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

The neutron-capture and -scattering measurements and gamma-cascade inv estigations for U-238 resonances have been carried out to study the na ture of U-238 neutron resonances with anomalously large fission width. Radiation widths for 721.58 eV and 1211.4 eV resonances have been det ermined. One and the same radiation-width value of the 1211 eV resonan ce was obtained from capture and scattering data. This value turned ou t 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 t han 5 times less than a radiation width of the known resonances of all nuclei, was obtained from capture measurements. The scattering data d o not contradict this result. But due to their insufficiently high acc uracy it is not excepted that the radiation width of the 721 eV resona nce could have a routine value (23 meV) if we assume the presence of a n additional previously unknown decay channel for this resonance. The multiplicity spectra and energy spectra of capture gamma-quanta for bo th mentioned resonances are similar within the experimental errors (si milar 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 barri er.