Clarification of large-strength transitions in the beta decay of 11Be

The shape and normalisation of the beta-delayed alpha spectrum from 11Be was measured by implanting 11Be ions in a segmented Si detector. The spectrum is found to be dominated by a well-known transition to the 3/2+ state at Ex = 9.87MeV in 11B. A significant increase in the observed decay strength towards the higher end of the Q window means, however, that the 9.87MeV state cannot alone be responsible for the transition. Using the R-matrix framework we find that the inclusion of an extra 3/2+ state at Ex = 11.49(10)MeV is required in order to obtain a satisfactory description of the spectrum. Both states show large widths towards alpha decay, exhausting significant fractions of the Wigner limit, a typical signature of alpha clusterisation. The observed Gamow-Teller strength indicate large overlaps between the two states and the ground state of 11Be.