An actinide nucleus such as uranium-238 (238U) is known to be deformed into the shape of a lemon. When such a nucleus is bombarded with high-energy heavy ions supplied by an accelerator, two kinds of interactions occur. One is the reaction at the tips of the lemon-shaped nucleus, and the other is the reaction at the flattened side of the lemon. For the first case, the Coulomb potential energy between the two nuclei is low. This implies the possible occurrence of fusion reactions even with low reaction energy (sub-barrier energy). Up to now, no precise measurements have been made of sub-barrier fusion reactions involving an actinide target.
In the present work, we have investigated the fusion reaction between oxygen-16 (16O) and 238U. The cross sections for fermium isotopes 250,249,248Fm (atomic number 100) are measured as direct evidence of successful fusion.
16O ions with various energies are supplied by the JAERI tandem accelerator and are directed to bombard a 238U target. By taking advantage of the alpha-decay character of Fm isotopes, such decays were used to determine the cross sections for each isotope. The results are shown in Fig. 8-3. From the experimental data, the fusion probabilities have been determined (Fig. 8-4). In this figure, we show model calculations assuming 238U to be spherical(dotted), and those incorporating the deformation of 238U (solid). The effects of the lemon-like deformation of 238U on the fusion enhancement are clearly visible in the sub-barrier energy region. In this case, the excitation energy of compound nuclei is suppressed to some degree. This indicates the possibility of producing new fusion products which have not been made with a proper neutron emission channel.
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