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Subbarrier photofission cross-sections and the fission half-lives have been calculated for "Th in terms of a suitable three-hump fission barrier model consistent with the suggestion of Moiler 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 Moiler and Nix.NUCLEAR REACTIONS Fission, calculated subbarrier photofission cross-sec-tions and fission half-lives using a three-hump fission barrier in Th. Results reproduce the observed "shelf" in the cross-sections indeep-subbarrier energy region.

The concept of resonant tunneling is invoked to explain the sharp drop in the measured spontaneousfission half-life when going from 256 Fm to 258 Fm. Various consequences of such a suggestion on the other observed characteristics of the bimodal symmetric fission of 258 Fm are briefly discussed.PACS numbers: 25.85.Ca, 27.90.+b Bimodal symmetric fission of the heaviest known elements has been of considerable interest 1 " 9 over the past few years. It exhibits two distinctly different components in the total-kinetic-energy (TKE) distributions of the fission fragments occurring concurrently in the symmetric mass division in the spontaneous fission of 258 Fm and of a few other elements in this region of the actinide nuclei. Such results are qualitatively understood in terms of two distinguishable modes of spontaneous fission, each of which is derived from the effects of the shell structure. The first mode corresponds to the effects of the shell structure in the parent fissioning nucleus and leads to fission along the so-called "old" valley resulting in liquid-drop-type elongated scission shapes similar to the scission configuration for the lighter actinides, with relatively low TKE values centered around 200 MeV and a broadly symmetric fission-fragment mass distribution. On the other hand, the second mode produces a sharply symmetric fission-fragment mass distribution with rather large values of the TKE of the fragments ( -235 MeV) approaching the Q value for the fission reaction. This mode is believed to occur because of the strong shell effects in the emerging fission fragments approaching the doubly magic l32 Sn nuclei with closed proton and neutron shells (Z=50, TV =82). Such fragment-shelldirected symmetric fission is found to occur along the so-called "new" valley leading to very compact scission configuration of two touching spheres. The two such valleys have been found in recent potential-energy-surface calculations 2,4 " 6 to be separated by a rather high ridge which serves to prevent reequilibration during the descent to scission. Recent results from the neutron multiplicity measurements 10 in the spontaneous fission of 260 Md provide rather conclusive evidence in support of explaining the bimodal symmetric fission in terms of two such valleys corresponding to distinctly different scission configurations.Spontaneous-fission properties of the actinide nuclei are known to vary fairly smoothly as one goes from uranium (Z =92) to fermium (Z = 100). An increase in the proton number of the fissioning nucleus leads to a gradual decrease in fission-fragment mass asymmetry, an increase in fission-fragment kinetic energies, and a decrease in fission half-lives. Except for the fission halflives these quantities vary fairly slowly with the neutron number of the fissioning nucleus. However, at 258 Fm there are sudden dramatic changes in all of these quantities. Fission becomes symmetric with a very narrow mass distribution, the average TKE values are about 35 MeV higher than in the asymmetric fission of 256 Fm, and th...

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