Heavy-ion induced transfer reactions with spherical and deformed nuclei

Abstract

Cross sections for the quasielastic channels have been measured at energies slightly above the Coulomb barrier using 58Ni and 112,116Sn projectiles on targets of 117,118Sn and 161,162,163,164Dy. The deexcitation γ-rays were detected in a 4π NaI array (the Spin Spectrometer of Oak Ridge National Laboratory) in kinematic coincidence with both target-like and projectile-like particles; thus providing a measure of the total energy and multiplicity of the entry state. The reaction channels were identified by characteristic γ-deexcitation transitions measured with Ge detectors substituted for array elements in the Spin Spectrometer. The angular distribution for inelastic scattering is well described by both semiclassical and quantal calculations. Two general features for few-neutron transfer channels are observed; (i) the dependence of the total cross section on the ground-state Q-value, (ii) the “coldness” of the reaction mechanism indicated by the measured entry states. For one-neutron transfer the distorted-wave Born approximation seems to give a correct overall qualitative description of the reaction mechanism for spherical nuclei, and reproduces the cross sections within a factor of two. The ground-to-ground two-neutron transfer probability for the reaction between Sn isotopes was deduced and found to be strongly enhanced ( F ≈ 760), which is consistent with the expectation of strong pairing correlations. Two-neutron transfer to the ground-state band in Dy was partially resolved from transfer to other intrinsic states by using the Spin Spectrometer. The oscillating radial dependence of the probability for populating the ground-state band was found to be related to the nuclear deformation. This leads to an explanation of the “slope anomaly” where the measured radial dependence of the total two-neutron transfer probability has too flat a slope compared to the prediction based on a simple binding energy argument. The probability for two-neutron transfer to the ground-state band is found to be similar to the probability for ground-to-ground transfer in the Sn+Sn system, suggesting a comparable strength for pairing correlations in the two systems.