Halo neutrons and the $\beta$ decay of ${}^{11}\mathrm{Li}$

The $\beta$ decay of ${}^{11}\mathrm{Li}$ has been investigated at TRIUMF-ISAC using a high-efficiency array of Compton suppressed $\mathrm{HPGe}$ detectors. From a line-shape analysis of the Doppler-broadened peaks observed in the ${}^{10}\mathrm{Be}$ $\gamma$ spectrum, both the half-lives of states in ${}^{10}\mathrm{Be}$ and the energies of the $\beta$-delayed neutrons feeding those states were obtained. Furthermore, it was possible to determine the excitation energies of the parent states in ${}^{11}\mathrm{B}\mathrm{e}$ with uncertainties comparable to those obtained from neutron spectroscopy experiments. These data suggest that the $\beta$ decay to the $8.81\mathrm{MeV}$ state in ${}^{11}\mathrm{Be}$ occurs in the ${}^9\mathrm{Li}$ core and that one neutron comprising the halo of ${}^{11}\mathrm{Li}$ survives in a halolike configuration after the $\beta$-delayed neutron emission from this level.

Figure 1

Compton-suppressed $\gamma$ spectrum following the $\beta$ decay of ${}^{11}\mathrm{Li}$. Only the relevant parts that contain the $\gamma$ transitions observed in ${}^{10}\mathrm{Be}$ and in ${}^{11}\mathrm{Be}$ are shown (room background subtracted).

Figure 2

Decay scheme of ${}^{11}\mathrm{Li}$ deduced from this work. The energies of the first two excited states in ${}^{10}\mathrm{Be}$ were taken from [11]. All transitions are labeled with $\gamma$-ray intensities, normalized to the $3367\mathrm{keV}$ transition (100). The intensity breakdown of the decay of the $\left(1^{-},2_2^{+}\right)$ doublet can be found in Table .

Figure 3

(Color online) Comparison between the experimental data and the best-fit obtained by the Monte Carlo simulation (black line). For the transitions involving the $\left(1^{-},2_2^{+}\right)$ doublet, the three contributions discussed in the text are also shown.