Single and pair neutron transfers at sub-barrier energies

Multinucleon transfer cross sections in the ⁹⁶Zr+⁴⁰Ca system have been measured, in inverse kinematics, at bombarding energies ranging from the Coulomb barrier to ~25$%$ below. Targetlike recoils have been identified in A, Z and velocity with the large solid angle magnetic spectrometer PRISMA. The experimental data for one- and two-neutron transfer channels have been compared with semiclassical microscopic calculations. For the two-neutron transfer channels the relevance of the transitions to the ground state and to the 0⁺ excited states of ⁴²Ca are discussed by employing, for the reaction mechanism, the successive approximation. It is found that the transition to the 0⁺ state at ~6 MeV, whose wave function is dominated by the two neutrons in the 2p3/2 shell, is much larger than the ground state one. The comparison with the inclusive data reveals that transitions to states with high multipolarity and non-natural parity are important. This suggests that more complex two-particle correlations have to be incorporated in the treatment of the transfer process.