Nucleus-nucleus proximity potential and superheavy nuclei

Using up-to-date values of nuclear radii and of the nuclear surface tension, the 1977 proximity treatment of nucleus-nucleus interaction is confronted with 113 measured fusion barriers. The ≈4% overestimate of theory with respect to experiment, seen in a similiar comparison in 1981, is no longer present. The calculated proximity barriers, when applied to fusion reactions used to produce heavy elements with atomic number $Z=102$--118, suggest that the unexpectedly large cross section observed in the reaction ⁸⁶Kr+²⁰⁸→Pb²⁹³118+1n may be due to the sinking of the Coulomb barrier below the level of the bombarding energy. Tests of this hypothesis are suggested. Some consequences of the appearance of such ``unshielded'' reactions for very heavy systems are discussed. An Appendix supplies very accurate analytic formulas for the universal nuclear proximity force and potential functions φ and Φ. This does away with the need to consult the tables published in 1977.