Researchers in the Theoretical Physics Group at Institute of Modern Physics (IMP), Chinese Academy of Sciences, have investigated the neutron skin thickness in ²⁰⁸Pb with a more practicable strategy.    

Accurate knowledge about the neutron skin thickness in ²⁰⁸Pb has far-reaching implications for different communities of nuclear physics and astrophysics. It is closely related to the neutron star structure and cooling, pulsar glitch. Many measurements have been focusing on it with different methods, such as exotic atoms, pygmy dipole resonance, proton elastic scattering, proton inelastic scattering and electron dipole polarizability. However, systematic uncertainties associated with various model assumptions are unavoidable in these mentioned approaches. The novel parity-violating electron elastic scattering experiment (PREX) recently has been suggested as a model-independent probe to the neutron skin thickness, whereas it may yield a result carrying a large uncertainty.    

In the present work, the researchers have introduced a practicable strategy (namely, J-a_sym) to constrain the neutron skin thickness, as shown in Fig.1, which turns out to be more effective than the parity-violating asymmetry A_PV in the current PREX. The J-a_sym(A) can be well determined based on the researchers’ previous work (PRC 88 (2013) 014302; PRC 89 (2014) 017305) thanks to a wealth of the measured data on nuclear masses and on beta decay energies. Compared with the parity-violating asymmetry A_PV in the PREX, the reliably experimental information on the J-a_sym is much more easily available attributed to the experimentally measured data on nuclear masses and on decay energies.  

On the other hand, to reach the same error level of the neutron skin thickness ΔR_np, say, ±0.021 fm, the J -a_sym with 20% accuracy is accurate enough, whereas the A_PV in the PREX should be measured at least up to 2% accuracy. Thus, it is indicated that the J-a_sym is currently a much more effective probe to the ΔR_np compared with the A_PV in the PREX. The ΔR_np of ²⁰⁸Pb is calculated to be 0.176 ± 0.021 fm in the present work. This result is expected to be meaningful to discriminate between the models and predictions relevant for the description of nuclear properties and neutron stars.  

The researchers proposed for a new method (or concept) in nuclear physics called ‘tomoscan’ to grasp richer information on the neutron skin thickness ΔR_np . They got the conclusion that, to obtain the ΔR_np, one simply needs to measure the nucleon densities in ²⁰⁸Pb from R_m = 7.61 ± 0.04 fm and the densities in the range of r < R_m have no contribution to the ΔR_np. Thus, the measurement on the ΔR_np could be simplified in the hadron scattering experiments.  

It has been widely believed that the nuclear surface structure is well constrained in nuclear energy-density functionals, while in experimental measurements, the interior profile is ambiguous. However, the researchers have showed that it is not justified. To grasp the ΔR_np, one must especially concentrate on the dilute matter located in nuclear surface which results in the dominant uncertainty. Moreover, this method could also be used to explore other intriguing issues, such as the halo structure in exotic nuclei.   

In the present work, the density dependence of the symmetry energy at subsaturation and suprasaturation densities has also been constrained with the J-a_sym. With the derived values of the J-a_sym, the density dependence of the symmetry energy near the saturation density, is constrained well, particularly the slope parameter L at ρ=0.11fm^-3 (the correlation coefficient is up to 0.995). The obtained results can be applied to explore some intriguing problems in nuclear astrophysics and have important impacts on its relevant problems. For example, the transition density from a solid neutron star crust to the liquid interior is estimated to be 0.091±0.008 fm⁻³.    

The work has been published in Phys. Rev. C91, 034315 (2015). 

Fig.1 Neutron skin thickness ΔR_np in ²⁰⁸Pb against the J − a_sym with different nuclear energy-density functionals. (Image by IMP)