Status of experimental knowledge on the unbound nucleus $^{13}$Be

TL;DR

This paper reviews four decades of experimental and theoretical research on the unbound nucleus $^{13}$Be, highlighting new data analysis methods and comparing different models to understand its structure and resonance states.

Contribution

It introduces a new analysis of recent $^{12}$Be(d,p) reaction data using GEANT4 simulations and Bayesian fitting, and compares it with previous measurements to clarify the structure of $^{13}$Be.

Findings

Good agreement between recent transfer and nucleon removal measurements.

Two scenarios proposed for the low-energy strength: a single p-wave resonance or a mixture involving an s-wave virtual state.

Analysis supports the existence of specific resonance states near the neutron separation energy.

Abstract

The structure of the unbound nucleus $^{13}$Be is important to understanding the Borromean, two-neutron halo nucleus $^{14}$Be. The experimental studies conducted over the last four decades are reviewed in the context of the beryllium chain of isotopes and some significant theoretical studies. One focus of this paper is the comparison of new data from a $^{12}$Be(d,p) reaction in inverse kinematics, which was analyzed using GEANT4 simulations and a Bayesian fitting procedure, with previous measurements. Two possible scenarios to explain the strength below 1~MeV above the neutron separation energy were proposed in that study: a single $p$-wave resonance, or a mixture of an $s$-wave virtual state with a weaker either $p$-wave or $d$-wave resonance. Comparisons of recent invariant mass and the (d,p) experiments show good agreement between the transfer measurement and the two most recent high-energy nucleon removal measurements.