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Register a new userCharge radius of the short-lived $^{68}$Ni and correlation with the dipole polarizability
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Wilfried N\\\"ortersh\\\"auser
Publication date
2020
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English
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We present the first laser spectroscopic measurement of the neutron-rich nucleus $^{68}$Ni at the mbox{$N=40$} subshell closure and extract its nuclear charge radius. Since this is the only short-lived isotope for which the dipole polarizability $alpha_{rm D}$ has been measured, the combination of these observables provides a benchmark for nuclear structure theory. We compare them to novel coupled-cluster calculations based on different chiral two- and three-nucleon interactions, for which a strong correlation between the charge radius and dipole polarizability is observed, similar to the stable nucleus $^{48}$Ca. Three-particle--three-hole correlations in coupled-cluster theory substantially improve the description of the experimental data, which allows to constrain the neutron radius and neutron skin of $^{68}$Ni.
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The nuclear root-mean-square charge radius of $^{54}$Ni was determined with collinear laser spectroscopy to be $R(^{54}$Ni) = 3.737,(3)~fm. In conjunction with the known radius of the mirror nucleus $^{54}$Fe, the difference of the charge radii was extracted as $Delta R_{rm ch}$ = 0.049,(4)~fm. Based on the correlation between $Delta R_{rm ch}$ and the slope of the symmetry energy at nuclear saturation density ($L$), we deduced $20 le L le 70$,MeV. The present result is consistent with the $L$ from the binary neutron star merger GW170817, favoring a soft neutron matter EOS, and barely consistent with the PREX-2 result within 1$sigma$ error bands. Our result indicates the neutron-skin thickness of $^{48}$Ca as 0.15,-,0.19,fm.
The dipole polarizability of stable even-mass tin isotopes 112,114,116,118,120,124 was extracted from inelastic proton scattering experiments at 295 MeV under very forward angles performed at RCNP. Predictions from energy density functionals cannot account for the present data and the polarizability of 208Pb simultaneously. The evolution of the polarizabilities in neighboring isotopes indicates a kink at 120Sn while all model results show a nearly linear increase with mass number after inclusion of pairing corrections.
The electric dipole strength distribution in Ca-48 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 for the first time the extraction of the electric dipole polarizability alpha_D(Ca-48) = 2.07(22) fm^3. Remarkably, the dipole response of Ca-48 is found to be very similar to that of Ca-40, consistent with a small neutron skin in Ca-48. 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 Ca-48 of 0.14 - 0.20 fm.
The electric dipole strength distribution in 120Sn between 5 and 22 MeV has been determined at RCNP Osaka from a polarization transfer analysis of proton inelastic scattering at E_0 = 295 MeV and forward angles including 0{deg}. Combined with photoabsorption data an electric dipole polarizability alpha_D(120Sn) = 8.93(36) fm^3 is extracted. The dipole polarizability as isovector observable par excellence carries direct information on the nuclear symmetry energy and its density dependence. The correlation of the new value with the well established alpha_D(208Pb) serves as a test of its prediction by nuclear energy density functionals (EDFs). Models based on modern Skyrme interactions describe the data fairly well while most calculations based on relativistic Hamiltonians cannot.
The neutron is a cornerstone in our depiction of the visible universe. Despite the neutron zero-net electric charge, the asymmetric distribution of the positively- (up) and negatively-charged (down) quarks, a result of the complex quark-gluon dynamics, lead to a negative value for its squared charge radius, $langle r_{rm n}^2 rangle$. The precise measurement of the neutrons charge radius thus emerges as an essential part of unraveling its structure. Here we report on a $langle r_{rm n}^2 rangle$ measurement, based on the extraction of the neutron electric form factor, $G_{rm E}^{rm n}$, at low four-momentum transfer squared $(Q^2)$ by exploiting the long known connection between the $N rightarrow Delta$ quadrupole transitions and the neutron electric form factor. Our result, $langle r_{rm n}^2 rangle = -0.110 pm0.008~({rm fm}^2)$, addresses long standing unresolved discrepancies in the $langle r_{rm n}^2 rangle$ determination. The dynamics of the strong nuclear force can be viewed through the precise picture of the neutrons constituent distributions that result into the non-zero $langle r_{rm n}^2 rangle$ value.
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