FISSION COMPONENTS IN 234U RESONANCES

The fission cross section of 234U has been measured in the energy range below 20 keV and shows three regions of strong fission yield with essentially no fission at the intervening energies. In the first group, which covers the energy range up to 740 eV, all the 20 resonances known to exist below 370 eV in the total cross section have a measurable fission width with an average 〈Γf〉 = 0.098 meV. The second and third groups are centred at 8.33 keV and 13.9 keV and extend over about 1 keV. The area under the fission cross section curve σ0Γf is 97.7 b · eV for the 29 resonances below 740 eV, 52 b · eV for the group at 8.33 keV and 32 b · eV for the group at 13.9 keV. An analysis of the distribution of fission widths for the 20 resonances below 370 eV shows that they fit a χ2 distribution with v = 1.39±0.37 degrees of freedom. This evidence for a grouping of sub-threshold fission resonances is similar to that already found in 240Pu and 237Np and according to Weigmann and Lynn is a result of the existence of the second minimum in the fission potential barrier as predicted by Strutinsky. The well depth corresponding to a level density of 7 keV indicates that the second minimum lies at 3 MeV above the ground state of 234U. © 1968.