Speaker
Description
The equation of state (EOS) of nuclear matter is essential not only for describing the structure and collisions of atomic nuclei but also for understanding various astrophysical phenomena such as the mechanism of supernova explosions and the structure of neutron stars. The EOS includes components that depend on the difference between proton and neutron densities, which are represented by the symmetry energy. It has been shown that the first-order coefficient of the density dependence of the symmetry energy is closely related to the neutron-skin thickness, which is the difference between the neutron and proton distribution radii in nuclei[1].
In this study, we measured the interaction cross sections $\sigma_{\rm I}$ of $^{58-77}\mathrm{Ni}$ isotopes on a carbon target at 250 MeV/nucleon in order to determine the matter radius. These $\sigma_\rm I$ data constitute the first systematic dataset for the Ni isotope chain in this mass region. Using a modified Glauber model based on the optical limit approximation, we successfully extracted the RMS nuclear matter radii $\langle r^2 \rangle_\rm m^{1/2}$. By combining these results with charge radii previously measured via isotope shift methods, we derived the neutron-skin thickness $r_{\rm np}$. The measurement of charge radii using the isotope shift method has only been performed in the range of $^{58-68, 70}\mathrm{Ni}$[2], in which region the neutron skin thickness was derived in this work.
From the slope of the neutron-skin thickness as a function of the relative neutron excess $\delta$=(N−Z)/A, we extracted the EOS parameter L. The result was estimated as L=81(63) MeV, and this value is consistent with previous estimations[3,4]. In the future, precise measurements of the charge radii in the neutron-rich region of $^{71-77}\mathrm{Ni}$ are highly anticipated. These measurements are expected to enable the direct determination of neutron-skin thicknesses, and nuclear structure theories capable of reliably extracting the EOS parameter L from the resulting data are also eagerly awaited.
[1] M. Centelles et al., Phys. Rev. Lett. 102 (2009) 122502.
[2] S. Malbrunot-Ettenauer et al., Phys. Rev. Lett. 128 (2022) 022502.
[3] Bao-An Li et al., Phys. Lett. B 727 (2013) 276-281.
[4] Brendan T. Reed et al., Phys. Rev. Lett. 126 (2021) 172503.
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