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.