First evidence for non 1− pygmy dipole resonance in neutron-rich nuclei

Abstract We predict the existence of medium spin PDR and find experimental evidence in 80 Ge using β -decay of 80 Ga. This nucleus was produced in the photo-fission of 238 U. A hybrid γ -ray spectrometer was developed using a plastic detector for β -tagging, HPGe detectors for low-energy γ rays detection and phoswich detectors of PARIS for high-energy γ rays. The low-high energy coincidence spectrum shows two peaks at 7181(53) and 7337(53) keV in the β -decay of the 80m Ga isomeric state only, in coincidence with the 2 1 + → 0 + transition of 80 Ge. Combined with decay information, electric dipole character (E1) were assigned to these γ rays which de-excite 7.84 MeV and 7.996 MeV states. Based on this fact and the comparison with theoretical calculations, we conclude that these enhanced E1 transitions are driven by neutron-skin oscillation of 80 Ge. The oscillation is triggered by a conversion of a deeply-bound neutron from the Z=N=28 proton-neutron saturated core into a proton in the Fermi surface. These results provide an evidence for a PDR state with spin different from 1 − and built on a low-energy excited state. Consequently, it extends the PDR’s boundary along spin dimension for the first time and provides experimental data for testing the validity of the Brink-Axel hypothesis in the PDR region.