Structure and neutron decay of the most neutron-rich beryllium isotopes

Abstract:  The study of neutron-rich nuclei provides a powerful insight on nuclear structure and its evolution towards extreme configurations. Unusual properties and new exotic decay modes are encountered as we move further away from stability. Near the dripline, where the energy needed to remove one nucleon is low enough or even negative, neutron emission from neutron-rich nuclei is a characteristic phenomenon. Ground state two-neutron decays are a special case that may occur once we go beyond the dripline. So far, and since the neutron dripline is only attainable for light nuclei, only two clear candidates are found:26O[1] and 16Be[2]. With this aim, we have investigated the spectroscopy and neutron decay of the most neutron-rich isotopes of Beryllium, 14Be, 15Be and especially 16Be, during different experimental campaigns at RIBF- RIKEN using the SAMURAI setup and the NEBULA neutron array. The high beam intensities and large neutron acceptance, coupled in the 16Be case to the high luminosity and resolution provided by the MINOS target, let us probe their spectroscopy over a wide energy range with good statistics. The detailed treatment of multi-neutron events has given access to the analysis of the three-body correlations of the several core+n + n decays from the populated two-neutron unbound states. In close collaboration with theorists, the characteristic low-energy nn correlations observed have been linked to the three-body wave function and therefore, the internal correlations of those systems. In particular, the role that the n-n interaction plays in the structure of the 16Be ground state, and in its further decay, has been identified through a direct comparison of theory and experiment. The results obtained for the three Beryllium isotopes will be presented, and the perspectives opened by this work discussed. References:[1] Y. Kondo et al. Phys. Rev. Lett. 116, 102503 (2016).[2] A. Spyrou et al., Phys. Rev. Lett. 108, 102501 (2012).