Mass, Spectroscopy, and Two-Neutron Decay of ${}^{16}\mathrm{Be}$

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

Figure 1

Relative-energy spectrum of ${}^{14}\mathrm{Be}+n+n$ events ($E_{fnn}$) following the ${}^{17}\mathrm{B}(p,2p)$ reaction. The red line represents the best fit, incorporating all experimental effects, up to 4 MeV (${\chi }^2/\mathrm{ndf}=1.3$) including ${}^{16}\mathrm{Be}$ resonances at 0.84 and 2.15 MeV (dotted lines). The upper panel displays the overall detection efficiency, including the effects of the neutron crosstalk rejection filter.

Figure 2

Low-lying level schemes of the most neutron-rich beryllium isotopes. The shaded bands represent the error bars of the measured energies. The three columns on the right correspond to ${}^{16}\mathrm{Be}$ present results, shell-model calculations (WBP) [11], and the result of Ref. [11].

Figure 3

Top: experimental Dalitz plots of the normalized energies ${}^{14}\mathrm{Be}\text{−}n$ (${ϵ}_{fn}$) vs $n\text{−}n$ (${ϵ}_{nn}$) for the decay of the two observed states in ${}^{16}\mathrm{Be}$, selected by gates in $E_{fnn}$ between 0–1.2 and 1.7–3.0 MeV (see Fig. 1). Bottom: theoretical spatial probability distributions $P(r_x,r_y)$ for each state in terms of the distances ${}^{14}\mathrm{Be}\text{−}nn$ ($r_y$) vs $n\text{−}n$ ($r_x$), with the color scale ranging from 0 to $0.12{\mathrm{fm}}^{-2}$.

Figure 4

Normalized $n\text{−}n$ energy distributions for the $0^{+}$ and $2^{+}$ levels compared to the predictions of the three-body model incorporating a realistic description of the decay (red lines, see text).