DREB Conference 2024

Relativistic Coulomb excitation in inverse kinematics can be utilized to study the electric dipole response of projectile neutron-rich nuclei. In such conditions, collective excitations arise where neutron and proton densities of the excited nucleus are displaced with respect to each other. Additionally, access to greater isospin asymmetries on the neutron-rich side of the nuclide chart provide a suitable environment to probe the symmetry energy, a crucial yet still fairly unknown ingredient of the nuclear equation of state.

In Ref. [1] a novel approach to constrain the slope of the symmetry energy $L$, i.e. the linear coefficient in the expansion of the symmetry energy around saturation density, is explored for the first time by measuring the Coulomb-excitation cross section $\sigma_C$ of neutron-rich nuclei at relativistic energies. This particular cross section correlates with the dipole polarizability $\alpha_D$, and through the established correlation between $\alpha_D$ and $L$, enables constraining the symmetry energy by measuring $\sigma_C$. The advantage of using $\sigma_C$ instead of $\alpha_D$ lies in simpler measurement and analysis procedure.

This approach was further examined in the experiment carried out using the large acceptance spectrometer R$^3$B-GLAD at the GSI accelerator facility as a part of the FAIR Phase-0 campaign [2]. Tin isotopes in the mass range 124-134 were produced as a secondary beam in the fragmentation and fission reactions at energies close to 1 GeV/u and impinged onto the lead target which provided a Lorentz-contracted field to induce Coulomb excitations. De-excitation followed through the emission of gammas and neutrons, which were detected using the CALIFA gamma calorimeter [3] and the NeuLAND neutron detector [4]. The remaining fragment nuclei were detected by tracking detectors located before and after the GLAD magnet, altogether providing a kinematically complete measurement.

[1] A. Horvat, Doctoral thesis, Technische Universität Darmstadt (2019).

[2] R$^3$B Collaboration, https://www.r3b-nustar.de/.

[3] H. Alvarez-Pol, et al., Nucl. Instrum. Meth. A, 767, 453-466 (2014).

[4] K. Boretzky, et al., Nucl. Instrum. Meth. A, 1014, 165701 (2021).