Two-Photon Lithium Spectroscopy for the Charge Radius Determination of the Halo Nucleus Li-11

Motivation

Experiments with first-generation beams of accelerated unstable ions in the mid 1980's led to the discovery of a remarkable feature in some light exotic nuclei: ⁶He, ¹¹Li, and ¹¹Be exhibit an extended spatial distribution of neutron matter. The best known and most studied examples are ¹¹Li and ⁶He. This nucleus consists of a ⁹Li core and two weakly bound halo neutrons. Due to the small binding energy of only approximately 300 keV, these neutrons are smeared out over a huge range and their root-means square (rms) matter radius is as large as the rms matter radius of ²⁰⁸Pb. The outer reaches of¹¹Li, a few fm away from the⁹Li core, comprises a low-density, diffuse, spatially extended weakly bound region of nearly pure neutron matter. Thus, ¹¹Li not only represents a new topology but also a new form of matter.

The strong increase in the matter radius between ⁹Li and ¹¹Li together with the observations that the break-up of this nucleus always removes both neutrons from the system and that there is no bound state of ¹⁰Li, lead to the picture of two-neutron halos as "Borromean" systems. This name corresponds to the heraldic symbol of the italian Borromeo family that showed three rings, which are interlocked in such a way that if any one of them is being removed, the other two would also fall apart. The three intertwined Borromean rings are now widely used as a logo of the halo field and are included in the ToPLiS logo.

The matter distribution in ¹¹Li extends very far out of the ⁹Li core. The rms matter radius is as large as that of ⁴⁸Ca, and the radius of the halo neutrons as large as for the outermost neutrons in ²⁰⁸Pb. The theory is challenged to account for both, the extension and the granularity. Thus, many different models have been used to describe this phenomenon, e.g., Greens Function Monte Carlo calculations, large basis shell model calculations, (relativistic) mean-field theories, stochastic variational multi-cluster  approaches and the dynamic correlation model. However, the origin of halos is only partly understood. An important aspect with regard to a better insight into these structures is the question of the interaction between the core and the halo nucleons, e.g., how much are the protons inside the core affected by the presence of the halo neutrons. This question could be resolved by measuring the differences in the rms charge radii of ⁹Li and ¹¹Li, the goal of our project.