STUDY OF U-238 NEUTRON RESONANCES WITH ANOMALOUSLY LARGE FISSION WIDTH

The neutron-capture and -scattering measurements and gamma-cascade investigations for U-238 resonances have been carried out to study the nature of U-238 neutron resonances with anomalously large fission width. Radiation widths for 721.58 eV and 1211.4 eV resonances have been determined. One and the same radiation-width value of the 1211 eV resonance was obtained from capture and scattering data. This value turned out to be equal to 23 +/- 3 meV and corresponded to an average radiation width for U-238 resonances. That means that the 1211 eV resonance is not distinguished by its radiation width. For the 721 eV resonance the anomalously small radiation width of 3.2 +/- 0.5 meV, which is more than 5 times less than a radiation width of the known resonances of all nuclei, was obtained from capture measurements. The scattering data do not contradict this result. But due to their insufficiently high accuracy it is not excepted that the radiation width of the 721 eV resonance could have a routine value (23 meV) if we assume the presence of an additional previously unknown decay channel for this resonance. The multiplicity spectra and energy spectra of capture gamma-quanta for both mentioned resonances are similar within the experimental errors (similar to 15%) and do not differ from appropriate spectra for ordinary s-resonances. The data obtained indicate that the 721 eV resonance is an extraordinary one and its extraordinarity is unlikely caused by the state of the second potential well of the double-humped fission barrier.