Observation of a Near-Threshold Proton Resonance in B-11

A near-threshold proton resonance in B-11 at E-ex = 11.44 +/- 0.04 MeV is observed via the reaction Be-10(d, n)Be-11 -> Be-10 + p in inverse kinematics, measured with a beam of the radioactive isotope Be-10. The resonance energy at E-res = 211 (40) keV is consistent with a proton signal observed by Ayyad et al. in the beta-delayed proton decay of 11Be. By comparison to a distorted wave Born approximation calculation, a 0.27(6) spectroscopic factor is extracted and a tentative (l = 0) character is assigned for this resonance. The significant cross section in the proton-transfer (d, n)reaction, as well as the observation of its protondecay signal, point to the threshold-resonance character of this state. The position of this state, its structure, and strong coupling to the s-wave continuum represent an ideal case to study quantum near-threshold many-body dynamics of unstable states. The presence of this state is an important step toward understanding the excessively large beta-delayed proton-decay branch of Be-11.