Measurement of the Dipole Polarizability of the Unstable Neutron-Rich NucleusNi68

Abstract

The E1 strength distribution in 68Ni has been investigated using Coulomb excitation in inverse kinematics at the R3B-LAND setup and by measuring the invariant mass in the one- and two-neutron decay channels. The giant dipole resonance and a low-lying peak (pygmy dipole resonance) have been observed at 17.1(2) and 9.55(17) MeV, respectively. The measured dipole polarizability is compared to relativistic random phase approximation calculations yielding a neutron-skin thickness of 0.17(2) fm. A method and analysis applicable to neutron-rich nuclei has been developed, allowing for a precise determination of neutron skins in nuclei as a function of neutron excess.

Neutron skin thickness from the measured electric dipole polarizability inNi68,Sn120, andRoca-Maza1, Viñas2, Centelles3 et al. 2015Phys. Rev. C225322543View full textAdd to dashboardCite

“…The electric dipole response of 68 Ni has been measured in the energy interval between 7.8 MeV and 28.4 MeV and the dipole polarizability α D ( 68 Ni) = 3.40 ± 0.23 fm 3 has been reported [18]. However, to compare with RPA calculations, the measured dipole response has to be extrapolated to lower and higher energy regions to cover the full range between zero and some upper limit at which the contribution to the dipole polarizability becomes negligible (this limit is expected to lie much below the pion production threshold).…”

“…However, to compare with RPA calculations, the measured dipole response has to be extrapolated to lower and higher energy regions to cover the full range between zero and some upper limit at which the contribution to the dipole polarizability becomes negligible (this limit is expected to lie much below the pion production threshold). The strength below the neutron threshold, which was not accessible in the experiment [18], is estimated from the tail of a Lorentzian-plus-Gaussian fit to the deconvoluted data [42]. The Lorentzian extrapolates the giant dipole resonance to low energies and the Gaussian takes care of the PDS contribution to the strength.…”

“…It is expected that the uncertainty accounts for possible deviations of the hypothesis assumed in the extrapolation method. The strength above the upper experimental limit of 28.4 MeV [18] is also extrapolated from the same Lorentzian fit of the GDR strength. Such an extrapolation to energies above 30 MeV implies, in general, a rather small contribution of the dipole strength to the total polarizability.…”

“…Combining this new measurement with existing photo-absorption data up to 135 MeV [16], a value of α D ( 120 Sn) = 8.93 ± 0.36 fm 3 was obtained [15]. Finally, turning to exotic nuclei, the electric dipole response of the unstable 68 Ni isotope has been recently investigated at GSI using both Coulomb excitation in in-verse kinematics and by measuring the invariant mass in the one-and two-neutron decay channels [17,18]. From these measurements, which cover the range between 7.8 and 28.4 MeV, both the giant and pygmy dipole strength have been identified and the dipole polarizability of α D ( 68 Ni) = 3.40 ± 0.23 fm 3 has been obtained [18].…”

“…Finally, turning to exotic nuclei, the electric dipole response of the unstable 68 Ni isotope has been recently investigated at GSI using both Coulomb excitation in in-verse kinematics and by measuring the invariant mass in the one-and two-neutron decay channels [17,18]. From these measurements, which cover the range between 7.8 and 28.4 MeV, both the giant and pygmy dipole strength have been identified and the dipole polarizability of α D ( 68 Ni) = 3.40 ± 0.23 fm 3 has been obtained [18]. Note that neither the high nor the low energy tails of the dipole response of 68 Ni are experimentally known, so their contribution was not taken into account in the published value of the polarizability.…”