Onset of neutron single-particle strengths in the tin isotopic chain

The stripping reaction (alpha, He-3) at 183 MeV bombarding energy has been used to study the neutron-particle response function on Sn-124, Sn-122, Sn-120, Sn-118, Sn-116, Sn-112 target nuclei up to 25 MeV excitation energy. Strong transitions to high-lying neutron states are observed beyond 4 MeV excitation energy. These transitions, arising from neutron stripping to high-spin outer subshells, appear as a sharp and strong peak located around 5 MeV excitation energy for the heavier tin isotopes (125, 123, and 121) and a broader structure extending up to 10 MeV. A clear broadening of the whole structure and the disappearance of the sharp component is observed with decreasing mass number, leading to a structureless spectrum in the case of Sn-113. Our investigation reveals a strong excitation of the 1i13/2 neutron strength, with possible mixture with the neighboring 1h9/2 and 2f subshells between 5 and 10 MeV excitation energy. The empirical data are compared with the theoretical predictions from the quasiparticle-phonon nuclear model. The present investigation, on a wide range of the tin isotopic chain, allows us to establish a comparison between the damping of high-lying particle and deep-hole states in heavy nuclei.