Abstract
The double neutron/proton ratio of nucleon emissions taken from two reaction systems using four isotopes of the same element, namely, the neutron/proton ratio in the neutron-rich system over that in the more symmetric system, has the advantage of reducing systematically the influence of the Coulomb force and the normally poor efficiencies of detecting low energy neutrons. The double ratio thus suffers less systematic errors. Within the IBUU04 transport model the double neutron/proton ratio is shown to have about the same sensitivity to the density dependence of nuclear symmetry energy as the single neutron/proton ratio in the neutron-rich system involved. The double neutron/proton ratio is therefore more useful for further constraining the symmetry energy of neutron-rich matter.
Highlights
The density dependence of nuclear symmetry energy Esym(ρ) is still poorly known but very important for both nuclear physics and astrophysics [1, 2, 3, 4, 5, 6]
The double neutron/proton ratio of nucleon emissions taken from two reaction systems using four isotopes of the same element, namely, the neutron/proton ratio in the neutron-rich system over that in the more symmetric system, has the advantage of reducing systematically the influence of the Coulomb force and the normally poor efficiencies of detecting low energy neutrons
The double neutron/proton ratio of nucleon emissions taken from two reaction systems using four isotopes of the same element, namely, the neutron/proton ratio in the neutron-rich system over that in the more symmetric system, was recently proposed by Lynch et al.[23] as a candidate of such an observable
Summary
The density dependence of nuclear symmetry energy Esym(ρ) is still poorly known but very important for both nuclear physics and astrophysics [1, 2, 3, 4, 5, 6]. Some comparisons between the available experimental data and transport model calculations have been carried out recently These studies have allowed us to place important constraints on the density dependence of symmetry energy. By using the free-space experimental nucleon-nucleon (NN) cross sections within the transport model IBUU04 [7], a symmetry energy of Esym(ρ) ≈ 31.6(ρ/ρ0)1.1 for densities less than 1.2ρ0 was extracted from the MSU data on isospin diffusion [8, 9]. While using in-medium NN cross sections calculated within an effective-mass scaling approach [10], a symmetry energy of Esym(ρ) ≈ 31.6(ρ/ρ0)0.69 was found most acceptable in comparison with both the MSU isospin diffusion data and the presently acceptable neutron-skin thickness in 208Pb [10, 11]. Complementary observables sensitive to the Esym(ρ), more desirably, studies on correlations of several such observables, are still very much needed to further constrain the symmetry energy
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