Decay study of ¹¹Be with an Optical TPC detector

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The β decay of one-neutron halo nucleus ¹¹Be was investigated using the Warsaw Optical Time Projection Chamber (OTPC) detector to measure β-delayed charged particles. The results of two experiments are reported. In the first one, carried out in LNS Catania, the absolute branching ratio for β-delayed α emission was measured by counting incoming ¹¹Be ions stopped in the detector and the observed decays with the emission of α 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 ¹¹Be ions were implanted into the OTPC detector followed by the detection of decays with the emission of charged particles. The energy spectrum of β-delayed α particles was determined in the full energy range. It was analysed 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 ¹¹B are involved in the transition. The search for β-delayed emission of protons was undertaken. Only the upper limit for the branching ratio for this process of (2.2 ± 0.6ₛₜₐₜ ± 0.6sys) × 10⁻⁶ could be determined. This value is in conflict with the result published in [Ayyad et al. Phys. Rev. Lett. 123, 082501 (2019)] but does agree with the limit reported in [Riisager et al., Eur. Phys. J. A (2020) 56:100]