Mass, Spectroscopy, and Two-Neutron Decay of ¹⁶Be

The structure and decay of the most neutron -rich beryllium isotope, 16Be, has been investigated following proton knockout from a high-energy 17B beam. Two relatively narrow resonances were observed for the first time, with energies of 0.84(3) and 2.15(5) MeV above the two -neutron decay threshold and widths of 0.32(8) and 0.95(15) MeV, respectively. These were assigned to be the ground (J pi = 0+) and first excited (2+) state, with Ex = 1.31(6) MeV. The mass excess of 16Be was thus deduced to be 56.93 (13) MeV, some 0.5 MeV more bound than the only previous measurement. Both states were observed to decay by direct two -neutron emission. Calculations incorporating the evolution of the wave function during the decay as a genuine three -body process reproduced the principal characteristics of the neutron -neutron energy spectra for both levels, indicating that the ground state exhibits a strong spatially compact dineutron component, while the 2+ level presents a far more diffuse neutron -neutron distribution.