  We have observed beta-delayed proton emission from the neutron-rich nucleus
11Be by analysing a sample collected at the ISOLDE facility at CERN with
accelerator mass spectrometry (AMS). With a branching ratio of (8.4 +- 0.6)
10^{-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.
