First Observation of Ground State Dineutron Decay: ${}^{16}\mathrm{Be}$

We report on the first observation of dineutron emission in the decay of ${}^{16}\mathrm{Be}$. A single-proton knockout reaction from a $53\mathrm{MeV}/\mathrm{u}$ ${}^{17}\mathrm{B}$ beam was used to populate the ground state of ${}^{16}\mathrm{Be}$. ${}^{16}\mathrm{Be}$ is bound with respect to the emission of one neutron and unbound to two-neutron emission. The dineutron character of the decay is evidenced by a small emission angle between the two neutrons. The two-neutron separation energy of ${}^{16}\mathrm{Be}$ was measured to be 1.35(10) MeV, in good agreement with shell model calculations, using standard interactions for this mass region.

Figure 1

Level and decay scheme for neutron-rich isotopes of beryllium. The solid lines correspond to experimental data from 17, 15, and the present Letter, while the dashed lines represent shell model calculations in the $s\mathrm{\text{−}}p\mathrm{\text{−}}sd\mathrm{\text{−}}pf$ model space using the WBP interaction [18].

Figure 2

(a) Three-body decay energy from the reconstruction of ${}^{14}\mathrm{Be}+n+n$. (b),(c) Two-body decay energy of ${}^{14}\mathrm{Be}+n$, where (b) corresponds to the highest decay energy and (c) to the lowest. (d) Two-body relative energy of the two neutrons. (e) Opening angle ${\theta }_{n-n}$ between the two neutrons in the center-of-mass frame of ${}^{16}\mathrm{Be}$. (f) Opening angle (${\theta }_{{}^{14}\mathrm{Be}-n1}$) between the ${}^{14}\mathrm{Be}$ fragment and the first neutron in the center-of-mass frame of ${}^{16}\mathrm{Be}$. In all figures, the experimental data are shown in black triangles, the sequential emission in dotted lines, the three-body decay in dashed lines, and the dineutron decay in solid lines.