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.
@article{arxiv.1402.1645,
title = {11Be(beta-p), a quasi-free neutron decay?},
author = {K. Riisager and O. Forstner and M. J. G. Borge and J. A. Briz and M. Carmona-Gallardo and L. M. Fraile and H. O. U. Fynbo and T. Giles and A. Gottberg and A. Heinz and J. G. Johansen and B. Jonson and J. Kurcewicz and M. V. Lund and T. Nilsson and G. Nyman and E. Rapisarda and P. Steier and O. Tengblad and R. Thies and S. R. Winkler},
journal= {arXiv preprint arXiv:1402.1645},
year = {2014}
}