Preliminary Study of Two-Neutron States via the (O-18,O-16) Reaction at 84 MeV

A study of light neutron-rich nuclei has been performed at the INFN-LNS laboratory in Catania (Italy) by the (O-18,O-16) reaction at 84 MeV incident energy on Be-9, B-11 and C-13 targets. The O-16 ejectiles have been momentum analyzed and detected at forward angles by the MAGNEX magnetic spectrometer. The energy spectra of C-15 show several known low lying states up to about 7 MeV excitation energy and unknown resonant structures at higher excitation energy. The strong excitation of these latter together with the measured width of about 2 MeV FWHM could indicate the presence of collective modes of excitation connected to the transfer of a correlated neutron pair. Also considerations regarding the reaction mechanism and the nuclei involved in the transfer seem to confirm such a conclusion. Similar features characterize the energy spectra of the Be-11 and B-13 nuclei. The achieved mass resolution (about 1/160) guarantying the identification of the reaction products corresponding to different reaction channels has allowed to study the relative yields for the different reaction channels. The measured yields show an enhancement of the two-neutron transfer channel compared to the one-neutron transfer. This result seems to indicate that the (O-18, O-16) reaction proceeds mainly by the direct transfer of the neutron pair and that the second order processes are negligible.