³U, ²³⁵U, and ²³⁸U. Measured delayed-neutron characteristics are discussed in terms of yield systematics, fission mass and charge distributions, and their dependence on energy of the neutron inducing fission.