Clarification of large-strength transitions in the β decay of 11Be.

The shape and normalization of the β-delayed α 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 Qβ 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 to obtain a satisfactory description of the spectrum. Both states show large widths towards α decay, exhausting significant fractions of the Wigner limit, a typical signature of α clusterization. The observed Gamow-Teller strength indicates large overlaps between the two states and the ground state of 11Be. [ABSTRACT FROM AUTHOR]