First experimental evidence of the Giant Pairing Vibration in atomic nuclei

Abstract

The main purpose of the present work is the understanding of the effects of the pairing force between nucleons in the structure of the atomic nuclei and in the nuclear reactions. This research is focused on the study of two-neutron transfer reactions, which are among the most effective probes to excite the nuclear pairing field. In particular, the (18O,16O) reactions at 84 MeV incident energy on 12C and 13C targets are investigated. A vast literature is available on the subject of the pairing correlations and two-neutron transfer reactions, but some key questions need still to be answered. The collective character due to the pairing interaction was extended to specific excitations at higher energies. The elementary excitation mode known as Giant Pairing Vibration (GPV) was predicted by the theory in the 70's. The excitation of the GPV should be induced by two-nucleon transfer reactions where the angular momentum transfer is L = 0 and a correlated pair of nucleons in a relative S-wave is transferred. Despite the strong theoretical indications in favor of the existence of the GPV mode and the large amount of proves of giant modes in the p-h side, the GPV has never been clearly identified in any nucleus. The puzzling question about the existence of the GPV has important consequences on the quantum mechanics particle-hole symmetry, which would be broken if the non-existence of the GPV was proven. The discovery of the GPV and the study of its properties has also important implications in the understanding of the structure of atomic nuclei, which is one of the most challenging problems in physics since decades. In the present work the 12C(18O,16O)14C and 13C(18O,16O)15C two-neutron transfer reactions at 84 MeV incident energy were investigated. The 18O beam was delivered by the Tandem van der Graaf installed at the Laboratori Nazionali del Sud (LNS-INFN) in Catania. The ejectiles were momentum analyzed and detected by the MAGNEX large acceptance magnetic spectrometer. Spectra up to ~22 MeV excitation energy were extracted for the residual nucleus. The population of the ground plus several known bound and resonant states of the residual nuclei are observed. In addition, two unknown broad structures are observed in the higher part of the excitation energy spectra, namely at 16.9 (FWHM = 1.2 MeV) in the 14C spectrum and at 13.7 MeV (FWHM = 2.5 MeV) in the 15C one. The cross section angular distributions are extracted for the most excited transitions and for the unknown bumps in a wide angular range (6° < theta_cm < 55°). From the reaction mechanism point of view, the direct one-step transfer of a correlated pair of neutrons is found to be dominant in (18O,16O) reactions, at least for the energies considered in this work. In the second part of the data analysis the origin of the two unknown structures at 16.9 MeV in 14C and 13.7 MeV in 15C is investigated. The obtained results demonstrate that they correspond to the excitation of the GPV resonance. First of all, the break-up component in the spectra are evaluated assuming an uncorrelated removal of the two neutrons, which is not able to reproduce the bumps, thus indicating that the inclusion of n-n correlations is required to describe such structures. Another signature comes from QRPA calculations of the 0+ excitations in the 14C energy spectra. A collective structure at about 16.5 MeV excitation energy with respect to the 14Cg.s. is found in the calculated strength function. Supplemental analysis of the features of the two structures in 14C and 15C provided other signatures. Indeed, they are found at the right predicted excitation energy and width and they are populated with an L = 0 transfer (deduced from their cross section angular distribution) and a strength comparable with that of the 14Cg.s.. The obtained results that the two observed bumps correspond to the first experimental excitation of the long searched GPV.