Be11(βp), a quasi-free neutron decay?

We have observed β^--delayed proton emission from the neutron-rich nucleus ¹¹Be by analyzing a sample collected at the ISOLDE facility at CERN with accelerator mass spectrometry (AMS). With a branching ratio of (8.3±0.9)ṡ10^-6 the strength of this decay mode, as measured by the B_GT-value, is unexpectedly high. The result is discussed within a simple single-particle model and could be interpreted as a quasi-free decay of the ¹¹Be halo neutron into a single-proton state.