Effective K Quantum Numbers in Fission of Oriented 235U

The angular anisotropy of fission fragments produced in neutron-induced fission of aligned 235U nuclei has been measured for neutron energies between 0.3 eV and 175 eV, using time-of-flight techniques at the pulsed 45 MeV electron linear accelerator at Saclay. The low-temperature nuclear-alignment apparatus used was a modified version of the apparatus described at the Salzburg conference, and gave an average temperature of 0.61 K for the four UO2Rb(NO3)3 single crystal samples during a measurement period of approximately 220 h. The flight path was 5 m and the election pulse length was 100 ns. Multilevel fits to the observed 0° and 90° fission cross-sections have been made using the Adler and Adler formalism with the aid of a program developed by G. de Saussure. The most striking result obtained in the analysis of some 16 of 100 levels or groups of levels is a strong correlation between small fission width and an effective value of K ≅ J. AH 5 resonances in the group of 16 for which results are final, for which ≅ = J is deduced, have exceptionally small widths. This result suggests that the small widths are associated with a large rotational energy and consequent diminution in available deformation energy, so that these 5 resonances are effectively sub-threshold resonances; such a suggestion is quite in accord with the ideas of Bohr. The preponderance of other resonances so far analysed have effective K values of 1 or 2. An analysis of all resolvable resonances is presented. (author)