$\beta$-Delayed Deuteron Emission from ${}^{11}\mathrm{Li}$: Decay of the Halo

The deuteron-emission channel in the $\beta$ decay of the halo nucleus ${}^{11}\mathrm{Li}$ was measured at the Isotope Separator and Accelerator facility at TRIUMF by implanting postaccelerated ${}^{11}\mathrm{Li}$ ions into a segmented silicon detector. The events of interest were identified by correlating the decays of ${}^{11}\mathrm{Li}$ with those of the daughter nuclei. This method allowed the energy spectrum of the emitted deuterons to be extracted, free from contributions from other channels, and a precise value for the branching ratio $B_d=1.30(13)\times {10}^{-4}$ to be deduced for $E_{\mathrm{c}.\mathrm{m}.}>200\mathrm{keV}$. The results provide the first unambiguous experimental evidence that the decay takes place essentially in the halo of ${}^{11}\mathrm{Li}$ and that it proceeds mainly to the ${}^9\mathrm{Li}+d$ continuum, opening up a new means to study the halo wave function of ${}^{11}\mathrm{Li}$.

Figure 1

Energy spectrum of $(E_1,t_1)$ events, defined as those following a ${}^{11}\mathrm{Li}$ implantation in the same pixel within 40 ms: These are ${}^{11}\mathrm{Li}$ decays. The inset shows the time behavior of all events following an implantation: The short-time component has the expected ${}^{11}\mathrm{Li}$ half-life.

Figure 2

Example of a fit of $(E_2,t_2)$ events, with contributions from daughter decays in channels (a), (f), and (g), and uncorrelated ${}^{11}\mathrm{Li}$ decays. This spectrum was obtained for $t_2-t_1<200\mathrm{ms}$ and for $E_1$ between 0.5 and 2.0 MeV. The inset shows the number of ${}^9\mathrm{Li}$ decay events in the $(E_2,t_2)$ spectra for $t_2-t_1<\Delta t$ as function of $\Delta t$. The fitted time behavior is in agreement with the expected ${}^9\mathrm{Li}$ half-life $T_{1/2}=178.3(4)\mathrm{ms}$ [19].

Figure 3

Transition probability for the ${}^{11}\mathrm{Li}\stackrel{\beta }{\to }{}^9\mathrm{Li}+d$ decay channel. The procedures to obtain the experimental data are explained in the text. The curves are predictions from Refs. 7 ($V_C$) and 8 ($V_a$ and $V_b$).