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

The structure and decay of the most neutron-rich beryllium isotope, Be-16, has been investigated following proton knockout from a high-energy B-17 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 E-x=1.31(6) MeV. The mass excess of Be-16 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.