Anti-analog giant dipole resonances and the neutron skin of nuclei

Abstract

We examine a method to determine the neutron-skin thickness of nuclei using data on the charge-exchange anti-analog giant dipole resonance (AGDR). Calculations performed using the relativistic proton-neutron quasiparticle random-phase approximation (RQRPA) reproduce the isotopic trend of the excitation energies of the AGDR, as well as that of the spin-flip giant dipole resonances (IVSGDR), in comparison to available data for the even-even isotopes 112-124Sn. It is shown that the excitation energies of the AGDR, obtained using a set of density-dependent effective interactions which span a range of the symmetry energy at saturation density, supplemented with the experimental values, provide a stringent constraint on the value of the neutron-skin thickness. For 124Sn, in particular, we determine the value DRpn=0.21(5) fm. The result of the present study shows that a measurement of the excitation energy of the AGDR in (p,n) reactions using rare-isotope beams in inverse kinematics, provides a valuable method for the determination of the neutron-skin thickness in exotic nuclei

Nuclear equation of state from ground and collective excited state properties of nuclei

“…Comparison with the experimental data for 124 Sn, E(AGDR) − E(IAS) = 10.93 ± 0.20 MeV, obtained from the measured value E(AGDR) − E(IAS) = 10.60 ± 0.20 MeV [499] after correcting for the admixture of the spin dipole resonance, provided a constraint on the neutron skin thickness, ∆r np = 0.205 ± 0.050 fm [498]. In Ref.…”

“…[502], the excitation energy of the AGDR depends sensitively on the neutron skin thickness ∆r np . More recently it has been confirmed by microscopic calculations based on the EDFs that the energy difference between the AGDR and isobaric analog state (IAS) is very sensitively related to the corresponding neutron-skin thickness [498]. By comparing the energy differences between AGDR and IAS, calculated using various EDFs with the experimental data, the values of ∆r np , as well as the symmetry energy parameters could be constrained.…”

“…57). However, from the consideration of relevant Clebsch-Gordan coefficients, the T 0 -1 component (AGDR) is favored compared to the T 0 and T 0 +1, by factors of T 0 and 2T 0 2 [498]. Experimental data on the AGDR have been extracted from (p, n) [499,473,500] and ( 3 He, t) reactions [393], because the observed ∆L = 1 resonance in general corresponds to the superposition of possible spin-flip dipole (IVSGDR) and non-spin-flip dipole (AGDR) modes.…”

“…In Ref. [498], a family of density dependent meson-exchange covariant EDFs [79,231] has been used within the relativistic QRPA to determine the properties of Figure 59: Differences between the AGDR and IAS excitation energies and neutron skin thicknesses for 124 Sn calculated using DDME family of interactions [79,231] (left panel). The same differences are shown for 208 Pb, with SAMi-J [258] and DDME [79] families of interactions (right panel).…”

“…Fig. 59 shows the differences between the AGDR and IAS excitation energies for 124 Sn calculated using proton-neutron relativistic QRPA using five effective interactions spanning the range of values of the symmetry energy at saturation, J =30,32,34,36, and 38 MeV [498].The estimate of theoretical uncertainty of 10% is denoted by two parallel lines in the figure.…”