[en] The purpose of this investigation is to examine the process of photofission in ²³⁸U near and below threshold. The absolute photofission cross section of ²³⁸U is known to have peaks at 5.2, 5.7, 6.2, 7.1, and 7.8 MeV of gamma energy. It is difficult to understand the existence of such structure in terms of liquid drop model with a single humped barrier against fission with respect to nuclear deformation. However, Strutinsky has recently suggested the existence of a double humped barrier in fission which has an additional potential minimum called the ''second well'' and it appears possible that the above noted resonance structure can possibly be explained in terms of quasi stable states formed in this second well. A detailed study of penetration through a double humped barrier has been made. A model calculation has been performed within the framework of quasi-classical approximation for a potential barrier which also includes the primary potential well of the bound nucleus. Photofission cross sections have been calculated and an attempt has been made to reconcile the apparent resonance structure observed in photofission experiments and the isomeric and ground state spontaneous fission halflives in terms of a single suitable set of parameters of a double humped barrier for ²³⁸U. The results indicate that fission halflives and resonances in photofission cross sections near threshold are satisfactorily reproduced. Our calculation also predicts several low energy resonances in photofission cross sections of ²³⁸U. (U.S.)

[en] Penetrability through a three-humped barrier has been calculated in quasi-classical approximation and a plausible explanation of sub-barrier fission characteristics of thorium is suggested. (Auth.)

No abstract available

[en] The adequacy of using smoothly joined parabolic segments to parametrize the multihumped fission barriers has been tested by examining its simultaneous consistency with the three relevant fission observables, namely, the near-barrier fission cross sections, isomeric half-lives, and the ground-state spontaneous fission half-lives of a wide variety of a total of 25 actinide nuclides. The penetrabilities through such multihumped fission barriers have been calculated in the Wentzel-Kramers-Brillouin approximation, and the various fission half-lives have been determined using the formalism given earlier by Nix and Walker. The results of our systematic analysis of these actinide nuclides suggest that such a parametrization is quite adequate at least for the even-even nuclei, as it reproduces satisfactorily their various observed fission characteristics. Major difficulties remain, however, for the odd mass and for the doubly odd nuclei where the calculated ground-state spontaneous fission half-lives are found to be several orders of magnitude larger than those measured. Possible reasons for such discrepancies are discussed. Fission branching ratios of the decay of the shape isomers in various actinide nuclides have also been calculated and are compared with their measured values

[en] The ²³⁸U nucleus was studied measuring the electrofission yield and angular distributions of fission fragments, in the energy range of 5.5 to 28.3 MeV, using a new method of analysis. An E2 isoscalar giant resonance was found in the photofission cross section of ²³⁸U. This resonance exhausts (71⁺-7)% of the EWSR and is located at 9.9⁺-0.2 MeV with a width of 6.8⁺-0.4 MeV. The position of this resonance is in reasonable agreement with the Bohr and Mottelson prediction (58.Asup(^-1/3MeV). The width of 6.8⁺-0.4 MeV is compatible with a possible triple splitting of the resonance. From the angular distributions of photofission fragments and yield measurements of multipoles other than E1, evidence of an M1 mixture in the energy region 6-7 MeV was found

[en] Subbarrier photofission cross sections and the fission half-lives have been calculated for ²³⁸U in terms of a suitable double-hump fission barrier model. The competition due to the gamma deexcitation to the shape isomeric state in the second well (and its consequent fission) has been included by introducing an absorptive part in the potential in the second well region. The calculated cross sections reproduce satisfactorily the recently observed ''shelf'' in the deep-subbarrier energy region and result in a resonance structure in the threshold region consistent with that observed. The calculation also predicts several low energy resonances in the cross sections and a detailed competition of the prompt and the delayed fission contributions suggests that an angular anisotropy measurement might be more sensitive in detecting the relatively small contribution of delayed fission in the energy region 4.5--5.5 MeV

[en] Subbarrier photofission cross-sections and the fission half-lives have been calculated for ²³²Th in terms of a suitable three-hump fission barrier model consistent with the suggestion of Moeller and Nix. The competition due to the gamma deexcitation to the shape-isomeric state in the second well (and its consequent fission) has been included by introducing an absorptive part in the potential in the second well region. The calculated cross sections reproduce satisfactorily the recently observed ''shelf '' in the deep-subbarrier energy region and provide a further quantitative evidence in favor of resolving the ''thorium anomaly'' along the lines suggested by Moeller and Nix

[en] The systematics of the fission barrier shapes of a total of 47 doubly even actinide and transactinide nuclei have been studied using the double-humped fission barrier model. The fission barrier has been parametrized in terms of four smoothly joined parabolic segments. The penetrabilities through such double-humped fission barriers have been calculated in the Wentzel-Kramers-Brillouin approximation, and the various fission half-lives have been determined using the formalism given earlier by Nix and Walker. The various parameters of such fission barriers have been deduced by requiring their simultaneous consistency with the various relevant fission observables, namely, the near-barrier fission cross sections, isomeric energies and isomeric half-lives, where available, and the ground-state spontaneous fission half-lives in the region 90≤Z≤98, and such model calculations with some further justifiable asssumptions have been extended to the region of the still heavier nuclei with Z≥100. The results of our systematic study of the heights of the inner and the outer barriers of the double-humped fission barriers corresponding to such doubly even nuclei suggest that while the height of the inner barrier remains approximately constant in the entire region of such nuclei, the deduced heights of the outer barrier decrease rather sharply and continuously with the increase in the value of the fissility parameter until one reaches the element Rf (Z=104)

[en] The concept of resonant tunneling is invoked to explain the sharp drop in the measured spontaneous-fission half-life when going from ²⁵⁶Fm to ²⁵⁸Fm. Various consequences of such a suggestion on the other observed characteristics of the bimodal symmetric fission of ²⁵⁸Fm are briefly discussed

[en] The double-humped potential barriers in actinide nuclei in the fission direction have been parametrized using three different procedures, namely, the smoothly joined parabolic segments, third-degree polynomials passing through and with zero slopes at the successive extremum points, and straight-line segments connecting the successive extremum points. The fission penetrabilities through the barriers and the ground-state spontaneous fission half-lives for a wide variety of 25 actinide nuclides have been calculated for these different parametrizations. Our results clearly indicate that while the third-degree polynomial and the straight-line parametrizations of the double-humped fission barrier lead to approximately similar results on the fission penetrability and fission half-lives, the corresponding results using the smoothly joined parabolic segment parametrization differ significantly by almost two to five orders of magnitude depending on the specific type of the fissioning nucleus and on the parameters of its corresponding double-humped fission barrier