arXiv:1209.0156·v4·Nuclear Experiment
Beyond the Neutron Drip-Line: The Unbound Oxygen Isotopes 25O and 26O
C. Caesar (1 and 2) · J. Simonis (1 and 3) · T. Adachi (4) · Y. Aksyutina (2 and 3) · J. Alcantara (5) · S. Altstadt (6) · H. Alvarez-Pol (5) · N. Ashwood (7) · T. Aumann (1 and 2) · V. Avdeichikov (8) · M. Barr (7) · S. Beceiro (5)
Abstract
The very neutron-rich oxygen isotopes 25O and 26O are investigated experimentally and theoret- ically. In this first R3B-LAND experiment, the unbound states are populated at GSI via proton- knockout reactions from 26F and 27F at relativistic energies around 450 MeV/nucleon. From the kinematically complete measurement of the decay into 24O plus one or two neutrons, the 25O ground- state energy and lifetime are determined, and upper limits for the 26O ground state are extracted. In addition, the results provide evidence for an excited state in 26O at around 4 MeV. The ex- perimental findings are compared to theoretical shell-model calculations based on chiral two- and three-nucleon (3N) forces, including for the first time residual 3N forces, which are shown to be amplified as valence neutrons are added.