The neutron total cross section of ${}_{92}{}^{}{}_{}{}^{}\mathrm{U}_{}^{235}{}_{}{}^{}$ has been studied for neutron energies between 1.5 and 60 ev by using the Brookhaven fast chopper. The transmission data were analyzed by the "area" method and the Breit-Wigner parameters obtained for the resonances up to 35 ev. The observed level spacing of 0.65 ev is the smallest yet reported for any isotope. The best available fission data have been combined with total cross-section data to obtain ${\Gamma }_{\gamma }$. The values of ${\Gamma }_{\gamma }$ are approximately constant and are consistent with those of neighboring heavy nuclei, whereas the ${{\Gamma }_n}^0\text{'}\mathrm{s}$ have a very broad distribution similar to that observed in a number of other elements. The distribution of ${\Gamma }_f\text{'}\mathrm{s}$ is also rather broad and resembles the neutron widths much more than the radiation widths in this respect. Average values of these parameters are presented, together with a discussion of the implications of these findings for current theories of fission.

• Received 28 May 1956

DOI:https://doi.org/10.1103/PhysRev.103.1342