Investigation of the Pygmy dipole resonance across the shell closure in chromium isotopes

Abstract

For several nuclei in the vicinity of shell closures an accumulation of electric dipole strength around the particle separation threshold has been reported. This strength accumulation is often referred to as pygmy dipole resonance (PDR). As part of the systematic survey of the PDR, the dipole responses of the even-even chromium isotopes 50, 52, 54 Cr have been measured using the phenomenon of nuclear resonance fluorescence. Spin and parity quantum numbers as well as excitation energies and transition strengths were measured for numerous known and previously unknown states. The comparison to calculations within the quasi-particle phonon model and between the isotopes provide new insight about the onset of the PDR and its evolution to heavier and more neutron rich nuclei.