Nuclear clustering is one of the most puzzling phenomena in subatomic physics. Numerous examples of such structures include the ground state of the 11Li nucleus with a halo of two neutrons or the famous Hoyle resonance at 12C, which plays a vital role in the synthesis of heavier elements in stars. The widespread presence of narrow resonances near the particle emission threshold suggests that this is a universal phenomenon in open quantum systems in which bound and unbound states strongly mix, resulting in the appearance of a collective state with the features of a nearby decay channel. A new spectacular example of this phenomenon is the β– delayed proton decay of the neutron halo ground state of 11Be. Studies within a shell model embedded in the continuum (SMEC) suggest the existence of a J𝜋 = 1/2+ collective resonance in 11B, carrying many characteristics of a nearby proton-decay channel, which explains this puzzling decay. The proximity of proton and tritium emission thresholds suggests that this resonance may also contain an admixture of the 3H cluster configuration To clarify the nature of this hypothetical 1/2+ resonance, study of 10Be(p,p)10Be reaction will be needed.
The narrow 5/2+ resonance in 11B at 11.600(20) MeV, which lies slightly above the neutron emission threshold and breaks down by the emission of the neutron or α particle, has a crucial effect on the huge value of the 10B neutron capture cross-section. This suggests that the wave function of this resonance is strongly modified by the coupling to a nearby neutron emission channel. Indeed, in the SMEC calculations, there is a 5/26+ state near the neutron emission threshold, which strongly couples in L=2 partial wave to the channel [10B(3+)+n]5/2+. The theoretically determined maximal collectivization for this state is found ~110 keV above the neutron emission threshold and close to the experimental energy of the 5/2+ state. In the future, to clarify the impact of the virtual neutron state on the 10B(n,𝛾)11B reaction cross section, studies of the reaction 10Be(d,p)11B will be needed.
J.Okołowicz, M. Płoszajczak, W. Nazarewicz
Convenient location of a near-threshold proton-emitting resonance in 11Be
Physical Review Letters 124, 042502 (2020)