Reexamining the heavy-ion reactions238U+238U and238U+248Cm and actinide production close to the barrier

Abstract

Recent theoretical work has renewed interest in radiochemically determined isotope distributions in reactions of ${}^{238}$U projectiles with heavy targets that had previously been published only in parts. These data are being reexamined. The cross sections \ensuremath{\sigma}($Z$) below the uranium target have been determined as a function of incident energy in thick-target bombardments. These are compared to predictions by a diffusion model whereby consistency with the experimental data is found in the energy intervals 7.65--8.30 MeV/u and 6.06--7.50 MeV/u. In the energy interval 6.06--6.49 MeV/u, the experimental data are lower by a factor of 5 compared to the diffusion model prediction indicating a threshold behavior for massive charge and mass transfer close to the barrier. For the intermediate energy interval, the missing mass between the primary fragment masses deduced from the generalized ${Q}_{\mathrm{gg}}$ systematics including neutron pair-breaking corrections and the centroid of the experimental isotope distributions as a function of $Z$ have been used to determine the average excitation energy as a function of $Z$. From this, the $Z$ dependence of the average total kinetic-energy loss ($\overline{\mathrm{TKEL}}$) has been determined. This is compared to that measured in a thin-target counter experiment at 7.42 MeV/u. For small charge transfers, the values of $\overline{\mathrm{TKEL}}$ of this work are typically about 30 MeV lower than in the thin-target experiment. This difference is decreasing with increasing charge transfer developing into even slightly larger values in the thick-target experiment for the largest charge transfers. This is the expected behavior which is also found in a comparison of the partial cross sections for quasielastic and deep-inelastic reactions in both experiments. The cross sections for surviving heavy actinides, e.g., ${}_{98}$Cf, ${}_{99}$Es, and ${}_{100}$Fm indicate that these are produced in the low-energy tails of the dissipated energy distributions, however, with a low-energy cutoff at about 35 MeV. Excitation functions show that identical isotope distributions are populated independent of the bombarding energy indicating that the same bins of excitation energy are responsible for the production of these fissile isotopes. A comparison of the survival probabilities of the residues of equal charge and neutron transfers in the reactions of ${}^{238}$U projectiles with either ${}^{238}$U or ${}^{248}$Cm targets is consistent with such a cutoff as evaporation calculations assign the surviving heavy actinides to the 3$n$ and/or 4$n$ evaporation channels.