Slow Neutron Resonances inU235

Abstract

The total and fission cross sections of ${\mathrm{U}}^{235}$ have been measured in the energy range 0.1 to 10 ev on the BNL crystal spectrometer. The capture cross section is derived by combining these data with published scattering cross section data. Analysis of the low-energy resonance structure depends strongly on the interpretation of the background cross section which dominates the low-energy region. This background cannot be accounted for in terms of observed resonances; however, if it is attributed to the presence of a bound level, approximate parameters for such a level can be obtained. It is seen that the strength ${{\ensuremath{\Gamma}}_{n}}^{0}$ of the hypothetical bound level is anomalously large.The capture component of the resonances is found to be symmetrical, i.e., of Breit-Wigner shape. However, the fission component of the same resonances is asymmetrical, which indicates that a multilevel formula is necessary for fitting the resonances in the fission cross section. A good but not perfect fit to the data between 0 and 2 ev has been achieved using a multilevel formula which applies to the case of a single fission channel and many capture channels. The remaining small discrepancies in fitting the data indicate that the assumption of a single fission channel is too restrictive, and that one or more additional channels contribute weakly to slow neutron fission in ${\mathrm{U}}^{235}$.