Two-nucleon emitters within a pseudostate method: the case of Be-6 and Be-16

Background: Since the first experimental observation, two-nucleon radioactivity has gained renewed attention since the early 2000s. The 6 Be system is the lightest two-proton ground-state emitter, while 16 Be was recently proposed to be the first two-neutron ground-state emitter ever observed. A proper understanding of their properties and decay modes requires a reasonable description of the three-body continuum. Purpose: Study the ground-state properties of 6 Be and 16 Be within a general three-body model and investigate their nucleon-nucleon correlations in the continuum. Method: The pseudostate (PS) method in hyperspherical coordinates, using the analytical transformed harmonic oscillator (THO) basis for three-body systems, is used to construct the 6 Be and 16 Be ground-state wave functions. These resonances are approximated as a stable PS around the known two-nucleon separation energy. Effective core- N potentials, constrained by the available experimental information on the binary subsystems 5 Li and 15 Be , are employed in the calculations. Results: The ground state of 16 Be is found to present a strong dineutron configuration, with the valence neutrons occupying mostly an l = 2 state relative to the core. The results are consistent with previous R -matrix calculations for the actual continuum. The case of 6 Be shows a clear symmetry with respect to its mirror partner, the two-neutron halo 6 He : The diproton configuration is dominant, and the valence protons occupy an l = 1 orbit. Conclusions: The PS method is found to be a suitable tool in describing the properties of unbound core + N + N ground states. For both 16 Be and 6 Be , the results are consistent with previous theoretical studies and confirm the dominant dinucleon configuration. This favors the picture of a correlated two-nucleon emission.