Electric Dipole Polarizability ofCa48and Implications for the Neutron Skin

Abstract

The electric dipole strength distribution in 48 Ca between 5 and 25 MeV has been determined at RCNP, Osaka, from proton inelastic scattering experiments at forward angles. Combined with photoabsorption data at higher excitation energy, this enables the first extraction of the electric dipole polarizability αD( 48 Ca) = 2.07(22) fm 3 . Remarkably, the dipole response of 48 Ca is found to be very similar to that of 40 Ca, consistent with a small neutron skin in 48 Ca. The experimental results are in good agreement with ab initio calculations based on chiral effective field theory interactions and with state-of-the-art density-functional calculations, implying a neutron skin in 48 Ca of 0.14 − 0.20 fm.Introduction.-The equation of state (EOS) of neutronrich matter governs the properties of neutron-rich nuclei, the structure of neutron stars, and the dynamics of corecollapse supernovae [1,2]. The largest uncertainty of the EOS at nuclear densities for neutron-rich conditions stems from the limited knowledge of the symmetry energy J, which is the difference of the energies of neutron and nuclear matter at saturation density, and the slope of the symmetry energy L, which is related to the pressure of neutron matter. The symmetry energy also plays an important role in nuclei, where it contributes to the formation of neutron skins in the presence of a neutron excess. Calculations based on energy density functionals (EDFs) pointed out that J and L can be correlated with isovector collective excitations of the nucleus such as pygmy dipole resonances [3] and giant dipole resonances (GDRs) [4], thus suggesting that the neutron skin thickness, the difference of the neutron and proton root-mean-square radii, could be constrained by studying properties of collective isovector observables at low energy [5]. One such observable is the nuclear electric dipole polarizability α D , which represents a viable tool to constrain the EOS of neutron matter and the physics of neutron stars [6][7][8][9][10][11].While correlations among α D , the neutron skin and the symmetry energy parameters have been studied extensively with EDFs [12][13][14][15][16], only recently have ab initio calculations based on chiral effective field theory (χEFT) interactions successfully studied such correlations in medium-mass nuclei [17,18]. By using a set of chiral two-plus three-nucleon interactions [19,20] and

Influence of complex configurations on the properties of the pygmy dipole resonance in neutron-rich Ca isotopes

“…The same evolution is obtained with other Skyrme EDF's [6]. In the case of 48 Ca, the experimental neutron skin thickness (0.14 − 0.20 fm) has been determined from the E1 strength distribution which is extracted from proton inelastic scattering [52]. HF-BCS analysis gives the neutron skin ∆R np of 48 Ca to be 0.16 fm and 0.14 fm with the SLy5 and SLy5+T EDF's, respectively.…”

“…The agreement between the SLy5 results and data is reasonable, the deviations being less than 1%. On the other hand, comparing SLy5 and SLy5+T results shows that the maximum relative difference between the binding energies is about 2% for the case of 52 Ca. In the case of the SLy5+T EDF, these deviations are connected with the central Skyrme parameters which have not been refitted after including the tensor terms [50].…”

“…The neutron ν2p experimental data show a different A-behavior, namely, the factor 5 reduction of S 1n values and the 7-time reduction of S 2n values from 40 Ca to 58 Ca. In general, both the SLy5 and SLy5+T interactions give an excellent description of S 1n and S 2n for 50,52 Ca isotopes. Most notably, the major shell closure at the magic neutron number N = 20 is too pronounced.…”

“…The strength V 0 is taken equal to −270 MeVfm 3 . This value of the pairing strength is fitted to reproduce the experimental neutron pairing energies of 50,52,54 Ca obtained from binding energies of neighboring isotopes. This choice of the pairing interaction has also been used for a satisfactory description of the experimental data of 70,72,74,76 Ni [48], 90,92 Zr and 92,94 Mo [41].…”

“…[6,[15][16][17]. Moreover, recent experimental studies indicate N = 32 as a new magic number in Ca isotopes due to the high energy of the first 2 + state in 52 Ca [18] and the trend obtained for the two-neutron separation energies [19]. The first experimental spectroscopic study of low-lying states in 54 Ca have been performed at RIKEN [20]: the 2 + 1 energy in 54 Ca was found to be only ∼ 500 keV below that in 52 Ca, suggesting a N = 34 new shell closure.…”