Assessment of the beta-delayed proton decay rate of ¹¹Be

Abstract

The 11Be neutron halo nucleus appears to decay into 10Be with a rate that exceeds expectations. Neutron disappearance into dark matter, beta decay of a halo neutron, or beta-delayed proton decay have been offered as explanations. In this work we study the latter option; we carry out shell model calculations and sequential decay analysis examining the beta-delayed proton decay going through a resonance in 11B. The narrow energy window, lack of states with sufficient spectroscopic strength, overwhelming alpha decay branch, all make reconciling the observed rate with beta-delayed proton decay difficult.

Beta decay of halo nuclei

“…The theoretical calculations suffer from a lack of knowledge of the potential in the final state between 10 Be and the proton. The first results [19,30] gave branching ratios around 3 • 10 −8 , but higher values (up to around 10 −5 ) may be obtained if a resonance is present in the Q-window for the decay [95,96], a conclusion verified by now via several different model calculations [8,37,121]. A convincing theoretical prediction must reproduce both the β α and β p branches, as attempted with the shell model embedded in the continuum [77,78].…”

$β^-{\rm p}$ and $β^-α$ decay of the $^{11}$Be neutron halo ground state

“…In the psdu calculations of Ref. [6], the contribution of the 3/2 + resonances is 0.054%, and the 1/2 + 3 resonance adds to the branching ratio 9•10 −4 % for Γ α = 10 keV. The calculated b r (β − α) branching ratio varies very slowly with V 0 .…”

“…[3,5] support the β − p decay scenario while Ref. [6] concludes that the reported half-life for the beta-delayed proton decay is virtually impossible to explain.…”

“…[10] and fsu [11] SM interactions used in Ref. [6]. The continuum coupling of SMEC is V0 = −37 MeV fm 3 with the energy scale adjusted to the energy of 1/2 + 3 state taken from [3].…”

“…Figure 1 shows experimental and calculated (our SMEC model and the psdu and fsu models of Ref. [6]) states of 11 B around the β − p window of 11 Be. Our calculations predict two 1/2 + and two 3/2 + excited states that can be fed by the β − decay of 11 Be.…”