Fission barrier determinations and fragment angular correlations for the 244Pu, 242Pu, 240U, 238U, 234Th, and 232Th compound nuclei from (t, pf) reactions

Fission probabilities and the angular distribution of the fission fragments have been measured for six even-even compound nuclear systems using the (t, pf) reaction. Angular correlations of fission fragments obtained in these experiments provide information about the low-lying collective excitations or transition states at the fission barrier. The (t, p) reaction in particular leads to neutron-rich residual nuclei unobtainable by other methods. The absence of spin coupling for (t, p) reactions on even-even targets provides angular distributions with well defined structure in the region of the fission barrier. The experimental data were obtained using an 18-MeV triton beam on targets of 242Pu, 240Pu, 238U, 236U, 232Th and 230Th at Los Alamos Van-de-Graaff accelerator facility. Outgoing protons were detected at 140 deg relative to the incident triton beam. Excitation energies ranging from 3.0 to 9.0 MeV were obtained in these experiments. Fission fragment angular distributions were measured at 24 angles from 0 deg to 140 deg relative to the kinematic recoil angle. The data were fitted to a series of even Legendre polynomials W(θ) = A0 [1 + ΣL gL PL (cos θ)] and the coefficients g2 through g12 and A0 were determined as a function of excitation energy. The fission probability Pf was obtained from the ratio of A0 to the (t, p) cross-section for the target nucleus. The results exhibit well defined structure in the angular coefficients which correlates with structure in the fission probability for most of the nuclei studied. In an attempt to interpret this observed structure the experimentally determined fitting parameters Pf and g are compared with calculated results of a microscopic model. This model takes into consideration the penetrability and angular dependence of fission through each member of the various transition bands at the saddle point and appropriately sums the results for comparison with the data. The effects of barrier penetration through a two-peaked deformation potential are explored in this analysis. This survey of the fission properties of several nuclei indicates some general trends relating the saddle point deformation to the character of the transition level structure. (author)