Decay study of ${}_{11}\mathrm{Be}$ with an optical time-projection chamber

The $\beta$ decay of one-neutron halo nucleus ${}_{11}\mathrm{Be}$ was investigated using the Warsaw Optical Time Projection Chamber (OTPC) detector to measure $\beta$-delayed charged particles. The results of two experiments are reported. In the first one, carried out in LNS Catania, the absolute branching ratio for $\beta$-delayed $\alpha$ emission was measured by counting incoming ${}_{11}\mathrm{Be}$ ions stopped in the detector and the observed decays with the emission of $\alpha$ particle. The result of 3.27(46)% is in good agreement with the literature value. In the second experiment, performed at the HIE-ISOLDE facility at CERN, bunches containing several hundreds of ${}_{11}\mathrm{Be}$ ions were implanted into the OTPC detector followed by the detection of decays with the emission of charged particles. The energy spectrum of $\beta$-delayed $\alpha$ particles was determined in the full energy range. It was analyzed in the R-matrix framework and was found to be consistent with the literature. The best description of the spectrum was obtained assuming that the two $3/2^{+}$ and one $1/2^{+}$ states in ${}_{11}B$ are involved in the transition. The search for $\beta$-delayed emission of protons was undertaken. Only the upper limit for the branching ratio for this process of $(2.2\pm 0.6_{\mathrm{stat}}\pm 0.6_{\mathrm{sys}})\times {10}^{-6}$ could be determined. This value is in conflict with the result published by Ayyad et al. [Phys. Rev. Lett. 123, 082501 (2019)] but does agree with the limit reported by Riisager et al. [Eur. Phys. J. A 56, 100 (2020)].