Mass, Spectroscopy, and Two-Neutron Decay of Be16

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 (Jπ=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.