Pygmy dipole strength in 90Zr

TL;DR

This study investigates the low-energy dipole response of 90Zr using photon-scattering experiments, revealing a pygmy dipole resonance likely caused by neutron vibrations against the core, with results matching photoabsorption data.

Contribution

First detailed measurement of the dipole response in 90Zr up to 12.9 MeV, identifying pygmy dipole strength and linking it to neutron vibrations via quasiparticle-phonon model calculations.

Findings

Identified 189 energy levels up to 12.9 MeV.

Observed enhancement of E1 strength between 6 and 11 MeV.

Results agree with photoabsorption cross sections from (gamma,n) data.

Abstract

The dipole response of the N=50 nucleus 90Zr was studied in photon-scattering experiments at the electron linear accelerator ELBE with bremsstrahlung produced at kinetic electron energies of 7.9, 9.0, and 13.2 MeV. We identified 189 levels up to an excitation energy of 12.9 MeV. Statistical methods were applied to estimate intensities of inelastic transitions and to correct the intensities of the ground-state transitions for their branching ratios. In this way we derived the photoabsorption cross section up to the neutron-separation energy. This cross section matches well the photoabsorption cross section obtained from (gamma,n) data and thus provides information about the extension of the dipole-strength distribution toward energies below the neutron-separation energy. An enhancement of E1 strength has been found in the range of 6 MeV to 11 MeV. Calculations within the framework of the quasiparticle-phonon model ascribe this strength to a vibration of the excessive neutrons against the N = Z neutron-proton core, giving rise to a pygmy dipole resonance.