Probing Halo Structure in Excited States of ¹⁷C with the Gamma-decay Transmission Method

Studies near the neutron dripline have revealed unique phenomena known as halos, where valence neutrons move in spatially extended orbits. Halo nuclei serve as benchmarks for understanding weakly-bound and continuum effects on the evolution of single-particle energies and particle correlations at and beyond the dripline. However, direct evidence of halo structures in nuclear excited states has remained elusive, thus limiting the cases to investigate halo formation near the threshold.

A new technique, based on modifications to the recently proposed Recoil Distance Transmission Method, has been developed to confirm the presence or absence of halos in excited states. This approach combines gamma-ray spectroscopy with the transmission method by comparing gamma-ray yields with and without a reaction target to determine the interaction cross section.

The 1/2⁺ excited state of ¹⁷C, characterized by a small one-neutron separation energy and a significant s-wave strength, is a strong halo candidate and therefore ideal to showcase the new method. An experimental study using this technique was conducted at FRIB utilizing GRETINA, the S800 spectrograph, and a dedicated target assembly to produce ¹⁷C. This talk will present an overview of the method and preliminary results.