Transfer and compound-nucleus reactions that lead to the nucleiCf245andCf244: Interactions ofC12withPu239andU238

The nuclear reactions ${}_{}{}^{239}{}_{}{}^{}\mathrm{Pu}({}_{}{}^{12}{}_{}{}^{}\mathrm{C},\alpha 2n-\alpha 3n)$ and ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}({}_{}{}^{12}{}_{}{}^{}\mathrm{C},5n-6n)$ leading to the same radioactive products ${}_{}{}^{245}{}_{}{}^{}\mathrm{Cf}$ and ${}_{}{}^{244}{}_{}{}^{}\mathrm{Cf}$ have been studied by measuring excitation functions and recoil range and angular distributions. As expected, the U + C data are consistent with predictions for complete-fusion (CF) reactions; for example, the range distributions are Gaussian with mean values that increase with increasing bombarding energy, and the angular distributions are forward peaked in the laboratory system. The results for the Pu + C reactions differ markedly from those for U + C. The range distributions are asymmetric, with high energy tails, and have centroids that decrease monotonically with increasing ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ energy, from 1.6 times the expected CF value at 67 MeV to 0.6 of the CF value at 97 MeV; the angular distributions are characterized by a maximum at ∼ 17° (lab) at energies well above the Coulomb barrier. The cross sections for the $\mathrm{Pu}(\mathrm{C},\alpha 2n \mathrm{and} \alpha 3n)$ reactions are much larger than those of the $\mathrm{Pu}(\mathrm{C},2n \mathrm{to} 4n)$ reactions, also indicating that noncompound processes are involved in the production of the Cf nuclides, since evaporation of charged particles is expected to be negligible in such heavy nuclei. All of these results for the ($\mathrm{C},\alpha \mathrm{xn}$) reactions are consistent with models of transfer reactions in which an aggregate is transferred from the projectile to the target nucleus, followed by evaporation of neutrons from the resulting heavy nucleus. In particular, the average recoil energies and angles are reproduced reasonably well by models that consider the kinematics of the transfer process.