One-neutron removal reactions on Al isotopes around the N=20 shell closure

The one-neutron removal cross sections of neutron-rich Al isotopes and longitudinal momentum distributions of the residues have been measured for A = 33 to 36 at relativistic energies (approximate to 900 MeV/u). The inclusive data have been interpreted within the eikonal approximation. The evolution of the single-particle occupancy in the ground state of Al-33,Al-34,Al-35 has been studied and compared with shell model predictions. The inferred 2s(1/2) neutron occupancy in the Al-33 ground-state wave function is 20% to 40% lower than the predicted one. The inclusive data do not exclude the presence of intruder states. Some intruder l = 1 occupancy is found in Al-34, similarly to Mg-33. The single-particle 1f(7/2) occupancy shows a gradual increase at N = 22. Correspondingly, a decrease of the 1d(3/2) strength has been observed.