Abstract
The fission cross section of U234 over the energy range of 180 keV to 6 MeV has been measured in high resolution experiment by neutron time of flight technique on the Synchrocyclotron at Harwell. The results show intermediate structure in the fission cross section near the vibrational resonance at 318 keV. The narrow intermediate structure below the threshold region (below 20 key) was first observed by James and Rae ref. (1.11). The parameters of many of the fine structure levels comprising the resonances at 638 eV were determined by James and Slaughter, ref. (1.12) who showed that the fission widths of the fine structure have a Lorentzian energy dependence. A broad resonance in the fission cross section of U234 was first observed by Lamphere ref. (1.2) in the subthreshold region at ~ 325 keV. The present measurements have confirmed the existence of the broad, resonance which is found to be centered at 318 keV. The fission cross section at 318 keV is obtained to be 0.l6b and is consistent with the value obtained by Lamphere ref. (1.2). It has been suggested by Lynn ref. (1.24) that the break observed at 318 keV is associated with a vibrational level occurring in the second minimum of the potential energy expressed as a function of nuclear deformation. It has also been suggested by Lynn that the narrow intermediate structure at low energy observed by James and Rae could be associated with the wing of a vibrational level which could be the one observed at 318 keV. The present measurements were carried out to discover whether intermediate structure exists in the fission cross section of U234 at near subthreshold energy of 318 keV, and also to decide whether the properties of this structure were consistent with the parameters of the resonances at 638 eV, so verifying Lynn's suggestion. The structure is expected, from the low energy data, to have a spacing which is somewhat greater than the experimental energy resolution, ~1keV at 318 keV. Significant fluctuations in the cross section have been observed and the evidence of the structure at this energy has been established. To support the evidence, the fission cross section has been simulated using a model based upon Lynn's theory of Intermediate Structure. From the simulation, the height of outer barrier of the double well potential curve has been estimated to be ~ 892 KeV. The simulated fission cross section yields a structure, the average properties of which are consistent with the average properties of the observed structure at 318 keV. The low energy parameters have been deduced from the simulated fission cross section and compared with the values obtained experimentally by James and Rae. This showed that the square root of the Class-II fission width, (rfII)½, deduced from the simulation is 1.7 standard deviation away from that of the experimentally observed fission width by James and Rae.