Role of a resonance in the (β–p) decay of 11Be

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 J^𝜋 = 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)

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.042502